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In 2011, a Europa mission was recommended by the U.S. Planetary Science Decadal Survey. In response, NASA commissioned concept studies of a Europa lander in 2011, along with concepts for a Europa flyby (Europa Clipper), and a Europa orbiter. The orbiter element option concentrates on the "ocean" science, while the multiple-flyby element (Clipper) concentrates on the chemistry and energy science. On 13 January 2014, the House Appropriations Committee announced a new bipartisan bill that includes $80 million in funding to continue the Europa mission concept studies.
In July 2013 an updated concept for a flyby Europa mission called Europa Clipper was presented by the Jet Propulsion Laboratory (JPL) and the Applied Physics Laboratory (APL). In May 2015, NASA announced that it had accepted development of the Europa Clipper mission, and revealed the instruments it would use. The aim of Europa Clipper is to explore Europa in order to investigate its habitability, and to aid in selecting sites for a future lander. The Europa Clipper would not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude flybys of Europa during its envisioned mission. The probe would carry an ice-penetrating radar, short-wave infrared spectrometer, topographical imager, and an ion- and neutral-mass spectrometer. The mission was launched on 14 October 2024 aboard a Falcon Heavy.
Future missions
Conjectures regarding extraterrestrial life have ensured a high profile for Europa and have led to steady lobbying for future missions. The aims of these missions have ranged from examining Europa's chemical composition to searching for extraterrestrial life in its hypothesized subsurface oceans. Robotic missions to Europa need to endure the high-radiation environment around Jupiter. Because it is deeply embedded within Jupiter's magnetosphere, Europa receives about 5.40 Sv of radiation per day.
Europa Lander is a recent NASA concept mission under study. 2018 research suggests Europa may be covered in tall, jagged ice spikes, presenting a problem for any potential landing on its surface.
Old proposals | Europa (moon) | Wikipedia | 430 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
In the early 2000s, Jupiter Europa Orbiter led by NASA and the Jupiter Ganymede Orbiter led by the ESA were proposed together as an Outer Planet Flagship Mission to Jupiter's icy moons called Europa Jupiter System Mission, with a planned launch in 2020. In 2009 it was given priority over Titan Saturn System Mission. At that time, there was competition from other proposals. Japan proposed Jupiter Magnetospheric Orbiter.
Jovian Europa Orbiter was an ESA Cosmic Vision concept study from 2007. Another concept was Ice Clipper, which would have used an impactor similar to the Deep Impact mission—it would make a controlled crash into the surface of Europa, generating a plume of debris that would then be collected by a small spacecraft flying through the plume.
Jupiter Icy Moons Orbiter (JIMO) was a partially developed fission-powered spacecraft with ion thrusters that was cancelled in 2006. It was part of Project Prometheus. The Europa Lander Mission proposed a small nuclear-powered Europa lander for JIMO. It would travel with the orbiter, which would also function as a communication relay to Earth.
Europa Orbiter – Its objective would be to characterize the extent of the ocean and its relation to the deeper interior. Instrument payload could include a radio subsystem, laser altimeter, magnetometer, Langmuir probe, and a mapping camera. The Europa Orbiter received the go-ahead in 1999 but was canceled in 2002. This orbiter featured a special ice-penetrating radar that would allow it to scan below the surface.
More ambitious ideas have been put forward including an impactor in combination with a thermal drill to search for biosignatures that might be frozen in the shallow subsurface.
Another proposal put forward in 2001 calls for a large nuclear-powered "melt probe" (cryobot) that would melt through the ice until it reached an ocean below. Once it reached the water, it would deploy an autonomous underwater vehicle (hydrobot) that would gather information and send it back to Earth. Both the cryobot and the hydrobot would have to undergo some form of extreme sterilization to prevent detection of Earth organisms instead of native life and to prevent contamination of the subsurface ocean. This suggested approach has not yet reached a formal conceptual planning stage.
Habitability | Europa (moon) | Wikipedia | 468 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
So far, there is no evidence that life exists on Europa, but the moon has emerged as one of the most likely locations in the Solar System for potential habitability. Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean hydrothermal vents. Even if Europa lacks volcanic hydrothermal activity, a 2016 NASA study found that Earth-like levels of hydrogen and oxygen could be produced through processes related to serpentinization and ice-derived oxidants, which do not directly involve volcanism. In 2015, scientists announced that salt from a subsurface ocean may likely be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable. The likely presence of liquid water in contact with Europa's rocky mantle has spurred calls to send a probe there.
The energy provided by tidal forces drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. Although Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source. Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean. Should Europa's oceans be too cold, biological processes similar to those known on Earth could not occur; too salty, only extreme halophiles could survive in that environment. In 2010, a model proposed by Richard Greenberg of the University of Arizona proposed that irradiation of ice on Europa's surface could saturate its crust with oxygen and peroxide, which could then be transported by tectonic processes into the interior ocean. Such a process could render Europa's ocean as oxygenated as our own within just 12 million years, allowing the existence of complex, multicellular lifeforms. | Europa (moon) | Wikipedia | 440 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell, as well as pockets of water that form M-shaped ice ridges when the water freezes on the surface – as in Greenland. If confirmed, the lakes and pockets of water could be yet another potential habitat for life. Evidence suggests that hydrogen peroxide is abundant across much of the surface of Europa. Because hydrogen peroxide decays into oxygen and water when combined with liquid water, the authors argue that it could be an important energy supply for simple life forms. Nonetheless, on 4 March 2024, astronomers reported that the surface of Europa may have much less oxygen than previously inferred.
Clay-like minerals (specifically, phyllosilicates), often associated with organic matter on Earth, have been detected on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet. Some scientists have speculated that life on Earth could have been blasted into space by asteroid collisions and arrived on the moons of Jupiter in a process called lithopanspermia. | Europa (moon) | Wikipedia | 239 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. Sipuncula was once considered a phylum, but was demoted to a class of Annelida, based on recent molecular work.
Sipunculans vary in size but most species are under in length. The body is divided into an unsegmented, bulbous trunk and a narrower, anterior section, called the "introvert", which can be retracted into the trunk. The mouth is at the tip of the introvert and is surrounded in most groups by a ring of short tentacles. With no hard parts, the body is flexible and mobile. Although found in a range of habitats throughout the world's oceans, the majority of species live in shallow water habitats, burrowing under the surface of sandy and muddy substrates. Others live under stones, in rock crevices or in other concealed locations.
Most sipunculans are deposit feeders, extending the introvert to gather food particles and draw them into the mouth, and retracting the introvert when feeding conditions are unsuitable or danger threatens. With a few exceptions, reproduction is sexual and involves a planktonic larval stage. Sipunculid worms are used as food in some countries in south-east Asia.
Taxonomy
is a feminine variant of the now-obsolete genus name , itself a variant of the Latin ("little tube"), a diminutive of from Greek (síphōn, "tube, pipe").
The Swedish naturalist Carl Linnaeus first described the worm in his in 1767. In 1814, the French zoologist Constantine Samuel Rafinesque used the word "Sipuncula" to describe the family (now Sipunculidae), and in time, the term came to be used for the whole class. This is a relatively understudied group, and it is estimated there may be around 162 species worldwide. | Sipuncula | Wikipedia | 407 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
The phylogenetic placement of this group in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. These phyla have been included in a larger group, the Lophotrochozoa, that also includes the annelids, the ribbon worms and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Annelida (including echiurans and pogonophorans). It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.
Anatomy
Sipunculans are worms ranging from in length, with most species being under . The sipunculan body is divided into an unsegmented, bulbous trunk and a narrower, anterior section, called the "introvert". Sipunculans have a body wall somewhat similar to that of most other annelids (though unsegmented) in that it consists of an epidermis without cilia overlain by a cuticle, an outer layer of circular and an inner layer of longitudinal musculature. The body wall surrounds the coelom (body cavity) that is filled with fluid on which the body wall musculature acts as a hydrostatic skeleton to extend or contract the animal. When threatened, Sipunculid worms can contract their body into a shape resembling a peanut kernel—a practice that has given rise to the name "peanut worm". The introvert is pulled inside the trunk by two pairs of retractor muscles that extend as narrow ribbons from the trunk wall to attachment points in the introvert. It can be protruded from the trunk by contracting the muscles of the trunk wall, thus forcing the fluid in the body cavity forwards. The introvert can vary in size from half the length of the trunk to several times its length, but whatever their comparative sizes, it is fully retractable. | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
The mouth is located at the anterior end of the animal; in the subclass Sipunculidea, the mouth is surrounded by a mass of 18 to 24 ciliated tentacles, while in the subclass Phascolosomatidea, the tentacles are arranged in an arc above the mouth, surrounding the nuchal organ, also located at the tip of the introvert. The tentacles each have a deep groove along which food is moved to the mouth by cilia. They are used to gather organic detritus from the water or substrate, and probably also function as gills. In the family Themistidae the tentacles form an elaborate crown-like structure, the members of this group being specialized filter feeders, unlike the other groups of sipunculans which are deposit feeders. The tentacles are hollow and are extended via hydrostatic pressure in a similar manner as the introvert, but have a different mechanism from that of the rest of the introvert, being connected, via a system of ducts, to one or two contractile sacs next to the oesophagus. Hooks are often present near the mouth on the introvert. These are proteinaceous, non-chitinous specializations of the epidermis, either arranged in rings or scattered. They may be involved in scraping algae off rock, or alternatively provide anchorage. | Sipuncula | Wikipedia | 275 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Three genera (Aspidosiphon, Lithacrosiphon and Cloeosiphon) in the Aspidosiphonidae family possess epidermal structures, known as anal and caudal shields. These are patches of thickened, hard plates, and are used for boring into rock; the anal shield is near the anteriorly-located anus on the trunk, just below the introvert of the animal, while the caudal shield is at the posterior of the body. In Aspidosiphon and Lithacrosiphon the anal shield is restricted to the dorsal side, causing the introvert to emerge at an angle, whereas it surrounds the anterior trunk in Cloeosiphon with the introvert emerging from its center. In Aspidosiphon the shield is a hardened, horny structure; in Lithacrosiphon it is a calcareous cone; in Cloeosiphon it is composed of separate plates. When the introvert is retracted in these animals, the anal shield blocks the entrance to its burrow. At the posterior end of the trunk, a hardened caudal shield is sometimes present in Aspidosiphon; this may help with anchoring the animal in its burrow or may be used in the boring process.
Digestive system
The digestive tract of sipunculans starts with the esophagus, located between the introvert retractor muscles. In the trunk the intestine runs posteriorly, forms a loop and turns anteriorly again. The downward and upward sections of the gut are coiled around each other, forming a double helix. At the termination of the gut coil, the rectum emerges and ends in the anus, located in the anterior third of the trunk. Digestion is extra-cellular, taking place in the lumen of the intestine. A rectal caecum, present in most species, is a blind ending sac at the transition between intestine and rectum with unknown function. The anus is often not visible when the introvert is retracted into the trunk. | Sipuncula | Wikipedia | 420 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Circulation
Sipunculans do not have a vascular blood system. Fluid transport and gas exchange are instead accomplished by the coelom, which contains the respiratory pigment haemerythrin, and the separate tentacular system, the two being separated by an elaborate septum. The coelomic fluid contains five types of coelomic cells: haemocytes, granulocytes, large multinuclear cells, ciliated urn-shaped cells and immature cells. The ciliated urn cells may also be attached to the peritoneum and assist in waste filtering from the coelomic fluid. Nitrogenous waste is excreted through a pair of metanephridia opening close to the anus, except in Phascolion and Onchnesoma, which have only a single nephridium. A ciliated funnel, or nephrostome, opens into the coelomic cavity at the anterior end, close to the nephridiopore. The metanephridia have an osmoregulation function but it is unclear whether the mechanism is via filtration or secretion. They also serve as gamete storage and maintenance organs.
The tentacular coelom connects the tentacles at the tip of the introvert to a ring canal at their base, from which a contractile vessel runs along beside the esophagus and ends blindly posteriorly. Some evidence points towards the involvement of these structures in ultrafiltration. In crevice-dwelling sipunculans, respiration is mainly through the tentacular system, with oxygen diffusing into the trunk coelom from the tentacular coelom. However, in other species the skin is thin and respiration is mainly through the cuticle of the trunk, where oxygen uptake is assisted by the presence of dermal coelomic canals just beneath the epidermis.
Nervous system
The nervous system consists of dorsal cerebral ganglion, brain above the oesophagus and a nerve ring around the oesophagus, which links the brain with the single ventral nerve cord that runs the length of the body. Lateral nerves lead off this to innervate the muscles of the body wall. | Sipuncula | Wikipedia | 450 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
In some species, there are simple light-sensitive ocelli associated with the brain. Two organs, likely functioning as a unit for chemoreception are located near its anterior margin; the non-ciliated cerebral organ, which possesses bipolar sensory cells, and the nuchal organ, located posterior to the brain. Similar light-sensing tubes have been reported in the fauveliopsid annelids. In addition, all sipunculans have numerous sensory nerve endings on the body wall, especially at the forward end of the introvert which is used for exploring the surrounding environment.
Distribution and habitat
All sipunculid worms are marine and benthic; they are found throughout the world's oceans including polar waters, equatorial waters and the abyssal zone, but the majority of species occur in shallow water, where they are relatively common. They inhabit a range of habitats including burrowing in sand, mud, clay and gravel, hiding under stones, in rock crevices, in hollow coral heads, in wood, in empty seashells and inside the bones of dead whales. Some hide in kelp holdfasts, under tangles of eelgrass, inside sponges and in the empty tubes of other organisms, and some live among fouling organisms on man-made structures. Some bore into solid rocks to make a shelter for themselves.
They are common below the surface of the sediment on tidal flats. These worms may stay submerged in the sea bed for between 10 and 18 hours a day. They are sensitive to low salinities, and thus not commonly found near estuaries. They can also be abundant in coralline rock, and in Hawaii, up to seven hundred individuals have been found per square metre in burrows in the rock.
Reproduction
Both asexual and sexual reproduction can be found in sipunculans, although asexual reproduction is uncommon and has only been observed in Aspidosiphon elegans and Sipunculus robustus. These reproduce asexually through transverse fission, followed by regeneration of vital body components, with S. robustus also reproducing by budding. One species of sipunculan, Themiste lageniformis, has been recorded as reproducing parthenogenetically; eggs produced in the absence of sperm developed through the normal stages. | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Most sipunculan species are dioecious. Their gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment, where fertilisation takes place. In at least one species, Themiste pyroides, swarming behaviour occurs with adults creating compact masses among rocks immediately before spawning.
Although some species hatch directly into the adult form, many have a trochophore larva, which metamorphoses into the adult after anything from a day to a month, depending on species. In a few species, the trochophore does not develop directly into the adult, but into an intermediate pelagosphaera stage, that possesses a greatly enlarged metatroch (ciliated band). Metamorphosis occurs only in the presence of suitable habitat conditions, and is triggered by the presence of adults.
Behaviour
Most sipunculans are deposit feeders employing a number of different methods to obtain their foods. Those living in burrows extend their tentacles over the surface of the sediment. Food particles get trapped in mucous secretions and the beating of cilia transport the particles to the mouth. Among those that burrow through the sand, the tentacles are replaced by fluted folds which scoop up sediment and food particles. Most of this material is swallowed but larger particles are discarded. Species dwelling in crevices are able to withdraw their introverts, blocking the crevice entrance with their thickened trunks and presumably ingesting any food they have snared at the same time. One species, Thysanocardia procera is thought to be carnivorous, gaining entrance in some way to the interior of the sea mouse Aphrodita aculeata and sucking out its liquefied contents.
