Wikipedia

Tunicate

Atunicateis an exclusively marineinvertebrateanimal, a member of thesubphylumTunicata(/ˌtjuːnɪˈkeɪtə/TEW-nih-KAY-tə). This grouping is part of theChordata,aphylumwhich includes all animals withdorsal nerve cordsandnotochords(includingvertebrates). The subphylum was at one time calledUrochordata,and the termurochordatesis still sometimes used for these animals. They are the only chordates that have lost theirmyomericsegmentation, with the possible exception of the seriation of the gill slits.[8][9]However,doliolidsstill display segmentation of the muscle bands.[10]

Tunicates
Temporal range:
Cambrian Stage 3–Present,
518–0Ma[1](PossibleEdiacaranrecord, 557 Ma[2][3])
Gold-mouth sea squirt (Polycarpa aurata)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Olfactores
Subphylum: Tunicata
Lamarck,1816[4][5]
Classes and unplacedgenera[4][7]
Synonyms

UrochordataLankester, 1877

Some tunicates live as solitary individuals, but others replicate bybuddingand becomecolonies,[11]each unit being known as azooid.They are marinefilter feederswith a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During theirrespirationand feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Adult ascidian tunicates aresessile,immobile and permanently attached to rocks or other hard surfaces on the ocean floor.Thaliaceans(pyrosomes, doliolids, and salps) andlarvaceanson the other hand, swim in thepelagic zoneof the sea as adults.

Various species ofascidians,the most well-known class of tunicates, are commonly known assea squirts,sea pork, sea livers, orsea tulips.

The earliest probable species of tunicate appears in the fossil record in the earlyCambrian period.Despite their simple appearance and very different adult form, their close relationship to the vertebrates is evidenced by the fact that during their mobile larval stage, they possess anotochordor stiffening rod and resemble atadpole.Their name derives from their unique outer covering or "tunic", which is formed from proteins and carbohydrates, and acts as anexoskeleton.In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff.

Taxonomy

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Clavelina moluccensis,the bluebell tunicate
Botrylloides violaceusshowing oral tentacles at openings of buccal siphons

About 3,000 species of tunicate exist in the world's oceans, living mostly in shallow water. The most numerous group is theascidians;fewer than 100 species of these are found at depths greater than 200 m (660 ft).[12]Some are solitary animals leading asessileexistence attached to the seabed, but others arecolonialand a few arepelagic.Some are supported by a stalk, but most are attached directly to asubstrate,which may be a rock, shell, coral, seaweed,mangroveroot, dock, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest is a stalked sea tulip,Pyura pachydermatina,which can grow to be over 1 metre (3.3 ft) tall.[12]

The Tunicata were established byJean-Baptiste Lamarckin 1816. In 1881,Francis Maitland Balfourintroduced another name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates.[13]No doubt largely because of his influence, various authors supported the term, either as such, or as the slightly older "Urochordata", but this usage is invalid because "Tunicata" has precedence, and grounds for superseding the name never existed. Accordingly, the current (formally correct) trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, and the name Tunicata is almost invariably used in modern scientific works. It is accepted as valid by the World Register of Marine Species[14]but not by the Integrated Taxonomic Information System.[15]

Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks.[16]Sea squirts are so named because of their habit of contracting their bodies sharply and squirting out water when disturbed.[17]Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat.[18]

Classification

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Tunicates are more closely related tocraniates(includinghagfish,lampreys,and jawedvertebrates) than tolancelets,echinoderms,hemichordates,Xenoturbellaor otherinvertebrates.[19][20][21]

Thecladeconsisting of tunicates and vertebrates is calledOlfactores.[22]

The Tunicata contain roughly 3,051 described species,[12]traditionally divided into these classes:

Members of theSorberaceawere included in Ascidiacea in 2011 as a result ofrDNAsequencing studies.[7]Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group ofparaphyleticstatus.[23][24][25]A close relationship between Thaliacea and Ascidiacea, with the former possibly emerging from the latter, had already been proposed since the early 20th century under the name of Acopa.[26]

The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues.[27][25][28]

Tunicata

Fossil record

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The star-shaped holes (Catellocaula vallata) in this Upper Ordovician bryozoan may represent a tunicate preserved bybioimmurationin thebryozoanskeleton.