Fossil record
Because of their soft-bodied structure, fossils of sipunculans are extremely rare, and are only known from a few genera. Archaeogolfingia and Cambrosipunculus appear in the Cambrian Chengjiang biota in China. These fossils appear to belong to the crown group, and demonstrate that sipunculans have changed little (morphologically) since the early Cambrian, about 520 million years ago. | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
An unnamed sipunculid worm from the Cambrian period has been discovered in the Burgess Shale in Alberta, Canada, and Lecthaylus has been identified from the Granton Shrimp Bed, near Edinburgh, Scotland, dating to the Silurian period. Trace fossils of burrows that may have been formed by sipunculans have been found from the Paleozoic.
Some scientists once hypothesized a close relationship between sipunculans and the extinct hyoliths, operculate shells from the Palaeozoic with which they share a helical gut; but this hypothesis has since been discounted.
As food
Sipunculid worm jelly (土笋凍) is a delicacy in southeast China, originally from Anhai, near Quanzhou. A sipunculid worm dish is also considered a delicacy in the islands of the Visayas region, Philippines. The muscle is first prepared by soaking it in spiced vinegar and then served with other ingredients as a dish similar to ceviche. It is a basic food for local fisherman and is sometimes seen in city restaurants as an appetizer. This style of food preparation is locally called kilawin or kinilaw, and is also used for fish, conch and vegetables.
The worms, especially in dried form, are considered a delicacy in Vietnam as well, where they are caught on the coasts of Minh Chao island, in Van Don District. The relatively high market price of the worms have made them a significant source of income for the local population of fishermen families. | Sipuncula | Wikipedia | 320 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting a primitive metazoan phylum.
The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921). Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century, new species began to be added since 2018. So far, three other extant species have been described, in two distinct classes: Uniplacotomia (Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022) and Polyplacotomia (Polyplacotoma mediterranea, the most basal, in 2019). A single putative fossil species is known, the Middle Triassic Maculicorpus microbialis.
History
Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium at the Zoological Institute in Graz, Austria. The generic name is derived from the classical Greek (), meaning "hair", and (), "plate". The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination. Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behaviour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion. | Placozoa | Wikipedia | 445 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
In 1893, Italian zoologist Francesco Saverio Monticelli described another animal which he named Treptoplax, the specimens of which he collected from Naples. He gave the species name T. reptans in 1896. Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is ruled as doubtful, and the species rejected.
Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria). In 1907, Thilo Krumbach published a hypothesis that Trichoplax is not a distinct animal but that it is a form of the planula larva of the anemone-like hydrozoan Eleutheria krohni. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s.
The development of electron microscopy in the mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest in Trichoplax starting in 1966. The most important descriptions were made by Karl Gottlieb Grell at the University of Tübingen since 1971. That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa. Grell derived the name from the placula hypothesis, Otto Bütschli's notion on the origin of metazoans.
Biology
Placozoans do not have well-defined body plans, much like amoebas, unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob." An individual body measures about 0.55 mm in diameter. There are no body parts; as one of the researchers Michael Eitel described: "There's no mouth, there's no back, no nerve cells, nothing." Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells. | Placozoa | Wikipedia | 451 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower epithelia. There are at least six different cell types. The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a cilium. Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number of mitochondria. The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and endosymbiotic bacteria in the endoplasmic reticulum. The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a secretory granule.
The body axes of Hoilungia and Trichoplax are overtly similar to the oral–aboral axis of cnidarians, animals from another phylum with which they are most closely related. Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences. Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.
Trichoplax is a small, flattened, animal around across. An amorphous multi-celled body, analogous to a single-celled amoeba, it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more complex animals. The epithelial cells bear cilia, which the animal uses to help it creep along the seafloor.
The lower surface engulfs small particles of organic detritus, on which the animal feeds. All placozoans can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme.
Sexual reproduction has been reported to occur in one clade of placozoans, whose strain H8 was later found to belong to genus Cladtertia, where intergenic recombination was observed as well as other hallmarks of sexual reproduction. | Placozoa | Wikipedia | 497 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Some Trichoplax species contain Rickettsiales bacteria as endosymbionts.
One of the at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described Margulisbacteria, that lives inside cells used for algal digestion. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return.
Studies suggest that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity for spatial orientation.
Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators. Genes has been found in Trichoplax with a strong resemblance to the venom genes of some poisonous snakes, like the American copperhead and the West African carpet viper.
The Placozoa show substantial evolutionary radiation in regard to sodium channels, of which they have 5–7 different types, more than any other invertebrate species studied to date.
Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies.
In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity.
Evolutionary relationships
There is no convincing fossil record of the Placozoa, although the Ediacaran biota (Precambrian, ) organism Dickinsonia appears somewhat similar to placozoans. Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the Middle Triassic Muschelkalk (Germany). | Placozoa | Wikipedia | 410 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Traditionally, classification was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss and thus is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the sponges and the Eumetazoa. In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.
Their exact position on the phylogenetic tree would give important information about the origin of neurons and muscles. If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a sister group; once in the Ctenophora, once in the Cnidaria and once in the Bilateria. If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups.
Functional-morphology hypothesis
On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals (Metazoa) and are thus considered a sister taxon to all other metazoans:
According to a functional-morphology model, all or most animals are descended from a gallertoid, a free-living (pelagic) sphere in seawater, consisting of a single ciliated layer of cells supported by a thin, noncellular separating layer, the basal lamina. The interior of the sphere is filled with contractile fibrous cells and a gelatinous extracellular matrix. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes: | Placozoa | Wikipedia | 391 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Infolding of the epithelium led to the formation of an internal system of ducts and thus to the development of a modified gallertoid from which the sponges (Porifera), Cnidaria and Ctenophora subsequently developed.
Other gallertoids, according to this model, made the transition over time to a benthic mode of life; that is, their habitat has shifted from the open ocean to the floor (benthic zone). This results naturally in a selective advantage for flattening of the body, as of course can be seen in many benthic species.
While the probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the substrate, leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to the substrate. In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed placuloid. | Placozoa | Wikipedia | 244 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Three different life strategies have accordingly led to three different possible lines of development:
Animals that live interstitially in the sand of the ocean floor were responsible for the fossil crawling traces that are considered the earliest evidence of animals; and are detectable even prior to the dawn of the Ediacaran Period in geology. These are usually attributed to bilaterally symmetrical worms, but the hypothesis presented here views animals derived from placuloids, and thus close relatives of Trichoplax adhaerens, to be the producers of the traces.
Animals that incorporated algae as photosynthetically active endosymbionts, i.e. primarily obtaining their nutrients from their partners in symbiosis, were accordingly responsible for the mysterious creatures of the Ediacara fauna that are not assigned to any modern animal taxon and lived during the Ediacaran Period, before the start of the Paleozoic. However, recent work has shown that some of the Ediacaran assemblages (e.g. Mistaken Point) were in deep water, below the photic zone, and hence those individuals could not dependent on endosymbiotic photosynthesisers.
Animals that grazed on algal mats would ultimately have been the direct ancestors of the Placozoa. The advantages of an amoeboid multiplicity of shapes thus allowed a previously present basal lamina and a gelatinous extracellular matrix to be lost secondarily. Pronounced differentiation between the surface facing the substrate (ventral) and the surface facing away from it (dorsal) accordingly led to the physiologically distinct cell layers of Trichoplax adhaerens that can still be seen today. Consequently, these are analogous, but not homologous, to ectoderm and endoderm – the "external" and "internal" cell layers in eumetazoans – i.e. the structures corresponding functionally to one another have, according to the proposed hypothesis, no common evolutionary origin. | Placozoa | Wikipedia | 401 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Should any of the analyses presented above turn out to be correct, Trichoplax adhaerens would be the oldest branch of the multicellular animals, and a relic of the Ediacaran fauna, or even the pre-Ediacara fauna. Although very successful in their ecological niche, due to the absence of extracellular matrix and basal lamina, the development potential of these animals was of course limited, which would explain the low rate of evolution of their phenotype (their outward form as adults) – referred to as bradytely.
This hypothesis was supported by a recent analysis of the Trichoplax adhaerens mitochondrial genome in comparison to those of other animals. The hypothesis was, however, rejected in a statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only at which indicates a marginal level of statistical significance.
Epitheliozoa hypothesis
A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues (Eumetazoa). The taxon they share, called the Epitheliozoa, is itself construed to be a sister group to the sponges (Porifera):
The above view could be correct, although there is some evidence that the ctenophores, traditionally seen as Eumetazoa, may be the sister to all other animals.
This is now a disputed classification. Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650 million years ago (many of the genes expressed in modern neurons are absent in ctenopheres, although some of these missing genes are present in placozoans).
The principal support for such a relationship comes from special cell to cell junctions – belt desmosomes – that occur not just in the Placozoa but in all animals except the sponges: They enable the cells to join together in an unbroken layer like the epitheloid of the Placozoa. Trichoplax adhaerens also shares the ventral gland cells with most eumetazoans. Both characteristics can be considered evolutionarily derived features (apomorphies), and thus form the basis of a common taxon for all animals that possess them. | Placozoa | Wikipedia | 501 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in Trichoplax adhaerens evolved by reduction of the collars in the collar cells (choanocytes) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as the precursor to the true epithelial tissue of the eumetazoans.
In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm — the two basic embryonic cell layers of the eumetazoans. The digestive gastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals (Bilateria) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer (epidermis), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians.
As noted above, this hypothesis was supported in a statistical analysis of the Trichoplax adhaerens whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species.
Eumetazoa hypothesis
A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified eumetazoans. According to this, Trichoplax adhaerens is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as the basal lamina of the epithelium, were accordingly lost more recently by radical secondary simplification.
Various studies in this regard so far yield differing results for identifying the exact sister group: In one case, the Placozoa would qualify as the nearest relatives of the Cnidaria, while in another they would be a sister group to the Ctenophora, and occasionally they are placed directly next to the Bilateria. Currently, they are typically placed according to the cladogram below: | Placozoa | Wikipedia | 494 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
In this cladogram the Epitheliozoa and Eumetazoa are synonyms to each other and to the Diploblasts, and the Ctenophora are basal to them.
An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is only known for parasitic organisms, but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens.
This version is supported by statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species. However, Ctenophora was not included in the analyses, placing the placozoans outside of the sampled Eumetazoans.
Cnidaria-sister hypothesis
DNA comparisons suggest that placozoans are related to Cnidaria, derived from planula larva (as seen in some Cnidaria). The Bilateria also are thought to be derived from planuloids. The Cnidaria and Placozoa body axis are overtly similar, and placozoan and cnidarian cells are responsive to the same neuropeptide antibodies despite extant placozoans not developing any neurons. | Placozoa | Wikipedia | 287 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.
Taxonomy
Symbion was discovered in 1995 by Reinhardt Kristensen and Peter Funch on the mouthparts of the Norway lobster (Nephrops norvegicus). Other, related, species have since been discovered on:
the American lobster (Homarus americanus, host to Symbion americanus)
the European lobster (Homarus gammarus, host to an as yet unnamed species of Symbion)
The genus is so named because of its commensal relationship with the lobster (a form of symbiosis) – it feeds on the leftovers from the lobster's own meals.
They are peculiar microscopic animals, with no obvious close relatives, which were therefore given their own phylum, called Cycliophora. The phylogenetic position of Symbion is still not finally settled. Currently it is placed in the clade Polyzoa along with the phyla Ectoprocta and Entoprocta, based on genetic analysis.
Description
Symbion pandora has a bilateral, sac-like body with no coelom. There are three basic life stages:
Asexual Feeding Stage – At this stage, S. pandora is neither male nor female. It has a length of 347 μm and a width of 113 μm. On the posterior end of the sac-like body is a stalk with an adhesive disc, which attaches itself to the host. On the anterior end is a ciliated funnel (mouth) and an anus.
Sexual Stage
Male – S. pandora has a length of 84 μm and a width of 42 μm during this stage. It has no mouth or anus, which signifies the absence of a digestive system. It also has two reproductive organs.
Female – S. pandora is the same size as the male in this stage. It does, however, have a digestive system which collapses and reconstitutes itself as a larva. | Symbion | Wikipedia | 508 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
Reproduction
Symbion reproduces both asexually and sexually, and has a complex reproduction cycle, a strategy evolved to produce as many offspring as possible that can survive and find a new host when the lobster they live on sheds its shell. The asexual individuals are the largest ones. The sexual individuals do not eat. During the autumn they make copies of themselves, where a new individual grows inside the parent body, one offspring at the time. The new offspring attach themselves to an available spot on the lobster, begin to feed and eventually start making new copies of themselves.
In early winter, the asexual animals start producing males. When a male is born, it crawls away from its parent and glues itself to another asexual individual. Once attached, the male produces two dwarf males inside its body, which turns into a hollow pouch. Each of the two dwarf males are about one hundred times smaller than the asexual individual to which they are attached. Their bodies start out with about 200 cells, but this number has been reduced to just 47 by the time they reach maturity. Thirty-four of the cells form its nervous system, and three more become sensory cells used to help them feel their surroundings. Eight cells becomes mucous glands, which produce mucus that helps them move across the surface. The final two cells form the testes, which make the sperm that fertilize the female's egg. Most of the cells of the dwarf males also lose their nucleus and shrink to almost half their size, which is an adaptation that allows two mature individuals to fit inside the body of the parent male. Two males increases their chances to fertilize a female. | Symbion | Wikipedia | 341 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
By late winter, when the large feeding individuals in the colony have males attached to their bodies, they start making females. Each female has a single egg inside her. When she is about to be born, one of the two dwarf males fertilizes her when she comes out. The fertilized female finds herself a place on the host's whiskers where she attaches herself. Inside her the developing embryo extracts all the nutrients it needs to grow from its mother, and by the time it is ready to be born, all that remains of the mother is an empty husk. This new offspring is a strong swimmer unlike all the other forms in the colony, and those who succeed in finding a new host will attach themselves to its mouthparts, where it will grow a stomach and mouthparts, morphing into a large, feeding and asexual type, starting the cycle all over again.
The larval stage may be unscientifically referred to as sea worms. | Symbion | Wikipedia | 201 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry (pentamerous symmetry), and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in the deep sea, as well as shallower oceans. Most echinoderms are able to reproduce asexually and regenerate tissue, organs and limbs; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified dermal endoskeletons, which are major contributors to many limestone formations and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries. Further, some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution.
Etymology
The name echinoderm is .
The name Echinodermata was originated by Jacob Theodor Klein in 1734, but only in reference to echinoids. It was expanded to the phylum level by Jean Guillaume Bruguière, first informally in 1789 and then in formal Latin in 1791. In 1955, Libbie Hyman attributed the name to "Bruguière, 1791 [ex Klein, 1734]."
This attribution has become common and is listed by the Integrated Taxonomic Information System (ITIS), although some workers believe that the ITIS rules should result in attributing "Klein, 1778" due to a 2nd edition of his work published by Leske in that year. | Echinoderm | Wikipedia | 478 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
While Echinodermata has been in common use since the mid-1800s, several other names had been proposed. Notably, F. A. Bather called the phylum "Echinoderma" (apparently after Latreille, 1825) in his 1900 treatise on the phylum, but this name now refers to a fungus.
Diversity
There are about 7,600 extant species of echinoderm as well as about 13,000 extinct species.
All echinoderms are marine, but they are found in habitats ranging from shallow intertidal areas to abyssal depths. Five extant classes of echinoderms are generally recognized: the Asteroidea (starfish, with over 1900 species), Ophiuroidea (brittle stars, with around 2,300 species), Echinoidea (sea urchins and sand dollars, with some 900 species), Holothuroidea (sea cucumbers, with about 1,430 species), and Crinoidea (feather stars and sea lilies, with around 580 species).
Anatomy and physiology
Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial symmetry, their larvae are ciliated, free-swimming organisms with bilateral symmetry. Later, during metamorphosis, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. Within the Asterozoa, there can be a few exceptions from the rule. Most starfish in the genus Leptasterias have six arms, although five-armed individuals can occur. The Brisingida also contain some six-armed species. Amongst the brittle stars, six-armed species such as Ophiothela danae, Ophiactis savignyi, and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six.
Echinoderms have secondary radial symmetry in portions of their body at some stage of life, most likely an adaptation to a sessile or slow-moving existence. Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while the sea-lily Comaster schlegelii has two hundred. | Echinoderm | Wikipedia | 504 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Genetic studies have shown that genes directing anterior-most development are expressed along ambulacra in the center of starfish rays, with the next-most-anterior genes expressed in the surrounding fringe of tube feet. Genes related to the beginning of the trunk are expressed at the ray margins, but trunk genes are only expressed in interior tissue rather than on the body surface. This means that a starfish body can more-or-less be considered to consist only of a head.
Skin and skeleton
Echinoderms have a mesodermal skeleton in the dermis, composed of calcite-based plates known as ossicles. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom. Ossicles may be fused together, as in the test of sea urchins, or may articulate to form flexible joints as in the arms of sea stars, brittle stars and crinoids. The ossicles may bear external projections in the form of spines, granules or warts and they are supported by a tough epidermis. Skeletal elements are sometimes deployed in specialized ways, such as the chewing organ called "Aristotle's lantern" in sea urchins, the supportive stalks of crinoids, and the structural "lime ring" of sea cucumbers.
Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in the fossil record. On the other hand, sea urchins are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is continuous with the surrounding rock. On fracturing such rock, paleontologists can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test.
The epidermis contains pigment cells that provide the often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple. These cells may be light-sensitive, causing many echinoderms to change appearance completely as night falls. The reaction can happen quickly: the sea urchin Centrostephanus longispinus changes colour in just fifty minutes when exposed to light. | Echinoderm | Wikipedia | 460 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
One characteristic of most echinoderms is a special kind of tissue known as catch connective tissue. This collagen-based material can change its mechanical properties under nervous control rather than by muscular means. This tissue enables a starfish to go from moving flexibly around the seabed to becoming rigid while prying open a bivalve mollusc or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines upright as a defensive mechanism when attacked.
The water vascular system
Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the coelom (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like madreporite on the aboral (upper) surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or oesophagus. The ring canal branches into a set of radial canals, which in asteroids extend along the arms, and in echinoids adjoin the test in the ambulacral areas. Short lateral canals branch off the radial canals, each one ending in an ampulla. Part of the ampulla can protrude through a pore (or a pair of pores in sea urchins) to the exterior, forming a podium or tube foot. The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla. | Echinoderm | Wikipedia | 362 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The organisation of the water vascular system is somewhat different in ophiuroids, where the madreporite may be on the oral surface and the podia lack suckers. In holothuroids, the system is reduced, often with few tube feet other than the specialised feeding tentacles, and the madreporite opens on to the coelom. Some holothuroids like the Apodida lack tube feet and canals along the body; others have longitudinal canals. The arrangement in crinoids is similar to that in asteroids, but the tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth. In the asteroids, the same motion is employed to move the animal across the ground.
Other organs
Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with the anus located in the centre of the aboral body surface. With a few exceptions, the members of the order Paxillosida do not possess an anus. In many species of starfish, the large cardiac stomach can be everted to digest food outside the body. Some other species are able to ingest whole food items such as molluscs. Brittle stars, which have varying diets, have a blind gut with no intestine or anus; they expel food waste through their mouth. Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew their food, mainly algae. They have an oesophagus, a large stomach and a rectum with the anus at the apex of the test. Sea cucumbers are mostly detritivores, sorting through the sediment with modified tube feet around their mouth, the buccal tentacles. Sand and mud accompanies their food through their simple gut, which has a long coiled intestine and a large cloaca. Crinoids are suspension feeders, passively catching plankton which drift into their outstretched arms. Boluses of mucus-trapped food are passed to the mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine. | Echinoderm | Wikipedia | 489 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The coelomic cavities of echinoderms are complex. Aside from the water vascular system, echinoderms have a haemal coelom, a perivisceral coelom, a gonadal coelom and often also a perihaemal coelom. During development, echinoderm coelom is divided into the metacoel, mesocoel and protocoel (also called somatocoel, hydrocoel and axocoel, respectively). The water vascular system, haemal system and perihaemal system form the tubular coelomic system. Echinoderms are unusual in having both a coelomic circulatory system (the water vascular system) and a haemal circulatory system, as most groups of animals have just one of the two.
Haemal and perihaemal systems are derived from the original coelom, forming an open and reduced circulatory system. This usually consists of a central ring and five radial vessels. There is no true heart, and the blood often lacks any respiratory pigment. Gaseous exchange occurs via dermal branchiae or papulae in starfish, genital bursae in brittle stars, peristominal gills in sea urchins and cloacal trees in sea cucumbers. Exchange of gases also takes place through the tube feet. Echinoderms lack specialized excretory (waste disposal) organs and so nitrogenous waste, chiefly in the form of ammonia, diffuses out through the respiratory surfaces. | Echinoderm | Wikipedia | 323 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The coelomic fluid contains the coelomocytes, or immune cells. There are several types of immune cells, which vary among classes and species. All classes possess a type of phagocytic amebocyte, which engulf invading particles and infected cells, aggregate or clot, and may be involved in cytotoxicity. These cells are usually large and granular, and are believed to be a main line of defence against potential pathogens. Depending on the class, echinoderms may have spherule cells (for cytotoxicity, inflammation, and anti-bacterial activity), vibratile cells (for coelomic fluid movement and clotting), and crystal cells (which may serve for osmoregulation in sea cucumbers). The coelomocytes secrete antimicrobial peptides against bacteria, and have a set of lectins and complement proteins as part of an innate immune system that is still being characterised.
Echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnected neurons with no central brain, although some do possess ganglia. Nerves radiate from central rings around the mouth into each arm or along the body wall; the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet. Starfish have sensory cells in the epithelium and have simple eyespots and touch-sensitive tentacle-like tube feet at the tips of their arms. Sea urchins have no particular sense organs but do have statocysts that assist in gravitational orientation, and they too have sensory cells in their epidermis, particularly in the tube feet, spines and pedicellariae. Brittle stars, crinoids and sea cucumbers in general do not have sensory organs, but some burrowing sea cucumbers of the order Apodida have a single statocyst adjoining each radial nerve, and some have an eyespot at the base of each tentacle.
The gonads at least periodically occupy much of the body cavities of sea urchins and sea cucumbers, while the less voluminous crinoids, brittle stars and starfish have two gonads in each arm. While the ancestors of modern echinoderms are believed to have had one genital aperture, many organisms have multiple gonopores through which eggs or sperm may be released.
Regeneration | Echinoderm | Wikipedia | 501 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Many echinoderms have great powers of regeneration. Many species routinely autotomize and regenerate arms and viscera. Sea cucumbers often discharge parts of their internal organs if they perceive themselves to be threatened, regenerating them over the course of several months. Sea urchins constantly replace spines lost through damage, while sea stars and sea lilies readily lose and regenerate their arms. In most cases, a single severed arm cannot grow into a new starfish in the absence of at least part of the disc. However, in a few species a single arm can survive and develop into a complete individual, and arms are sometimes intentionally detached for the purpose of asexual reproduction. During periods when they have lost their digestive tracts, sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water.
The regeneration of lost parts involves both epimorphosis and morphallaxis. In epimorphosis stem cells, either from a reserve pool or those produced by dedifferentiation, form a blastema and generate new tissues. Morphallactic regeneration involves the movement and remodelling of existing tissues to replace lost parts. Direct transdifferentiation of one type of tissue to another during tissue replacement is also observed.
Reproduction
Sexual reproduction
Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. Almost all species have separate male and female sexes, though some are hermaphroditic. The eggs and sperm cells are typically released into open water, where fertilisation takes place. The release of sperm and eggs is synchronised in some species, usually with regard to the lunar cycle. In other species, individuals may aggregate during the reproductive season, increasing the likelihood of successful fertilisation. Internal fertilisation has been observed in three species of sea star, three brittle stars and a deep-water sea cucumber. Even at abyssal depths, where no light penetrates, echinoderms often synchronise their reproductive activity. | Echinoderm | Wikipedia | 425 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Some echinoderms brood their eggs. This is especially common in cold water species where planktonic larvae might not be able to find sufficient food. These retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body, or even in her cardiac stomach. Many brittle stars are hermaphrodites; they often brood their eggs, usually in special chambers on their oral surfaces, but sometimes in the ovary or coelom. In these starfish and brittle stars, development is usually direct to the adult form, without passing through a bilateral larval stage. A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines. Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back, where they are retained. In a very small number of species, the eggs are retained in the coelom where they develop viviparously, later emerging through ruptures in the body wall. In some crinoids, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them.
Asexual reproduction
One species of seastar, Ophidiaster granifer, reproduces asexually by parthenogenesis. In certain other asterozoans, adults reproduce asexually until they mature, then reproduce sexually. In most of these species, asexual reproduction is by transverse fission with the disc splitting in two. Both the lost disc area and the missing arms regrow, so an individual may have arms of varying lengths. During the period of regrowth, they have a few tiny arms and one large arm, and are thus often known as "comets".
Adult sea cucumbers reproduce asexually by transverse fission. Holothuria parvula uses this method frequently, splitting into two a little in front of the midpoint. The two halves each regenerate their missing organs over a period of several months, but the missing genital organs are often very slow to develop. | Echinoderm | Wikipedia | 462 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The larvae of some echinoderms are capable of asexual reproduction. This has long been known to occur among starfish and brittle stars, but has more recently been observed in a sea cucumber, a sand dollar and a sea urchin. This may be by autotomising parts that develop into secondary larvae, by budding, or by splitting transversely. Autotomised parts or buds may develop directly into fully formed larvae, or may pass through a gastrula or even a blastula stage. New larvae can develop from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms, or their rear ends.
Cloning is costly to the larva both in resources and in development time. Larvae undergo this process when food is plentiful or temperature conditions are optimal. Cloning may occur to make use of the tissues that are normally lost during metamorphosis. The larvae of some sand dollars clone themselves when they detect dissolved fish mucus, indicating the presence of predators. Asexual reproduction produces many smaller larvae that escape better from planktivorous fish, implying that the mechanism may be an anti-predator adaptation.
Larval development
Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often planktonic, but in some species the eggs are retained inside the female, while in some the female broods the larvae. | Echinoderm | Wikipedia | 361 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example, a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a 'bipinnaria' larva, which develops into a multi-armed 'brachiolaria' larva. A sea cucumber's larva is an 'auricularia' while a crinoid's is a 'vitellaria'. All these larvae are bilaterally symmetrical and have bands of cilia with which they swim; some, usually known as 'pluteus' larvae, have arms. When fully developed, they settle on the seabed to undergo metamorphosis, and the larval arms and gut degenerate. The left-hand side of the larva develops into the oral surface of the juvenile, while the right side becomes the aboral surface. At this stage, the pentaradial symmetry develops.
A plankton-eating larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using a yolk-feeding larva. The provision of a yolk-sac means that smaller numbers of eggs are produced, the larvae have a shorter development period and a smaller dispersal potential, but a greater chance of survival.
Distribution and habitat
Echinoderms are globally distributed in almost all depths, latitudes and environments in the ocean. Living echinoderms are known from between 0 to over 10,000 meters. Adults are mainly benthic, living on the seabed, whereas larvae are often pelagic, living as plankton in the open ocean. Some holothuroid adults such as Pelagothuria are pelagic. In the fossil record, some crinoids were pseudo-planktonic, attaching themselves to floating logs and debris. Some Paleozoic taxa displayed this life mode, before competition from organisms such as barnacles restricted the extent of the behaviour.
Mode of life
Locomotion | Echinoderm | Wikipedia | 456 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms primarily use their tube feet to move about, though some sea urchins also use their spines. The tube feet typically have a tip shaped like a suction pad in which a vacuum can be created by contraction of muscles. This combines with some stickiness from the secretion of mucus to provide adhesion. The tube feet contract and relax in waves which move along the adherent surface, and the animal moves slowly along.
Brittle stars are the most agile of the echinoderms. Any one of the arms can form the axis of symmetry, pointing either forwards or back. The animal then moves in a co-ordinated way, propelled by the other four arms. During locomotion, the propelling arms can made either snake-like or rowing movements. Starfish move using their tube feet, keeping their arms almost still, including in genera like Pycnopodia where the arms are flexible. The oral surface is covered with thousands of tube feet which move out of time with each other, but not in a metachronal rhythm; in some way, however, the tube feet are coordinated, as the animal glides steadily along. Some burrowing starfish have points rather than suckers on their tube feet and they are able to "glide" across the seabed at a faster rate.
Sea urchins use their tube feet to move around in a similar way to starfish. Some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate. If a sea urchin is overturned, it can extend its tube feet in one ambulacral area far enough to bring them within reach of the substrate and then successively attach feet from the adjoining area until it is righted. Some species bore into rock, usually by grinding away at the surface with their mouthparts.
Most sea cucumber species move on the surface of the seabed or burrow through sand or mud using peristaltic movements; some have short tube feet on their under surface with which they can creep along in the manner of a starfish. Some species drag themselves along using their buccal tentacles, while others manage to swim with peristaltic movements or rhythmic flexing. Many live in cracks, hollows and burrows and hardly move at all. Some deep-water species are pelagic and can float in the water with webbed papillae forming sails or fins. | Echinoderm | Wikipedia | 496 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The majority of feather stars (also called Comatulida or "unstalked crinoids") and some stalked forms are motile. Several stalked crinoid species are sessile, attached permanently to the substratum. Movement in most sea lilies is limited to bending (their stems can bend) and rolling and unrolling their arms; a few species can relocate themselves on the seabed by crawling. Feather stars are unattached and usually live in crevices, under corals or inside sponges with their arms the only visible part. Some feather stars emerge at night and perch themselves on nearby eminences to better exploit food-bearing currents. Many species can "walk" across the seabed, raising their body with the help of their arms, or swim using their arms. Most species of feather stars, however, are largely sedentary, seldom moving far from their chosen place of concealment.
Feeding
The modes of feeding vary greatly between the different echinoderm taxa. Crinoids and some brittle stars tend to be passive filter-feeders, enmeshing suspended particles from passing water. Most sea urchins are grazers; sea cucumbers are deposit feeders; and the majority of starfish are active hunters.
Crinoids catch food particles using the tube feet on their outspread pinnules, move them into the ambulacral grooves, wrap them in mucus, and convey them to the mouth using the cilia lining the grooves. The exact dietary requirements of crinoids have been little researched, but in the laboratory, they can be fed with diatoms.
Basket stars are suspension feeders, raising their branched arms to collect zooplankton, while other brittle stars use several methods of feeding. Some are suspension feeders, securing food particles with mucus strands, spines or tube feet on their raised arms. Others are scavengers and detritus feeders. Others again are voracious carnivores and able to lasso their waterborne prey with a sudden encirclement by their flexible arms. The limbs then bend under the disc to transfer the food to the jaws and mouth. | Echinoderm | Wikipedia | 449 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Many sea urchins feed on algae, often scraping off the thin layer of algae covering the surfaces of rocks with their specialised mouthparts known as Aristotle's lantern. Other species devour smaller organisms, which they may catch with their tube feet. They may also feed on dead fish and other animal matter. Sand dollars may perform suspension feeding and feed on phytoplankton, detritus, algal pieces and the bacterial layer surrounding grains of sand.
Sea cucumbers are often mobile deposit or suspension feeders, using their buccal podia to actively capture food and then stuffing the particles individually into their buccal cavities. Others ingest large quantities of sediment, absorb the organic matter and pass the indigestible mineral particles through their guts. In this way they disturb and process large volumes of substrate, often leaving characteristic ridges of sediment on the seabed. Some sea cucumbers live infaunally in burrows, anterior-end down and anus on the surface, swallowing sediment and passing it through their gut. Other burrowers live anterior-end up and wait for detritus to fall into the entrances of the burrows or rake in debris from the surface nearby with their buccal podia.
Nearly all starfish are detritus feeders or carnivores, though a few are suspension feeders. Small fish landing on the upper surface may be captured by pedicilaria and dead animal matter may be scavenged but the main prey items are living invertebrates, mostly bivalve molluscs. To feed on one of these, the starfish moves over it, attaches its tube feet and exerts pressure on the valves by arching its back. When a small gap between the valves is formed, the starfish inserts part of its stomach into the prey, excretes digestive enzymes and slowly liquefies the soft body parts. As the adductor muscle of the bivalve relaxes, more stomach is inserted and when digestion is complete, the stomach is returned to its usual position in the starfish with its now liquefied bivalve meal inside it. Other starfish evert the stomach to feed on sponges, sea anemones, corals, detritus and algal films.
Antipredator defence | Echinoderm | Wikipedia | 473 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Despite their low nutrition value and the abundance of indigestible calcite, echinoderms are preyed upon by many organisms, including bony fish, sharks, eider ducks, gulls, crabs, gastropod molluscs, other echinoderms, sea otters, Arctic foxes and humans. Larger starfish prey on smaller ones; the great quantity of eggs and larva that they produce form part of the zooplankton, consumed by many marine creatures. Crinoids, on the other hand, are relatively free from predation.
Antipredator defences include the presence of spines, toxins (inherent or delivered through the tube feet), and the discharge of sticky entangling threads by sea cucumbers. Although most echinoderm spines are blunt, those of the crown-of-thorns starfish are long and sharp and can cause a painful puncture wound as the epithelium covering them contains a toxin. Because of their catch connective tissue, which can change rapidly from a flaccid to a rigid state, echinoderms are very difficult to dislodge from crevices. Some sea cucumbers have a cluster of cuvierian tubules which can be ejected as long sticky threads from their anus to entangle and permanently disable an attacker. Sea cucumbers occasionally defend themselves by rupturing their body wall and discharging the gut and internal organs. Starfish and brittle stars may undergo autotomy when attacked, detaching an arm; this may distract the predator for long enough for the animal to escape. Some starfish species can swim away from danger.
Ecology
Echinoderms are numerous invertebrates whose adults play an important role in benthic ecosystems, while the larvae are a major component of the plankton. Among the ecological roles of adults are the grazing of sea urchins, the sediment processing of heart urchins, and the suspension and deposit feeding of crinoids and sea cucumbers. Some sea urchins can bore into solid rock, destabilising rock faces and releasing nutrients into the ocean. Coral reefs are also bored into in this way, but the rate of accretion of carbonate material is often greater than the erosion produced by the sea urchin. Echinoderms sequester about 0.1 gigatonnes of carbon dioxide per year as calcium carbonate, making them important contributors in the global carbon cycle. | Echinoderm | Wikipedia | 500 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms sometimes have large population swings which can transform ecosystems. In 1983, for example, the mass mortality of the tropical sea urchin Diadema antillarum in the Caribbean caused a change from a coral-dominated reef system to an alga-dominated one. Sea urchins are among the main herbivores on reefs and there is usually a fine balance between the urchins and the kelp and other algae on which they graze. A diminution of the numbers of predators (otters, lobsters and fish) can result in an increase in urchin numbers, causing overgrazing of kelp forests, resulting in an alga-denuded "urchin barren". On the Great Barrier Reef, an unexplained increase in the numbers of crown-of-thorns starfish (Acanthaster planci), which graze on living coral tissue, has greatly increased coral mortality and reduced coral reef biodiversity.
Taxonomy and evolution
The characteristics of adult echinoderms are the possession of a water vascular system with external tube feet and a stereom endoskeleton.
Stereom is a calcareous material consisting of ossicles connected by a mesh of collagen fibres, which is unique to this phylum.
Phylogeny
Echinoderm phylogeny has long been a contentious subject. While the relationships among extant taxa are well-understood, there is no broadly accepted consensus regarding the phylum's origins or the relationships among its extinct groups. Echinoderm evolution shows a high degree of homoplasy, meaning that many features have evolved multiple times independently. This means that many features initiatlly assumed to indicate a genetic connection do not, in fact, do so, which has obscured the true relationships of various groups.
External phylogeny
Echinoderms are bilaterians, meaning that their ancestors were mirror-symmetric. Among the bilaterians, they belong to the deuterostome division, meaning that the blastopore, the first opening to form during embryo development, becomes the anus instead of the mouth. | Echinoderm | Wikipedia | 438 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms are the sister group of the Hemichordata, with which they form the crown group Ambulacraria. Two taxa of uncertain placement, Vetulocystida and Yanjiahella, have each been proposed as either stem-group echinoderms or stem-group ambulacrarians. Vetulocystids have also been proposed as stem-group chordates, while Yanjiahella has also been proposed to be a stem-group hemichordate.
The Ambulacrarian context of the echinoderms is shown below, simplified from Li et al. 2023, with the possible ambulacrarian placements of the uncertian taxa shown with dashed lines and question marks:
Internal phylogeny: extant classes
The extant echinoderms consist of the Crinoidea and the Eleutherozoa, the latter of which is divided into the Asterozoa and the Echinozoa.
Internal phylogeny: total group
The lack of a consensus cladistic phylogeny incorporating extinct echinoderm groups has resulted in the continued use of terms from Linnaean taxonomies, even when the named taxa are known to be paraphyletic and/or polyphyletic.
Linnaean taxonomies
Three taxonomies introduced nearly all of the traditional subphyla and class divisions that continue to be referenced in cladistic work:
F. A. Bather produced the earliest widely referenced classification of both fossil and extant echinoderms in 1900, using a two-subphylum system.
In 1966, the Treatise on Invertebrate Paleontology, rejected Bather's classification, replacing it with a new four-subphylum scheme that had been previously proposed by H. B. Fell.
James Sprinkle which added a fifth subphylum to the Treatise taxonomy in 1973. His later class-level taxonomy of the five subphyla was the most recent approach cited in an early cladistic re-assessment of the phylum.
Other proposed classes not included at that rank in any of the above taxonomies include:
Cryptosyringida
Somasteroidea
Stenuroidea
Coronoidea
Concentricycloidea
There are also several common alternative names involving homalozoans: | Echinoderm | Wikipedia | 475 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Carpoidea for Homalozoa, giving rise to the term "carpoids"
Cincta as either the senior synonym of or sole order within Homostelea
Soluta as either the senior synonym of or sole order within Homoiostelea
Calcichordata , a subphylum effectively identical to Stylophora that was central to the now-disproven calcichordate hypothesis
Cladograms
According to 2024 review, there are two main schools of thought regarding echinoderm phylogeny: One that sees pentaradiality as a plesiomorphic trait of the phylum, and another that considers it a derived trait (apomorphy).
Note that neither cladogram shown below includes all of the traditional classes, or even all of the classes mentioned in accompanying text.
Pentaradiality as a plesiomorphy
Supporters of pentaradiality as an initial condition of the phylum note that radial forms are the first uncontested echinoderms to appear in the fossil record. They also define homologies of echinoderm anatomy based on a division of the skeleton into two parts: those that are or are not associated with the water vascular system.
The following cladogram is based on David & Mooi (1999) and David, Lefebvre, Mooi, and Parsley (2000):
In this theory, the controversial Ediacaran fossil Arkarua is tentatively placed as the sister to all other echinoderms. Helicoplacoidea and Edrioasteroidea join it in the stem group. Pelmatozoa, Eocrinoidea, and Cystoidea are shown to be paraphyletic while Homalozoa is polyphyletic.
Pentaradiality as an apomorphy | Echinoderm | Wikipedia | 381 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Those who find pentaradiality to be derived incorporate the recently-discovered fossils Ctenoimbricata (seen as a possible sister to all other echinoderms) and Helicocystis (seen as bridging the triradial helicoplacoids and the pentaradial crown group). They cite research indicating that the early appearance of pentaradial forms is likely due to an incomplete fossil record, as well as multiple studies showing non-radial forms as an early stem group, to argue that this is phylogeny represents an emerging consensus. They reject Arkarua as an echinoderm due to its lack of stereom and possession of true pentaradiality instead of the 2-1-2 pseudo-pentaradiality seen in all early forms.
The following cladogram is based on Rahman & Zamora (2024), incorporating class and subphylum names from the text:
Here, Homalozoa (with uncertain placement of Stylophora) is shown to be a paraphyletic assemblage along the stem group, followed by Helicoplacoidea and then Helicocystis as the sister of the crown group. The details of Blastozoa vs Crinozoa are not addressed, as they are represented only by the classes Eocrinoidea and Crinoidea, respectively, and the overall nature of Pelmatozoa remains unresolved. The four-way polytomy including the Eleutherozoa and Crinoidea shows either Camptostroma or Gogia or both could prove to be outside of the crown group.
Fossil history
Echinoderms have a rich fossil record due to their mineralized endoskeletons.
Possible early echinoderms
The three oldest known candidate echinoderms all lack stereom and other echinoderm apomorphies, making their inclusion in the phylum controversial. | Echinoderm | Wikipedia | 405 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The oldest potential echinoderm fossil is Arkarua from the late Ediacaran of Australia circa 555 Ma. These fossils are disc-like, with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, the fossils have no stereom or internal structure indicating a water vascular system, so they cannot be conclusively identified. Additionally, all known early pentaradial echinoderms are pseudo-pentaradial in a 2-1-2 pattern, with true pentaradiality as seen in Arkarua not seen until the emergence of the Eleutherozoa.
The next possible echinoderms are the vetulocystids, which date to the early to mid Cambrian, 541–501 Ma. While the youngest vetulocystid, Thylacocercus, displays some characteristics that could be interemediate between older vetulocystids and Yanjiahella, its discoverers consider vetulocystids more likely to be stem ambulacrarians than stem echinoderms.
Yanjiahella, from the Fortunian (circa 539–529 Ma), is unlike the older fossils in that it has a plated theca, albeit one without evidence of stereom. To some, this is a reason to place it as a stem ambulacrarian or stem hemichordate. Others argue that absence of evidence for stereom is not evidence of absence, and consider a stem echinoderm position more likely.
Echinoderms in the Cambrian and Ordovician
The first universally accepted echinoderms appear in the Lower Cambrian period; asterozoans appeared in the Ordovician, while the crinoids were a dominant group in the Paleozoic.
It is hypothesised that the ancestor of all echinoderms was a simple, motile, bilaterally symmetrical animal with a mouth, gut and anus. This ancestral organism adopted an attached mode of life with suspension feeding, and developed radial symmetry. Even so, the larvae of all echinoderms are bilaterally symmetrical, and all develop radial symmetry at metamorphosis. Like their ancestor, the starfish and crinoids still attach themselves to the seabed while changing to their adult form. | Echinoderm | Wikipedia | 472 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The first known echinoderms were non-motile, but evolved into animals able to move freely. These soon developed endoskeletal plates with stereom structure, and external ciliary grooves for feeding. The Paleozoic echinoderms were globular, attached to the substrate and were orientated with their oral surfaces facing upwards. These early echinoderms had ambulacral grooves extending down the side of the body, fringed on either side by brachioles, like the pinnules of a modern crinoid. Eventually, the mobile eleutherozoans reversed their orientation to become mouth-downward. Before this happened, the podia probably had a feeding function, as they do in the crinoids today. The locomotor function of the podia came later, when the re-orientation of the mouth brought the podia into contact with the substrate for the first time.
Use by humans
As food and medicine
In 2019, 129,052 tonnes of echinoderms were harvested. The majority of these were sea cucumbers (59,262 tonnes) and sea urchins (66,341 tonnes). These are used mainly for food, but also in traditional Chinese medicine. Sea cucumbers are considered a delicacy in some countries of southeast Asia; as such, they are in imminent danger of being over-harvested. Popular species include the pineapple roller Thelenota ananas (susuhan) and the red sea cucumber Holothuria edulis. These and other species are colloquially known as bêche de mer or trepang in China and Indonesia. The sea cucumbers are boiled for twenty minutes and then dried both naturally and later over a fire which gives them a smoky tang. In China, they are used as a basis for gelatinous soups and stews. Both male and female gonads of sea urchins are consumed, particularly in Japan and France. The taste is described as soft and melting, like a mixture of seafood and fruit. Sea urchin breeding trials have been undertaken to try to compensate for overexploitation.
In research | Echinoderm | Wikipedia | 444 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Because of their robust larval growth, sea urchins are widely used in research, particularly as model organisms in developmental biology and ecotoxicology. Strongylocentrotus purpuratus and Arbacia punctulata are used for this purpose in embryological studies. The large size and the transparency of the eggs enables the observation of sperm cells in the process of fertilising ova. The arm regeneration potential of brittle stars is being studied in connection with understanding and treating neurodegenerative diseases in humans. Genomic data relevant to echinoderm model organisms are collected in Echinobase. Currently, there are four species of echinoderms fully supported (gene pages, BLAST, JBrowse tracks, genome downloads) including Strongylocentrotus purpuratus (purple sea urchin), Lytechinus variegatus (green sea urchin), Patiria miniata (bat star) and Acanthaster planci (crown-of-thorns sea star). Partially supported species (no gene pages) include Lytechinus pictus (painted sea urchin), Asterias rubens (sugar star) and Anneissia japonica (feather star crinoid).
Other uses
The calcareous tests or shells of echinoderms are used as a source of lime by farmers in areas where limestone is unavailable and some are used in the manufacture of fish meal. 4,000 tons of the animals are used annually for these purposes. This trade is often carried out in conjunction with shellfish farmers, for whom the starfish pose a major threat by eating their cultured stock. Other uses for the starfish they recover include the manufacture of animal feed, composting and the preparation of dried specimens for the arts and craft trade. | Echinoderm | Wikipedia | 374 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine (except for two species), and the Chaetonotida, some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.
Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs, and a trunk with a simple gut and the reproductive organs. They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx. They are hermaphrodites, the marine species producing eggs which develop directly into miniature adults. The freshwater species are parthenogenetic, producing unfertilised eggs, and at least one species is viviparous. Gastrotrichs mature with great rapidity and have lifespans of only a few days.
Etymology and taxonomy
The name gastrotrich comes from Greek γαστήρ, gaster 'stomach' and θρίξ, thrix 'hair'. The name was coined by the Russian zoologist Élie Metchnikoff in 1865. The common name hairyback apparently arises from a mistranslation of gastrotrich. | Gastrotrich | Wikipedia | 402 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the Rotifera, or the Nematoda. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa. As of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the Macrodasyida and the Chaetonotida. Edward Ruppert et al. report that the Macrodasyida are wholly marine, but two rare and poorly known species, Marinellina flagellata and Redudasys fornerise, are known from fresh water. The Chaetonotida comprises both marine and freshwater species.
Anatomy
Gastrotrichs vary in size from about in body length. They are bilaterally symmetrical, with a transparent strap-shaped or bowling pin-shaped body, arched dorsally and flattened ventrally. The anterior end is not clearly defined as a head but contains the sense organs, brain and pharynx. Cilia are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the posterior end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body.
The body wall consists of a cuticle, an epidermis and longitudinal and circular bands of muscle fibres. In some primitive species, each epidermal cell has a single cilium, a feature shared only by the gnathostomulans. The whole ventral surface of the animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no coelom (body cavity) and the interior of the animal is filled with poorly differentiated connective tissue. In the macrodasyidans, Y-shaped cells, each containing a vacuole, surround the gut and may function as a hydrostatic skeleton. | Gastrotrich | Wikipedia | 495 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
The mouth is at the anterior end and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen, lined by myoepithelial cells. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The anus is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to the ventral surface; these contain valves and may allow egestion of any excess water swallowed while feeding.
In the chaetonotidans, the excretory system consists of a single pair of protonephridia, which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of the body. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation. Unusually, the protonephridia do not take the form of flame cells, but, instead, the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum. These cells, termed cyrtocytes, connect to a single outlet cell which passes the excreted material into the protonephridial duct.
As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two ganglia, one on either side of the pharynx, connected by a commissure. From these lead a pair of nerve cords which run along either side of the body beside the longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface which function as mechanoreceptors. There are also ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors.
Distribution and habitat | Gastrotrich | Wikipedia | 433 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land. They are also found in stagnant pools and anaerobic mud, where they thrive even in the presence of hydrogen sulfide. When pools dry up they can survive periods of desiccation as eggs, and some species are capable of forming cysts in harsh conditions. In marine sediments they have been known to reach 364 individuals per making them the third most common invertebrate in the sediment after nematodes and harpacticoid copepods. In freshwater they may reach a density of 158 individuals per and are the fifth most abundant group of invertebrates in the sediment.
Behaviour and ecology
In marine and freshwater environments, gastrotrichs form part of the benthic community. They are detritivores and are microphagous: they feed by sucking small dead or living organic materials, diatoms, bacteria and small protozoa into their mouths by the muscular action of the pharynx. They are themselves eaten by turbellarians and other small macrofauna.
Like many microscopic animals, gastrotrich locomotion is primarily powered by hydrostatics, but movement occurs through different methods in different members of the group. Chaetonotids only have adhesive glands at the back and, in them, locomotion typically proceeds in a smooth gliding manner; the whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. In the pelagic chaetonotid genus Stylochaeta, however, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body. By contrast, with chaetonotids, macrodasyidans typically have multiple adhesive glands and move forward with a creeping action similar to that of a "looper" caterpillar. In response to a threat, the head and trunk can be rapidly pulled backwards, or the creeping movement can be reversed. Muscular action is important when the animal turns sideways and during copulation, when two individuals twine around each other.
Reproduction and lifespan | Gastrotrich | Wikipedia | 457 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrich reproduction and reproductive behaviour has been little studied. That of macrodasiyds probably most represents that of the ancestral lineage and these more primitive gastrotrichs are simultaneous hermaphrodites, possessing both male and female sex organs. There is generally a single pair of gonads, the anterior portion of which contains sperm-producing cells and the posterior portion producing ova. The sperm is sometimes packaged in spermatophores and is released through male gonopores that open, often temporarily, on the underside of the animal, roughly two-thirds of the way along the body. A copulatory organ on the tail collects the sperm and transfers it to the partner's seminal receptacle through the female gonopore. Details of the process and the behaviour involved vary with the species, and there is a range of different accessory reproductive organs. During copulation, the "male" individual uses his copulatory organ to transfer sperm to his partner's gonopore and fertilisation is internal. The fertilised eggs are released by rupture of the body wall which afterwards repairs itself. As is the case in most protostomes, development of the embryo is determinate, with each cell destined to become a specific part of the animal's body. At least one species of gastrotrich, Urodasys viviparus, is viviparous.
Many species of chaetotonid gastrotrichs reproduce entirely by parthenogenesis. In these species, the male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. Though the eggs have a diameter of less than 50 μm, they are still very large in comparison with the animals' size. Some species are capable of laying eggs that remain dormant during times of desiccation or low temperatures; these species, however, are also able to produce regular eggs, which hatch in one to four days, when environmental conditions are more favourable. The eggs of all gastrotrichs undergo direct development and hatch into miniature versions of the adult. The young typically reach sexual maturity in about three days. In the laboratory, Lepidodermella squamatum has lived for up to forty days, producing four or five eggs during the first ten days of life. | Gastrotrich | Wikipedia | 478 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrichs demonstrate eutely, each species having an invariant genetically fixed number of cells as adults. Cell division ceases at the end of embryonic development and further growth is solely due to cell enlargement.
Classification
Gastrotricha is divided into two orders and a number of families: | Gastrotrich | Wikipedia | 61 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Order Macrodasyida Remane, 1925 [Rao and Clausen, 1970]
Family Cephalodasyidae Hummon & Todaro, 2010
Genus Cephalodasys Remane, 1926
Genus Dolichodasys Gagne, 1977
Genus Megadasys Schmidt, 1974
Genus Mesodasys Remane, 1951
Genus Paradasys Remane, 1934
Genus Pleurodasys Remane, 1927
Family Dactylopodolidae Strand, 1929
Genus Dactylopodola Strand, 1929
Genus Dendrodasys Wilke, 1954
Genus Dendropodola Hummon, Todaro & Tongiorgi, 1992
Family Lepidodasyidae Remane, 1927
Genus Lepidodasys Remane, 1926
Family Macrodasyidae Remane, 1926
Genus Macrodasys Remane, 1924
Genus Urodasys Remane, 1926
Family Planodasyidae Rao & Clausen, 1970
Genus Crasiella Clausen, 1968
Genus Planodasys Rao & Clausen, 1970
Family Redudasyidae Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012
Genus Anandrodasys Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012
Genus Redudasys Kisielewski, 1987
Family Thaumastodermatidae Remane, 1927
Subfamily Diplodasyinae Ruppert, 1978
Genus Acanthodasys Remane, 1927
Genus Diplodasys Remane, 1927
Subfamily Thaumastodermatinae Remane, 1927
Genus Hemidasys Claparède, 1867
Genus Oregodasys Hummon, 2008 =(Platydasys Remane, 1927)
Genus Pseudostomella Swedmark, 1956
Genus Ptychostomella Remane, 1926
Genus Tetranchyroderma Remane, 1926
Genus Thaumastoderma Remane, 1926
Family Turbanellidae Remane, 1927
Genus Desmodasys Clausen, 1965
Genus Dinodasys Remane, 1927
Genus Paraturbanella Remane, 1927
Genus Prostobuccantia Evans & Hummon, 1991
Genus Pseudoturbanella d'Hondt, 1968
Genus Turbanella Schultze, 1853
Family Xenodasyidae Todaro, Guidi, Leasi & Tongiorgi, 2006 | Gastrotrich | Wikipedia | 499 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Genus Chordodasiopsis Todaro, Guidi, Leasi & Tongiorgi, 2006
Genus Xenodasys Swedmark, 1967 | Gastrotrich | Wikipedia | 29 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Incertae sedis
Genus Marinellina Ruttner-Kolisko, 1955
Order Chaetonotida Remane, 1925 [Rao and Clausen, 1970]
Suborder Multitubulatina d'Hondt, 1971
Family Neodasyidae Remane, 1929
Genus Neodasys Remane, 1927
Suborder Paucitubulatina d'Hondt, 1971
Family Chaetonotidae Gosse, 1864
Subfamily Chaetonotinae Kisielewski, 1991
Genus Arenotus Kisielewski, 1987
Genus Aspidiophorus Voigt, 1903
Genus Caudichthydium Schwank, 1990
Genus Chaetonotus Ehrenberg, 1830
Genus Fluxiderma d'Hondt, 1974
Genus Ichthydium Ehrenberg, 1830
Genus Halichaetonotus Remane, 1936
Genus Heterolepidoderma Remane, 1927
Genus Lepidochaetus Kisielewski 1991
Genus Lepidodermella Blake, 1933
Genus Polymerurus Remane, 1927
Genus Rhomballichthys Schwank, 1990
Subfamily Undulinae Kisielewski 1991
Genus Undula Kisielewski 1991
Family Dasydytidae Daday, 1905
Genus Anacanthoderma Marcolongo, 1910
Genus Chitonodytes Remane, 1936
Genus Dasydytes Gosse, 1851
Genus Haltidytes Remane 1936
Genus Ornamentula Kisielewski 1991
Genus Setopus Grünspan, 1908
Genus Stylochaeta Hlava, 1905
Family Dichaeturidae Remane, 1927
Genus Dichaetura Lauterborn, 1913
Family Muselliferidae Leasi & Todaro, 2008
Genus Diuronotus Todaro, Kristensen & Balsamo, 2005
Genus Musellifer Hummon, 1969
Family Neogosseidae Remane, 1927
Genus Neogossea Remane, 1927
Genus Kijanebalola Beauchamp, 1932
Family Proichthydiidae Remane, 1927
Genus Proichthydium Cordero, 1918
Genus Proichthydioides Sudzuki, 1971
Family Xenotrichulidae Remane, 1927
Subfamily Draculiciterinae Ruppert, 1979
Genus Draculiciteria Hummon, 1974
Subfamily Xenotrichulinae Remane, 1927
Genus Heteroxenotrichula Wilke, 1954
Genus Xenotrichula Remane, 1927 | Gastrotrich | Wikipedia | 506 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The class Graptolithina, formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.
Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest secreted tubes) and are deposit feeders, but some species are pharyngeal filter feeders, while the family are free living detritivores. Many are well known for their production and accumulation of various halogenated phenols and pyrroles. Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium.
The discovery of the stem group hemichordate Gyaltsenglossus shows that early hemichordates combined aspects of the two morphologically disparate classes.
Anatomy
The body plan of hemichordates is characterized by a muscular organization. The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome.
The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles. The mouth is located between the proboscis and the collar. The trunk is the longest part of the animal. It contains the pharynx, which is perforated with gill slits (or pharyngeal slits), the oesophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile members of the acorn worm family Harrimaniidae. | Hemichordate | Wikipedia | 482 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium. The mesosome extends into one pair (in the genus Rhabdopleura) or several pairs (in the genus Cephalodiscus) of tentaculated arms used in filter feeding. The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of the colony, produced by asexual budding. In the genus Cephalodiscus, asexually produced individuals stay attached to the contractile stalk of the parent individual until completing their development. In the genus Rhabdopleura, zooids are permanently connected to the rest of the colony via a common stolon system.
They have a diverticulum of the foregut called a stomochord, previously thought to be related to the chordate notochord, but this is most likely the result of convergent evolution rather than a homology. A hollow neural tube exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes. Hemichordates have a nerve net and longitudinal nerves, but no brain.
Some species biomineralize in calcium carbonate.
Circulatory system
Hemichordates have an open circulatory system. The heart vesicle is located dorsally within the proboscis complex, and does not contain any blood. Instead it moves the blood indirectly by pulsating against the dorsal blood vessel.
Development
Together with the echinoderms, the hemichordates form the Ambulacraria, which are the closest extant phylogenetic relatives of chordates. Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly. Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm. | Hemichordate | Wikipedia | 479 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm. The Pterobranch genus most extensively studied is Rhabdopleura from Plymouth, England and from Bermuda.
The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly. | Hemichordate | Wikipedia | 171 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Ptychodera flava
P. flava’s early cleavage pattern is similar to that of S. kowalevskii. The first and second cleavages from the single cell zygote of P. flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. P. flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and four larger macromeres. Further divisions occur until P. flava finishes the blastula stage and goes on to gastrulation. The animal mesomeres of P. flava go on to give rise to the larva’s ectoderm, animal blastomeres also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues. Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development P. flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage. | Hemichordate | Wikipedia | 372 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Saccoglossus kowalevskii
Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm. | Hemichordate | Wikipedia | 303 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Genetic control of dorsal-ventral hemichordate patterning
Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii, and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4, which is homologous to Drosophila’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on the ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post-anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii, such as a netrin that groups with netrin gene class 1 and 2. Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.
Classification
Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which includes the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 120 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic. It is possible that the extinct organism Etacystis is a member of the Hemichordata, either within or with close affinity to the Pterobranchia.
There are 130 described species of Hemichordata and many new species are being discovered, especially in the deep sea. | Hemichordate | Wikipedia | 512 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Phylogeny
A phylogenetic tree showing the position of the hemichordates is:
The internal relationships within the hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. | Hemichordate | Wikipedia | 55 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Acanthocephala (Greek , 'thorn' + , 'head') is a group of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans have complex life cycles, involving at least two hosts, which may include invertebrates, fish, amphibians, birds, and mammals. About 1,420 species have been described.
The Acanthocephala were long thought to be a discrete phylum. Recent genome analysis has shown that they are descended from, and should be considered as, highly modified rotifers. This unified taxon is sometimes known as Syndermata, or simply as Rotifera, with the acanthocephalans described as a subclass of a rotifer class Hemirotatoria.
History
The earliest recognisable description of Acanthocephala – a worm with a proboscis armed with hooks – was made by Italian author Francesco Redi (1684). In 1771, Joseph Koelreuter proposed the name Acanthocephala. Philipp Ludwig Statius Müller independently called them Echinorhynchus in 1776. Karl Rudolphi in 1809 formally named them Acanthocephala.
Evolutionary history
The oldest known remains of acanthocephalans are eggs found in a coprolite from the Late Cretaceous Bauru Group of Brazil, around 70–80 million years old, likely from a crocodyliform. The group may have originated substantially earlier.
Phylogeny
Acanthocephalans are highly adapted to a parasitic mode of life, and have lost many organs and structures through evolutionary processes. This makes determining relationships with other higher taxa through morphological comparison problematic. Phylogenetic analysis of the 18S ribosomal gene has revealed that the Acanthocephala are most closely related to the rotifers. They are possibly closer to the two rotifer classes Bdelloidea and Monogononta than to the other class, Seisonidea, producing the names and relationships shown in the cladogram below.
The three rotifer classes and the Acanthocephala make up a clade called Syndermata. This clade is placed in the Gnathifera. | Acanthocephala | Wikipedia | 500 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
A study of the gene order in the mitochondria suggests that Seisonidea and Acanthocephala are sister clades and that the Bdelloidea are the sister clade to this group.
Currently the phylum is divided into four classes – Palaeacanthocephala, Archiacanthocephala, Polyacanthocephala and Eoacanthocephala. The monophyletic Archiacanthocephala are the sister taxon of a clade comprising Eoacanthocephala and the monophyletic Palaeacanthocephala.
Morphology
Several morphological characteristics distinguish acanthocephalans from other phyla of parasitic worms.
Digestion
Acanthocephalans lack a mouth or alimentary canal. This is a feature they share with the cestoda (tapeworms), although the two groups are not closely related. Adult stages live in the intestines of their host and uptake nutrients which have been digested by the host, directly, through their body surface. The acanthocephalans lack an excretory system, although some species have been shown to possess flame cells (protonephridia).
Proboscis
The most notable feature of the acanthocephala is the presence of an anterior, protrudable proboscis that is usually covered with spiny hooks (hence the common name: thorny or spiny headed worm). The proboscis bears rings of recurved hooks arranged in horizontal rows, and it is by means of these hooks that the animal attaches itself to the tissues of its host. The hooks may be of two or three shapes, usually: longer, more slender hooks are arranged along the length of the proboscis, with several rows of more sturdy, shorter nasal hooks around the base of the proboscis. The proboscis is used to pierce the gut wall of the final host, and hold the parasite fast while it completes its life cycle. | Acanthocephala | Wikipedia | 419 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Like the body, the proboscis is hollow, and its cavity is separated from the body cavity by a septum or proboscis sheath. Traversing the cavity of the proboscis are muscle-strands inserted into the tip of the proboscis at one end and into the septum at the other. Their contraction causes the proboscis to be invaginated into its cavity. The whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall.
Some of the acanthocephalans (perforating acanthocephalans) can insert their proboscis in the intestine of the host and open the way to the abdominal cavity.
Size
The size of these animals varies greatly, ranging from a few millimetres in length to Macracanthorhynchus hirudinaceus, which measures from . A curious feature shared by both larva and adult is the large size of many of the cells, e.g. the nerve cells and cells forming the uterine bell. Polyploidy is common, with up to 343n having been recorded in some species.
Skin
The body surface of the acanthocephala is peculiar. Externally, the skin has a thin tegument covering the epidermis, which consists of a syncytium with no cell walls. The syncytium is traversed by a series of branching tubules containing fluid and is controlled by a few wandering, amoeboid nuclei. Inside the syncytium is an irregular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is no endothelium. In their micro-structure the muscular fibres resemble those of nematodes. | Acanthocephala | Wikipedia | 382 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Except for the absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the proboscis are shut off from those of the body. The canals of the proboscis open into a circular vessel which runs round its base. From the circular canal two sac-like projections called the lemnisci run into the cavity of the body, alongside the proboscis cavity. Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a muscular coat. They seem to act as reservoirs into which the fluid which is used to keep the proboscis "erect" can withdraw when it is retracted, and from which the fluid can be driven out when it is wished to expand the proboscis.
Nervous system
The central ganglion of the nervous system lies behind the proboscis sheath or septum. It innervates the proboscis and projects two stout trunks posteriorly which supply the body. Each of these trunks is surrounded by muscles, and this nerve-muscle complex is called a retinaculum. In the male at least there is also a genital ganglion. Some scattered papillae may possibly be sense-organs.
Life cycles
Acanthocephalans have complex life cycles, involving a number of hosts, for both developmental and resting stages. Complete life cycles have been worked out for only 25 species.
Reproduction
The Acanthocephala are dioecious (an individual organism is either male or female). There is a structure called the genital ligament which runs from the posterior end of the proboscis sheath to the posterior end of the body. In the male, two testes lie on either side of this. Each opens in a vas deferens which bears three diverticula or vesiculae seminales. The male also possesses three pairs of cement glands, found behind the testes, which pour their secretions through a duct into the vasa deferentia. These unite and end in a penis which opens posteriorly. | Acanthocephala | Wikipedia | 435 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
In the female, the ovaries are found, like the testes, as rounded bodies along the ligament. From the ovaries, masses of ova dehisce into the body cavity, floating in its fluids for fertilization by male's sperm. After fertilization, each egg contains a developing embryo. (These embryos hatch into first stage larva.) The fertilized eggs are brought into the uterus by actions of the uterine bell, a funnel like opening continuous with the uterus. At the junction of the bell and the uterus there is a second, smaller opening situated dorsally. The bell "swallows" the matured eggs and passes them on into the uterus. (Immature embryos are passed back into the body cavity through the dorsal opening.) From the uterus, mature eggs leave the female's body via her oviduct, pass into the host's alimentary canal and are expelled from the host's body within feces.
Release
Having been expelled by the female, the acanthocephalan egg is released along with the feces of the host. For development to occur, the egg, containing the acanthor, needs to be ingested by an arthropod, usually a crustacean (there is one known life cycle which uses a mollusc as a first intermediate host). Inside the intermediate host, the acanthor is released from the egg and develops into an acanthella. It then penetrates the gut wall, moves into the body cavity, encysts, and begins transformation into the infective cystacanth stage. This form has all the organs of the adult save the reproductive ones.
The parasite is released when the first intermediate host is ingested. This can be by a suitable final host, in which case the cystacanth develops into a mature adult, or by a paratenic host, in which the parasite again forms a cyst. When consumed by a suitable final host, the cycstacant excysts, everts its proboscis and pierces the gut wall. It then feeds, grows and develops its sexual organs. Adult worms then mate. The male uses the excretions of its cement glands to plug the vagina of the female, preventing subsequent matings from occurring. Embryos develop inside the female, and the life cycle repeats.
Host control | Acanthocephala | Wikipedia | 497 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Thorny-headed worms begin their life cycle inside invertebrates that reside in marine or freshwater systems. One example is Polymorphus paradoxus. Gammarus lacustris, a small crustacean that inhabits ponds and rivers, is one invertebrate that P. paradoxus may occupy; ducks are one of the definitive hosts.
This crustacean is preyed on by ducks and hides by avoiding light and staying away from the surface. However, infection by P. paradoxus changes its behavior and appearance in a number of ways that increase its chance of being eaten. First, infection significantly reduces G. lacustris'''s photophobia; as a result, it becomes attracted toward light and swims to the surface. Second, an infected organism will even go so far as to find a rock or a plant on the surface, clamp its mouth down, and latch on, making it easy prey for the duck. Finally, infection reduces the pigment distribution and amount in G. lacustris, causing the host to turn blue; unlike their normal brown colour, this makes the crustacean stand out and increases the chance the duck will see it.
Experiments have shown that altered serotonin levels are likely responsible for at least some of these changes in behaviour. One experiment found that serotonin induces clinging behavior in G. lacustris similar to that seen in infected organisms. Another showed that infected G. lacustris had approximately 3 times as many serotonin-producing sites in its ventral nerve cord. Furthermore, experiments in closely-related species of Polymorphus and Pomphorhynchus infecting other Gammarus species confirmed this relation: infected organisms were considerably more attracted to light and had higher serotonin levels, while the phototropism could be duplicated by injections of serotonin.
Effects on hosts Polymorphus spp. are parasites of seabirds, particularly the eider duck (Somateria mollissima). Heavy infections of up to 750 parasites per bird are common, causing ulceration to the gut, disease and seasonal mortality. Recent research has suggested that there is no evidence of pathogenicity of Polymorphus spp. to intermediate crab hosts. The cystacanth stage is long lived and probably remains infectious throughout the life of the crab. | Acanthocephala | Wikipedia | 476 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Economic impact
Acanthocephalosis, a disease caused by Acanthacephalus infection, is prevalent in aquaculture, occurring in Atlantic salmon, rainbow and brown trout, tilapia, and tambaqui. Increasing occurrence in Brazilian farming of tambaqui has been reported, and in 2003 Acanthacephalus was first reported in cultured red snapper in Taiwan.
The life cycle of Polymorphus spp. normally occurs between sea ducks (e.g. eiders and scoters) and small crabs. Infections found in commercial-sized lobsters in Canada were probably acquired from crabs that form an important dietary item of lobsters. Cystacanths occurring in lobsters can cause economic loss to fishermen. There are no known methods of prevention or control.
Human infections
In humans, it causes the disease acanthocephaliasis. The earliest known infection was found in a prehistoric man in Utah. This infection was dated to 1869 ± 160 BC. The species involved was thought to be Moniliformis clarki which is still common in the area.
The first report of an isolate in historic times was by Lambl in 1859 when he isolated Macracanthorhynchus hirudinaceus from a child in Prague. Lindemann in 1865 reported that this organism was commonly isolated in Russia. The reason for this was discovered by Schneider in 1871 when he found that an intermediate host, the scarabaeid beetle grub, was commonly eaten raw.
The first report of clinical symptoms was by Calandruccio who in 1888 while in Italy infected himself by ingesting larvae. He reported gastrointestinal disturbances and shed eggs in two weeks. Subsequent natural infections have since been reported.
Eight species have been isolated from humans to date. Moniliformis moniliformis is the most common isolate. Other isolates include Acanthocephalus bufonis and Corynosoma strumosum''. | Acanthocephala | Wikipedia | 404 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Loricifera (from Latin, lorica, corselet (armour) + ferre, to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals with 43 described species. and approximately 100 more that have been collected and not yet described. Their sizes range from 100 μm to .
They are characterised by a protective outer case called a lorica and their habitat is in the spaces between marine gravel to which they attach themselves. The phylum was discovered in 1983 by R.M. Kristensen, near Roscoff, France. They are among the most recently discovered groups of animals. They attach themselves quite firmly to the substrate, and hence remained undiscovered for so long. The first specimen was collected in the 1970s, and described in 1983. They are found at all depths, in different sediment types, and in all latitudes.
Morphology
The animals have a head, mouth, and digestive system, as well as the lorica. The head (which contains the mouth and the brain), a trunk region surrounded by six plates that make up the 'lorica' or corselet and – in between these two – the neck region. Loricifera have a well developed brain and each scalid is individually connected to the brain by nerves.
The armor-like lorica consists of a protective external shell or case of encircling plicae. There is no circulatory system and no endocrine system. Many of the larvae are acoelomate, with some adults being pseudocoelomate, and some remaining acoelomate. Development is generally direct, though there are so-called Higgins larvae, which differ from adults in several respects. As adults, the animals are gonochoric. Very complex and plastic life cycles of pliciloricids include also paedogenetic stages with different forms of parthenogenetic reproduction. Most Loricifera are dioecious, meaning there are males and females. However, there are a few species known to be hermaphroditic, which means they contain both male and female reproductive organs. Fossils have been dated to the late Cambrian.
Taxonomic affinity | Loricifera | Wikipedia | 448 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
Morphological studies have traditionally placed the phylum in the Vinctiplicata with the Priapulida; this plus the Kinorhyncha constitutes the taxon Scalidophora. The three phyla share four characters in common – chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts. However, despite a 2015 study showing the phylum's closest relatives being the Panarthropoda, a 2022 study again showed that it belonged to the Scalidophora and told that further, more comprehensive genetic tests will be required to find its actual position in Ecdysozoa.
Evolutionary history
The loriciferans are believed to be miniaturized descendants of a larger organism, perhaps resembling the Cambrian fossil Sirilorica. However, the fossil record of the microscopic non-mineralized group is (perhaps unsurprisingly) scarce, so it is difficult to trace out the evolutionary history of the phylum in any detail.
The 2017 discovery of the Cambrian Eolorica deadwoodensis may shed some light on the group's history.
In anoxic environments
Three species of Loricifera have been found in the oxygen-free sediments at the bottom of the L'Atalante basin in Mediterranean Sea, more than 3,000 meters down, the first multicellular organisms known to spend their entire lives in an anoxic environment. Initially, it was thought that they are able to do this because their mitochondria act like hydrogenosomes, allowing them to respire anaerobically. However, by 2021, questions arose as to whether or not they have mitochondria. | Loricifera | Wikipedia | 352 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
The newly reported animals complete their life cycle in the total absence of light and oxygen, and they are less than a millimetre in size. They were collected from a deep basin at the bottom of the Mediterranean Sea, where they inhabit a nearly salt-saturated brine that, because of its density (> 1.2 g/cm3), does not mix with the waters above. As a consequence, this environment is completely anoxic and, due to the activity of sulfate reducers, contains sulphide at a concentration of 2.9 mM. Despite such harsh conditions, this anoxic and sulphidic environment is teeming with microbial life, both chemosynthetic prokaryotes that are primary producers, and a broad diversity of eukaryotic heterotrophs at the next trophic level.
Taxa | Loricifera | Wikipedia | 173 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
The Chaetognatha or chaetognaths (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, they are mostly nektonic; however about 20% of the known species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from .
Chaetognaths were first recorded by the Dutch naturalist Martinus Slabber in 1775. As of 2021, biologists recognize 133 modern species assigned to over 26 genera and eight families. Despite the limited diversity of species, the number of individuals is large.
Arrow worms are strictly related to and possibly belonging to Gnathifera, a clade of protostomes that do not belong to either Ecdysozoa or Lophotrochozoa.
Anatomy
Chaetognaths are transparent or translucent dart-shaped animals covered by a cuticle. They range in length between 1.5 mm to 105 mm in the Antarctic species Pseudosagitta gazellae. Body size, either between individuals in the same species or between different species, seems to increase with decreasing temperature. The body is divided into a distinct head, trunk, and tail. About 80% of the body is occupied by primary longitudinal muscles.
Head and digestive system
There are between four and fourteen hooked, grasping spines on each side of their head, flanking a hollow vestibule containing the mouth. The spines are used in hunting, and covered with a flexible hood arising from the neck region when the animal is swimming. Spines and teeth are made of α-chitin, and the head is protected by a chitinous armature. | Chaetognatha | Wikipedia | 385 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The mouth opens into a muscular pharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. Eukrohniid species possess an oil vacuole closely associated with the gut. This organ contains wax esters which may assist reproduction and growth outside of the production season for Eukrohnia hamata in Arctic seas. Owing to the position of the oil vacuole in the center of the tractus, the organ may also have implications for buoyancy, trim and locomotion.
Usually chaetognaths are not pigmented, however the intestines of some deep-sea species contain orange-red carotenoid pigments.
Nervous and sensory systems
The nervous system is reasonably simple and shows a typical protostome anatomy, consisting of a ganglionated nerve ring surrounding the pharynx. The brain is composed of two distinct functional domains: the anterior neuropil domain and the posterior neuropil domain. The former probably controls head muscles moving the spines and the digestive system. The latter is linked to eyes and the corona ciliata. A putative sensory structure of unknown function, the retrocerebral organ, is also hosted by the posterior neuropil domain. The dorsal ganglion is the largest, but nerves extend from all the ganglia along the length of the body. | Chaetognatha | Wikipedia | 346 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Chaetognaths have two compound eyes, each consisting of a number of pigment-cup ocelli fused together; some deep-sea and troglobitic species have unpigmented or absent eyes. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the lateral line in fish. An additional, curved, band of sensory bristles lies over the head and neck. Almost all chaetognaths have "indirect" or "inverted" eyes, according to the orientation of photoreceptor cells; only some Eukhroniidae species have "direct" or "everted" eyes. A unique feature of the chaetognath eye is the lamellar structure of photoreceptor membranes, containing a grid of 35–55 nm wide circular pores.
A significant mechanosensory system, composed of ciliary receptor organs, detects vibrations, allowing chaetognaths to detect the swimming motion of potential prey. Another organ on the dorsal part of the neck, the corona ciliata, is probably involved in chemoreception.
Internal organs
The body cavity is lined by peritoneum, and therefore represents a true coelom, and is divided into one compartment on each side of the trunk, and additional compartments inside the head and tail, all separated completely by septa. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no excretory or respiratory systems. While often said to lack a circulatory system, chaetognaths do have a rudimentary hemal system resembling those of annelids.
The arrow worm rhabdomeres are derived from microtubules 20 nm long and 50 nm wide, which in turn form conical bodies that contain granules and thread structures. The cone body is derived from a cilium.
Locomotion
The trunk bears one or two pairs of lateral fins incorporating structures superficially similar to the fin rays of fish, with which they are not homologous. Unlike those of vertebrates, these lateral fins are composed of a thickened basement membrane extending from the epidermis. An additional caudal fin covers the post-anal tail. Two chaetognath species, Caecosagitta macrocephala and Eukrohnia fowleri, have bioluminescent organs on their fins. | Chaetognatha | Wikipedia | 504 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering. Muscle movements have been described as among the fastest in metazoans. Muscles are directly excitable by electrical currents or strong K+ solutions; the main neuromuscular transmitter is acetylcholine.
Reproduction and life cycle
All species are hermaphroditic, carrying both eggs and sperm. Each animal possesses a pair of testes within the tail, and a pair of ovaries in the posterior region of the main body cavity. Immature sperm are released from the testes to mature inside the cavity of the tail, and then swim through a short duct to a seminal vesicle where they are packaged into a spermatophore.
During mating, each individual places a spermatophore onto the neck of its partner after rupture of the seminal vesicle. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place. The seminal receptacles and oviducts accumulate and store spermatozoa, to perform multiple fertilisation cycles. Some benthic members of Spadellidae are known to have elaborate courtship rituals before copulation, for example Paraspadella gotoi.
The eggs are mostly planktonic, except in a few species such as Ferosagitta hispida that attaches eggs to the substrate. In Eukrohnia, eggs develop in marsupial sacs or attached to algae. Eggs usually hatch after 1–3 days. Chaetognaths do not undergo metamorphosis nor they possess a well-defined larval stage, an unusual trait among marine invertebrates; however there are significant morphological differences between the newborn and the adult, with respect to proportions, chitinous structures and fin development.
The life spans of chaetognaths are variable but short; the longest recorded was 15 months in Sagitta friderici. | Chaetognatha | Wikipedia | 462 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Behaviour
Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat. It is known that they feed more frequently with higher temperatures. Planktonic chaetognaths often must swim continuously, with a "hop and sink" behaviour, to keep themselves in the desired location in the water layer, and swim actively to catch prey. They all tend to keep the body slightly slanted with the head pointing downwards. They often show a "gliding" behaviour, slowly sinking for a while, and then catching up with a quick movement of their fins. Benthic species usually stay attached to substrates such as rocks, algae or sea grasses, more rarely on top or between sand grains, and act more strictly as ambush predators, staying still until prey passes by. The prey is detected thanks to the ciliary fence and tuft organs, sensing vibrations – individuals of Spadella cephaloptera for example will attack a glass or metal probe vibrating at an adequate frequency. To catch prey, arrow worms jump forward with a strong stroke of the tail fin. Once in contact with prey, they withdraw the hood over the grasping spines, so that it forms a cage around the prey and bring it in contact with the mouth. They swallow their prey whole.
Ecology
Chaetognaths are found in all world's oceans, from the poles to tropics, and also in brackish and estuarine waters. They inhabit very diverse environments, from hydrothermal vents to deep ocean seafloor, to seagrass beds and marine caves. The majority are planktonic, and they are often the second most common component of zooplankton, with a biomass ranging between 10 and 30% that of copepods. In the Canada Basin, chaetognaths alone represent ~13% of the zooplankton biomass. As such, they are ecologically relevant and a key food source for fishes and other predators, including commercially relevant fishes such as mackerel or sardines. 58% of known species are pelagic, while about a third of species are epibenthic or meiobenthic, or inhabit the immediate vicinity of the substrate. Chaetognaths have been recorded up to 5000 and possibly even 6000 meters of depth. | Chaetognatha | Wikipedia | 467 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The highest density of chaetognaths is observed in the photic zone of shallow waters. Larger chaetognath species tend to live deeper in water, but spend their juvenile stages higher in the water column. Arrow worms however engage in diel vertical migration, spending the day at lower depths to avoid predators, and coming close to the surface at night. Their position in the water column can depend on light, temperature, salinity, age and food supply. They cannot swim against oceanic currents, and they are used as a hydrological indicator of currents and water masses.
All chaetognaths are ambush predators, preying on other planktonic animals, mostly copepods and cladocerans but also amphipods, krill and fish larvae. Adults can feed on younger individuals of the same species. Some species are also reported to be omnivores, feeding on algae and detritus.
Chaetognaths are known to use the neurotoxin tetrodotoxin to subdue prey, possibly synthesized by Vibrio bacterial species.
Genetics
Mitochondrial genome
The mtDNA of the arrow worm Spadella cephaloptera has been sequenced in 2004, and at the time it was the smallest metazoan mitochondrial genome known, being 11,905 base pairs long (it has now been surpassed by the mitchondrial genome of the ctenophore Mnemiopsis leidyi, which is 10,326 bp long). All mitochondrial tRNA genes are absent. The MT-ATP8 and MT-ATP6 genes are also missing. The mtDNA of Paraspadella gotoi, also sequenced in 2004, is even smaller (11,403 bp) and it shows a similar pattern, lacking 21 of the 22 usually present tRNA genes and featuring only 14 of the 37 genes normally present.
Chaetognaths show a unique mitochondrial genomic diversity within individual of the same species.
Phylogeny | Chaetognatha | Wikipedia | 396 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
External
The evolutionary relationships of chaetognaths have long been enigmatic. Charles Darwin remarked that arrow worms were "remarkable for the obscurity of their affinities". Chaetognaths in the past have been traditionally, but erroneously, classed as deuterostomes by embryologists due to deuterostome-like features in the embryo. Lynn Margulis and K. V. Schwartz placed chaetognaths in the deuterostomes in their Five Kingdom classification. However, several developmental features are at odds with deuterostomes and are either akin to Spiralia or unique to Chaetognatha.
Molecular phylogeny shows that Chaetognatha are, in fact, protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification. The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis—in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan. Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.
Studies of arrow worms' nervous systems suggests they should be placed within the protostomes. According to 2017 and 2019 papers, chaetognaths either belong to or are the sister group of Gnathifera.
Internal
Below is a consensus evolutionary tree of Chaetognatha, based on both morphological and molecular data, as of 2021. | Chaetognatha | Wikipedia | 379 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Fossil record
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described. Chaetognaths first appear during the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha ercainella Chen & Huang and Protosagitta spinosa Hu) and the Middle Cambrian Burgess Shale of British Columbia (Capinatator praetermissus.) A Cambrian stem-group chaetognath, Timorebestia, first described in 2024, was much larger than modern species, showing that chaetognaths occupied different roles in marine ecosystems compared to today. A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois.
Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, have been shown to be dental elements of vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then. There is evidence that chaetognaths were important components of the oceanic food web already in the Early Cambrian.
History
The first known description of a chaetognath has been published by Dutch naturalist Martinus Slabber in the 1770s; he also coined the name "arrow worm". The zoologist Henri Marie Ducrotay de Blainville also briefly mentioned probable chaetognaths but he understood them as pelagic mollusks. The first description of a currently accepted species of chaetognath, Sagitta bipunctata, is from 1827. Among the early zoologists describing arrow worms, there is Charles Darwin, who took notes about them during the voyage of the Beagle and in 1844 dedicated a paper to them. In the following year, August David Krohn published an early anatomical description of Sagitta bipunctata. | Chaetognatha | Wikipedia | 487 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The term "chaetognath" has been coined in 1856 by Rudolf Leuckart. He was also the first to propose that the genus Sagitta belonged to a separate group: «At the moment, it seems most natural to regard the Sagittas as representatives of a small group of their own that makes the transition from the real annelids (first of all the lumbricines) to the nematodes, and may not be unsuitably named Chaetognathi.»
The modern systematics of Chaetognatha begins in 1911 with Ritter-Záhony and is later consolidated by Takasi Tokioka in 1965 and Robert Bieri in 1991. Tokioka introduced the orders Phragmophora and Aphragmophora, and classified four families, six genera, for a total of 58 species – plus the extinct Amiskwia, classified as a true primitive chaetognath in a separate class, Archisagittoidea.
Chaetognaths were for a while considered as belonging or affine to the deuterostomes, but suspects of their affinities among Spiralia or other protostomes were already present as early as 1986. Their affinities with protostomes were clarified in 2004 by sequencing and analysis of mtDNA.
Infection by giant viruses | Chaetognatha | Wikipedia | 269 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a giant virus (Meelsvirus) infecting Adhesisagitta hispida; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped. In 2019, reanalysis of other previous studies has shown that structures that were taken in 1967 for bristles present on the surface of the species Spadella cephaloptera, and in 2003, for bacteria infecting Paraspadella gotoi, were in fact enveloped and spindle-shaped giant viruses with a cytoplasmic site of multiplication. The viral species infecting P. gotoi, whose maximum length is 3.1 μm, has been named Klothovirus casanovai (Klotho being the Greek name for one of the three Fates whose attribute was a spindle, and casanovai, in tribute to Pr J.-P. Casanova who devoted a large part of his scientific life to the study of chaetognaths). The other species has been named Megaklothovirus horridgei (in tribute to Adrian Horridge, the first author of the 1967 article). On a photograph, one of the viruses M. horridgei, although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteria Escherichia coli. Many ribosomes are present in virions but their origin remains unknown (cellular, viral or only partly viral). To date, giant viruses known to infect metazoans are exceptionally rare. | Chaetognatha | Wikipedia | 333 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Conodonts (Greek kōnos, "cone", + odont, "tooth") are an extinct group of jawless vertebrates, classified in the class Conodonta. They are primarily known from their hard, mineralised tooth-like structures called "conodont elements" that in life were present in the oral cavity and used to process food. Rare soft tissue remains suggest that they had elongate eel-like bodies with large eyes. Conodonts were a long-lasting group with over 300 million years of existence from the Cambrian (over 500 million years ago) to the beginning of the Jurassic (around 200 million years ago). Conodont elements are highly distinctive to particular species and are widely used in biostratigraphy as indicative of particular periods of geological time.
Discovery and understanding of conodonts
The teeth-like fossils of the conodont were first discovered by Heinz Christian Pander and the results published in Saint Petersburg, Russia, in 1856.
It was only in the early 1980s that the first fossil evidence of the rest of the animal was found (see below). In the 1990s exquisite fossils were found in South Africa in which the soft tissue had been converted to clay, preserving even muscle fibres. The presence of muscles for rotating the eyes showed definitively that the animals were primitive vertebrates.
Nomenclature and taxonomic rank
Through their history of study, "conodont" is a term which has been applied to both the individual fossils and to the animals to which they belonged. The original German term used by Pander was "conodonten", which was subsequently anglicized as "conodonts", though no formal latinized name was provided for several decades. MacFarlane (1923) described them as an order, Conodontes (a Greek translation), which Huddle (1934) altered to the Latin spelling Conodonta. A few years earlier, Eichenberg (1930) established another name for the animals responsible for conodont fossils: Conodontophorida ("conodont bearers"). A few other scientific names were rarely and inconsistently applied to conodonts and their proposed close relatives during 20th century, such as Conodontophoridia, Conodontophora, Conodontochordata, Conodontiformes, and Conodontomorpha. | Conodont | Wikipedia | 480 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Conodonta and Conodontophorida are by far the most common scientific names used to refer to conodonts, though inconsistencies regarding their taxonomic rank still persist. Bengtson (1976)'s research on conodont evolution identified three morphological tiers of early conodont-like fossils: protoconodonts, paraconodonts, and "true conodonts" (euconodonts). Further investigations revealed that protoconodonts were probably more closely related to chaetognaths (arrow worms) rather than true conodonts. On the other hand, paraconodonts are still considered a likely ancestral stock or sister group to euconodonts.
The 1981 Treatise on Invertebrate Paleontology volume on the conodonts (Part W revised, supplement 2) lists Conodonta as the name of both a phylum and a class, with Conodontophorida as a subordinate order for "true conodonts". All three ranks were attributed to Eichenberg, and Paraconodontida was also included as an order under Conodonta. This approach was criticized by Fåhraeus (1983), who argued that it overlooked Pander's historical relevance as a founder and primary figure in conodontology. Fåhraeus proposed to retain Conodonta as a phylum (attributed to Pander), with the single class Conodontata (Pander) and the single order Conodontophorida (Eichenberg). Subsequent authors continued to regard Conodonta as a phylum with an ever-increasing number of subgroups.
With increasingly strong evidence that conodonts lie within the phylum Chordata, more recent studies generally refer to "true conodonts" as the class Conodonta, containing multiple smaller orders. Paraconodonts are typically excluded from the group, though still regarded as close relatives. In practice, Conodonta, Conodontophorida, and Euconodonta are equivalent terms and are used interchangeably. | Conodont | Wikipedia | 421 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Conodont elements
For a long time, the function and arrangement of conodont elements was enigmatic, since the whole animal was soft-bodied, with the sole exception of the mineralized elements. Upon the conodont animal's demise, the soft tissues would decompose and the individual conodont elements would separate. However, in instances of exceptional preservation the conodont elements may be recovered in articulation. By closely observing these rare specimens, Briggs et al. (1983) were able to for the first time study the anatomy of the complexes formed by the conodont elements arranged as they were in life. Other researchers have continued to revise and reinterpret this initial description.
Lone elements
Conodont elements consist of mineralised teeth-like structures of varying morphology and complexity. The evolution of mineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodonts or the dermal skeleton of early agnathans.
The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth, i.e., coniform cones, ramiform bars, and pectiniform platforms, probably performed different functions.
For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometers to 5 millimeters in length), which occur commonly, but not always, in isolation and were not associated with any other fossil. Until the early 1980s, conodont teeth had not been found in association with fossils of the host organism, in a konservat lagerstätte. This is because the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.
These microfossils are made of hydroxylapatite (a phosphatic mineral). The conodont elements can be extracted from rock using adequate solvents. | Conodont | Wikipedia | 433 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
They are widely used in biostratigraphy. Conodont elements are also used as paleothermometers, a proxy for thermal alteration in the host rock, because under higher temperatures, the phosphate undergoes predictable and permanent color changes, measured with the conodont alteration index. This has made them useful for petroleum exploration where they are known, in rocks dating from the Cambrian to the Late Triassic.
Full apparatus
The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.
In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt and by Harold W. Scott in 1934.
Element types
The arrangement of elements in ozarkodinids and other complex conodonts was first reconstructed from extremely well-preserved taxa by Briggs et al. (1983), although loosely articulated conodont elements are reported as early as 1971. Conodont elements are organized into three different groups based upon shape. These groups of shapes are termed S, M, and P elements.
The S and M elements are ramiform, elongate, and comb-like structures. An individual element has a single row of many cusps running down the midline along its top side. These conodont elements are arranged towards the animal's anterior oral surface, forming an interlocking basket of cusps within the mouth. Cusp may point out towards the head of the animal, or back towards the tail. The number of S and M elements present as well as the direction they point may vary by taxonomic group. M (makellate) elements have a higher position in the mouth and commonly form a symmetrical shape akin to a horseshoe or pick. S elements are further divided into three subtypes:
S element - an unpaired symmetrical ramiform structure at the front of the mouth. Sometimes known as an S0 element.
S element - paired asymmetrical structures
S element - paired highly asymmetrical, bipennate structures | Conodont | Wikipedia | 430 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
In P elements, a pectiniform (comb-shaped) row of cusps transitions into a broad flat or ridged platform moving towards the base of the element. Platforms and cusps are only found along one side of the structure. Individual elements oriented vertically and arranged in pairs, with platforms and cusps pointing towards the animal's midline. They occur deeper in the throat than the S and M elements. P elements are further divided into two subtypes:
Pa element - blade-like structures
Pb element - arched structures
The conodont animal
Although conodont elements are abundant in the fossil record, fossils preserving soft tissues of conodont animals are known from only a few deposits in the world. One of the first possible body fossils of a conodont were those of Typhloesus, an enigmatic animal known from the Bear Gulch limestone in Montana. This possible identification was based on the presence of conodont elements with the fossils of Typhloesus. This claim was disproved, however, as the conodont elements were actually in the creature's digestive area. That animal is now regarded as a possible mollusk related to gastropods. As of 2023, there are only three described species of conodonts that have preserved trunk fossils: Clydagnathus windsorensis from the Carboniferous aged Granton Shrimp Bed in Scotland, Promissum pulchrum from the Ordovician aged Soom Shale in South Africa, and Panderodus unicostatus from the Silurian aged Waukesha Biota in Wisconsin. There are other examples of conodont animals that only preserve the head region, including eyes, of the animals known from the Silurian aged Eramosa site in Ontario and Triassic aged Akkamori section in Japan.
According to these fossils, conodonts had large eyes, fins with fin rays, chevron-shaped muscles and axial line, which were interpreted as notochord or the dorsal nerve cord. While Clydagnathus and Panderodus had lengths only reaching , Promissum is estimated to reach in length, if it had the same proportions as Clydagnathus.
Ecology | Conodont | Wikipedia | 452 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Diet
Because they are associated with the oral region of the conodont animal, it is accepted that conodont elements are used in the acquisition of food. Two primary hypotheses have arisen as to how this is accomplished. One hypothesis proposed that elements acted as support structures for filamentous soft-tissues. These small filaments (cilia) would be used to filter small planktonic organisms out of the water column, analogous to the cnidoblast cells of a coral or the lophophore of a brachiopod.
Another hypothesis contests that the conodont elements were used to actively catch and process prey. S and M elements could have been independently movable, allowing prey to be captured in the oral region of the animal. Modern hagfish and lampreys scrape at flesh using keratinous blades supported by a simple but effective pulley-like system, involving a string of muscles around a cartilaginous core. An equivalent system might have been present in conodonts. S and M elements would be able to open and close at will to firmly grasp or pinch at prey, before rotating back to consume the prey element. The blade-like P elements deeper in the throat would process the food by slicing against their counterparts like a pair of scissors, or grinding against each other like molar teeth.
Current consensus supports the latter hypothesis in which elements are used for predation, not suspension feeding. One line of evidence for this includes the isometric growth pattern exhibited by S, M, and P elements. If the conodont animal relied upon a filter feeding strategy then this growth pattern would not provide the necessary surface area needed to support ciliated tissue as the animal grew. There is some evidence for cartilaginous structures similar to those present in modern jawless fish, which are both predators and scavengers. Wear on some conodont elements suggests that they functioned like teeth, with both wear marks likely created by food as well as by occlusion with other elements.
It is possible that multiple feeding strategies may have arisen in different groups of conodonts, as they are a diverse clade. A 2009 paper suggested that the genus Panderodus may have utilized venom in the acquisition of prey. Evidence of longitudinal grooves are present on some conodont elements associated with the feeding apparatus of this particular animal. These sorts of grooves are analogous to those present in some extant groups of venomous vertebrates. | Conodont | Wikipedia | 502 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Lifestyle
Studies have concluded that conodonts taxa occupied both pelagic (open ocean) and nektobenthic (swimming above the sediment surface) niches. The preserved musculature suggests that some conodonts (Promissum at least) were efficient cruisers, but incapable of bursts of speed. Based on isotopic evidence, some Devonian conodonts have been proposed to have been low-level consumers that fed on zooplankton.
A study on the population dynamics of Alternognathus has been published. Among other things, it demonstrates that at least this taxon had short lifespans lasting around a month. A study Sr/Ca and Ba/Ca ratios of a population of conodonts from a carbonate platform from the Silurian of Sweden found that the different conodont species and genera likely occupied different trophic niches.
Classification and phylogeny
Affinities
, scientists classify the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.
Milsom and Rigby envision them as vertebrates similar in appearance to modern hagfish and lampreys, and phylogenetic analysis suggests they are more derived than either of these groups. However, this analysis comes with one caveat: the earliest conodont-like fossils, the protoconodonts, appear to form a distinct clade from the later paraconodonts and euconodonts. Protoconodonts are probably not relatives of true conodonts, but likely represent a stem group to Chaetognatha, an unrelated phylum that includes arrow worms.
Moreover, some analyses do not regard conodonts as either vertebrates or craniates, because they lack the main characteristics of these groups. More recently it has been proposed that conodonts may be stem-cyclostomes, more closely related to hagfish and lampreys than to jawed vertebrates.
Ingroup relations
Individual conodont elements are difficult to classify in a consistent manner, but an increasing number of conodont species are now known from multi-element assemblages, which offer more data to infer how different conodont lineages are related to each other. The following is a simplified cladogram based on Sweet and Donoghue (2001), which summarized previous work by Sweet (1988) and Donoghue et al. (2000): | Conodont | Wikipedia | 506 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Only a few studies approach the question of conodont ingroup relationships from a cladistic perspective, as informed by phylogenetic analyses. One of the broadest studies of this nature was the analysis of Donoghue et al. (2008), which focused on "complex" conodonts (Prioniodontida and other descendant groups):
Evolutionary history
The earliest fossils of conodonts are known from the Cambrian period. Conodonts extensively diversified during the early Ordovician, reaching their apex of diversity during the middle part of the period, and experienced a sharp decline during the late Ordovician and Silurian, before reaching another peak of diversity during the mid-late Devonian. Conodont diversity declined during the Carboniferous, with an extinction event at the end of the middle Tournaisian and a prolonged period of significant loss of diversity during the Pennsylvanian. Only a handful of conodont genera were present during the Permian, though diversity increased after the P-T extinction during the Early Triassic.
Diversity continued to decline during the Middle and Late Triassic, culminating in their extinction soon after the Triassic-Jurassic boundary. Much of their diversity during the Paleozoic was likely controlled by sea levels and temperature, with the major declines during the Late Ordovician and Late Carboniferous due to cooler temperatures, especially glacial events and associated marine regressions which reduced continental shelf area. However, their final demise is more likely related to biotic interactions, perhaps competition with new Mesozoic taxa. | Conodont | Wikipedia | 308 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Taxonomy
Conodonta taxonomy based on Sweet (1988), Sweet & Donoghue (2001), and Mikko's Phylogeny Archive.
Class Conodonta Pander, 1856 [Conodontophorida Eichenberg, 1930; "euconodonts" Bengtson, 1976]
Cavidonti Sweet, 1988
Order Belodellida? Sweet, 1988
Ansellidae? Fåhraeus & Hunter, 1985
Belodellidae Khodalevich & Tschernich, 1973
Dapsilodontidae? Sweet, 1988
Order Proconodontida Sweet, 1988
Cordylodontidae Lindström, 1970
Fryxellodontidae Miller, 1981
Pseudooneotodidae? Wang & Aldridge, 2010
Proconodontidae Lindström, 1970
Pygodontidae? Bergstrom, 1981
Conodonti Pander, 1856 non Branson, 1938
Order Protopanderodontida Sweet, 1988
Acanthodontidae Lindström, 1970
Clavohamulidae Lindström, 1970
Drepanoistodontidae? Fåhraeus, 1978 [Distacodontidae Bassler, 1925]
Protopanderodontidae Lindström, 1970 [Scolopodontidae Bergström, 1981; Oneotodontidae Miller, 1981; Teridontidae Miller, 1981]
Serratognathidae? Zhen et al., 2009
Strachanognathidae? Bergström, 1981 [Cornuodontidae Stouge, 1984]
Order Panderodontida Sweet, 1988
Panderodontidae Lindström, 1970
Order Prioniodontida Dzik, 1976 (paraphyletic)
Acodontidae? Dzik, 1993 [Tripodontinae Sweet, 1988]
Cahabagnathidae? Stouge & Bagnoli 1999
Distacodontidae? Bassler, 1925 emend. Ulrich & Bassler, 1926 [Drepanodontinae Fåhraeus & Nowlan, 1978; Lonchodininae Hass, 1959]
Gamachignathidae? Wang & Aldridge, 2010
Jablonnodontidae? Dzik, 2006
Nurrellidae? Pomešano-Cherchi, 1967
Paracordylodontidae? Bergström, 1981
Playfordiidae? Dzik, 2002
Ulrichodinidae? Bergström, 1981
Rossodus Repetski & Ethington, 1983 | Conodont | Wikipedia | 499 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Multioistodontidae Harris, 1964 [Dischidognathidae]
Oistodontidae Lindström, 1970 [Juanognathidae Bergström, 1981]
Periodontidae Lindström, 1970
Rhipidognathidae Lindström, 1970 sensu Sweet, 1988
Prioniodontidae Bassler, 1925
Phragmodontidae Bergström, 1981 [Cyrtoniodontinae Hass, 1959]
Plectodinidae Sweet, 1988
Pygodontidae? Bergstrom, 1981
Icriodontacea
Balognathidae (Hass, 1959)
Polyplacognathidae Bergström, 1981
Distomodontidae Klapper, 1981
Icriodellidae Sweet, 1988
Icriodontidae Müller & Müller, 1957
Order Prioniodinida Sweet, 1988
Oepikodontidae? Bergström, 1981
Xaniognathidae? Sweet, 1981
Chirognathidae Branson & Mehl, 1944
Prioniodinidae Bassler, 1925 [Hibbardellidae Mueller, 1956]
Bactrognathidae Lindström, 1970
Ellisoniidae Clark, 1972
Gondolellidae Lindström, 1970
Order Ozarkodinida Dzik, 1976 [Polygnathida]
Anchignathodontidae? Clark, 1972
Archeognathidae? Miller, 1969
Belodontidae? Huddle, 1934
Coleodontidae? Branson & Mehl, 1944 [Hibbardellidae Müller, 1956; Loxodontidae]
Eognathodontidae? Bardashev, Weddige & Ziegler, 2002
Francodinidae? Dzik, 2006
Gladigondolellidae? (Hirsch, 1994) [Sephardiellinae Plasencia, Hirsch & Márquez-Aliaga, 2007; Neogondolellinae Hirsch, 1994; Cornudininae Orchard, 2005; Epigondolellinae Orchard, 2005; Marquezellinae Plasencia et al., 2018; Paragondolellinae Orchard, 2005; Pseudofurnishiidae Ramovs, 1977]
Iowagnathidae? Liu et al., 2017
Novispathodontidae? (Orchard, 2005)
Trucherognathidae? Branson & Mehl, 1944
Vjalovognathidae? Shen, Yuan & Henderson, 2015
Wapitiodontidae? Orchard, 2005
Cryptotaxidae Klapper & Philip, 1971 | Conodont | Wikipedia | 499 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Spathognathodontidae Hass, 1959 [Ozarkodinidae Dzik, 1976]
Pterospathodontidae Cooper, 1977 [Carniodontidae]
Kockelellidae Klapper, 1981 [Caenodontontidae]
Polygnathidae Bassler, 1925 [?Eopolygnathidae Bardashev, Weddige & Ziegler, 2002]
Palmatolepidae Sweet, 1988
Hindeodontidae (Hass, 1959)
Elictognathidae Austin & Rhodes, 1981
Gnathodontidae Sweet, 1988
Idiognathodontidae Harris & Hollingsworth, 1933
Mestognathidae Austin & Rhodes, 1981
Cavusgnathidae Austin & Rhodes, 1981
Sweetognathidae Ritter, 1986 | Conodont | Wikipedia | 158 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.
Anatomy
Most gnathostomulids measure in length. They are often slender to thread-like worms, with a generally transparent body. In many Bursovaginoidea, one of the major group of gnathostomulids, the neck region is slightly narrower than the rest of the body, giving them a distinct head.
Like flatworms they have a ciliated epidermis, but in contrast to flatworms, they have one cilium per cell. The cilia allow the worms to glide along in the water between sand grains, although they also use muscles, allowing the body to twist or contract, for movement.
They have no body cavity, and no circulatory or respiratory system. The nervous system is simple, and restricted to the outer layers of the body wall. The only sense organs are modified cilia, which are especially common in the head region.
The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present. The basal plate is used to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera. The ultrastructure of the jaws made of rods with electron dense core in transmission electron microscopy sections also support their close relation with Rotifera and Micrognathozoa. The mouth opens into a blind-ending tube in which digestion takes place; there is no true anus. However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore. | Gnathostomulid | Wikipedia | 442 | 43177 | https://en.wikipedia.org/wiki/Gnathostomulid | Biology and health sciences | Spiralia | Animals |
Reproduction
Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary and one or two testes. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature version of the adult, without a larval stage.
Taxonomy
There are approximately 100 described species and certainly many more as yet undescribed. The known species are grouped in two orders. The filospermoids are very long and are characterized by an elongate rostrum. The bursovaginoids have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.
Gnathostomulids have no known fossil record, though there are (debatable) similarities between the jaws of modern gnathostomulids and certain conodont elements. (Ochietti & Cailleux, 1969; Durden et al, 1969)
They appear to be a sister clade to the Syndermata. | Gnathostomulid | Wikipedia | 230 | 43177 | https://en.wikipedia.org/wiki/Gnathostomulid | Biology and health sciences | Spiralia | Animals |
Lobopodians are members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as well. While the definition of lobopodians may differ between literatures, it usually refers to a group of soft-bodied, marine worm-like fossil panarthropods such as Aysheaia and Hallucigenia. However, other genera like Kerygmachela and Pambdelurion (which have features similar to other groups) are often referred to as “gilled lobopodians”.
The oldest near-complete fossil lobopodians date to the Lower Cambrian; some are also known from Ordovician, Silurian and Carboniferous Lagerstätten. Some bear toughened claws, plates or spines, which are commonly preserved as carbonaceous or mineralized microfossils in Cambrian strata. The grouping is considered to be paraphyletic, as the three living panarthropod groups (Arthropoda, Tardigrada and Onychophora) are thought to have evolved from lobopodian ancestors.
Definitions | Lobopodia | Wikipedia | 275 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The Lobopodian concept varies from author to author. Its most general sense refers to a suite of mainly Cambrian worm-like panarthropod taxa possessing lobopods – for example, Aysheaia, Hallucigenia, and Xenusion – which were traditionally united as "Xenusians" or "Xenusiids" (class Xenusia). Certain Dinocaridid genera, such as Opabinia, Pambdelurion, and Kerygmachela, may also be regarded as lobopodians, sometimes referred to more specifically as "gilled lobopodians" or "gilled lobopods". This traditional, informal usage of "Lobopodia" treats it as an evolutionary grade, including only extinct Panarthropods near the base of crown Panarthropoda. Crown Panarthropoda comprises the three extant Panarthropod phyla – Onychophora (velvet worms), Tardigrada (waterbears), and Arthropoda (arthropods) – as well as their most recent common ancestor and all of its descendants. Thus, in this usage, Lobopodia consists of various basal Panarthropods. This corresponds to "A" in the image to the left. | Lobopodia | Wikipedia | 268 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada, the two living panarthropod phyla which still bear lobopodous limbs. This definition, corresponding to "C", is a morphological one, depending on the superficial similarity of appendages (the "lobopods"). Thus, it is paraphyletic, excluding the Euarthropods, which are descendants of certain Lobopodians, on the basis of their highly divergent limb morphology. "Lobopodia" has also been used to refer to a proposed sister clade to Arthropoda, consisting of the extant Onychophora and Tardigrada, as well as their most recent common ancestor and all of its descendants. This definition renders Lobopodia a monophyletic taxon, if indeed it is valid (that is, if Tardigrades and Onychophora are closer to one another than either is to Arthropoda), but would exclude all the Euarthropod-line taxa traditionally considered Lobopodians. Its validity is uncertain, however, as there are a number of hypotheses regarding the internal phylogeny of Panarthropoda. The broadest definition treats Lobopodia as a monophyletic superphylum equivalent in circumscription to Panarthropoda. By this definition, represented by "D" in the image, Lobopodia is no longer treated as an evolutionary grade but as a clade, containing not only the early, superficially "Lobopodian" forms but also all of their descendants, including the extant Panarthropods.
Lobopodia has, historically, sometimes included Pentastomida, a group of parasitic panarthropod which were traditionally thought to be a unique phylum, but revealed by subsequent phylogenomic and anatomical studies to be a highly specialized taxon of crustaceans.
Representative taxa | Lobopodia | Wikipedia | 409 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The better-known genera include Aysheaia, which was discovered in the Canadian Burgess Shale, and Hallucigenia, known from both the Chenjiang Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has a morphology apparently basic for lobopodians — for example, a significantly annulated cuticle, a terminal mouth opening, specialized frontalmost appendages, and stubby lobopods with terminal claws. Hallucigenia sparsa is famous for having a complex history of interpretation — it was originally reconstructed with long, stilt-like legs and mysterious fleshy dorsal protuberances, and was long considered a prime example of the way in which nature experimented with the most diverse and bizarre body designs during the Cambrian. However, further discoveries showed that this reconstruction had placed the animal upside-down: interpreting the "stilts" as dorsal spines made it clear that the fleshy "dorsal" protuberances were actually elongated lobopods. More recent reconstruction even exchanged the front and rear ends of the animal: it was revealed that the bulbous imprint previously thought to be a head was actually gut contents being expelled from the anus.
Microdictyon is another charismatic as well as the speciose genus of lobopodians resembling Hallucigenia, but instead of spines, it bore pairs of net-like plates, which are often found disarticulated and are known as an example of small shelly fossils (SSF). Xenusion has the oldest fossil record amongst the described lobopodians, which may trace back to Cambrian Stage 2. Luolishania is an iconic example of lobopodians with multiple pairs of specialized appendages. The gill lobopodians Kerygmachela and Pambdelurion shed light on the relationship between lobopodians and arthropods, as they have both lobopodian affinities and characteristics linked to the arthropod stem-group.
Morphology
Most lobopodians were only a few centimeters in length, while some genera grew up to over 20 centimeters. Their bodies are annulated, although the presence of annulation may differ between position or taxa, and sometimes difficult to discern due to their close spacing and low relief on the fossil materials. Body and appendages are circular in cross-section.
Head | Lobopodia | Wikipedia | 483 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
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