Undisputed fossils of tunicates are rare. The best known and earliest unequivocally identified species isShankouclava shankouensefrom the LowerCambrianMaotianshan Shaleat Shankou village, Anning, nearKunming(South China).[29]There is also a commonbioimmuration,(Catellocaula vallata), of a possible tunicate found in UpperOrdovicianbryozoanskeletons of the upper midwestern United States.[30]A well-preserved Cambrian fossil,Megasiphon thylakos,shows that the tunicate basic body design had already been established 500 million years ago.[31]

Three Enigma tic species were also found from theEdiacaranperiod –Ausia fenestratafrom the Nama Group ofNamibia,the sac-likeYarnemia ascidiformis,and one from a second newAusia-like genus from the Onega Peninsula of northernRussia,Burykhia hunti.Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[32][33]AusiaandBurykhialived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.[33]The Russian Precambrian fossilYarnemiais identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those ofAusiaandBurykhia,so this identification has been questioned.

Fossils of tunicates are rare because their bodies decay soon after death, but in some tunicate families, microscopic spicules are present, which may be preserved as microfossils. These spicules have occasionally been found in Jurassic and later rocks, but, as few palaeontologists are familiar with them, they may have been mistaken forsponge spicules.[34]

In Permian and Triassic there were also forms with a calcareous exoskeleton. At first they were mistaken for corals.[35][36]

Hybridization studies

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A multi-taxonmolecular studyin 2010 proposed that sea squirts are descended from a hybrid between a chordate and aprotostomeancestor (before the divergence ofpanarthropodsandnematodes). This study was based on a quartet partitioning approach designed to revealhorizontal gene transferevents among metazoan phyla.[37]

Anatomy

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Body form

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Colonial tunicate with multiple openings in a single tunic.

Colonies of tunicates occur in a range of forms, and vary in the degree to which individual organisms, known aszooids,integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections calledstolons,which grow along the seabed. Other species have the zooids growing closer together in a tuft or clustered together and sharing a common base. The most advanced colonies involve the integration of the zooids into a common structure surrounded by the tunic. These may have separate buccal siphons and a single central atrial siphon and may be organized into larger systems, with hundreds of star-shaped units. Often, the zooids in a colony are tiny but very numerous, and the colonies can form large encrusting or mat-like patches.[12]

Body structure

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By far the largest class of tunicates is theAscidiacea.The body of an ascidiacean is surrounded by atestor tunic, from which thesubphylumderives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins, crosslinked by phenoloxidase reaction,[38]and complex carbohydrates, and includestunicin,a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as the animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed ofconnective tissue,musclefibres,blood vessels,andnerves.Two openings are found in the body wall: the buccal siphon at the top through which water flows into the interior, and the atrial siphon on the ventral side through which it is expelled. A large pharynx occupies most of the interior of the body. It is a muscular tube linking the buccal opening with the rest of the gut. It has a ciliated groove known as anendostyleon its ventral surface, and this secretes a mucous net which collects food particles and is wound up on the dorsal side of the pharynx. The gullet, at the lower end of the pharynx, links it to a loop of gut which terminates near the atrial siphon. The walls of the pharynx are perforated by several bands of slits, known as stigmata, through which water escapes into the surrounding water-filled cavity, the atrium. This is criss-crossed by various rope-likemesenterieswhich extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs.[12]

TheThaliacea,the other main class of tunicates, is characterised by free-swimming, pelagic individuals. They are all filter feeders using a pharyngeal mucous net to catch their prey. Thepyrosomesarebioluminouscolonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. About 10 species are known, and all are found in the tropics. The 23 species ofdoliolidsare small, mostly under 2 cm (0.79 in) long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies, and swim by jet propulsion. The 40 species ofsalpsare also small, under 4 cm (1.6 in) long, and found in the surface waters of both warm and cold seas. They also move by jet propulsion, and often form long chains by budding off new individuals.[12]

A third class, theLarvacea(or Appendicularia), is the only group of tunicates to retain their chordate characteristics in the adult state, a product of extensiveneoteny.The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. Thenotochordis retained, and the animals, mostly under 1 cm long, are propelled by undulations of the tail. They secrete an external mucous net known as a house, which may completely surround them and is very efficient at trapping planktonic particles.[12]

Physiology and internal anatomy

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Internal anatomy of a generalised tunicate
Section through the wall of an ascidian pyrosoma showing several zooids; (br) buccal siphon; (at) atrial siphon; (tp) test process;(br s) pharynx.

Like all otherchordates,tunicates have anotochordduring their early development, but it is lost by the time they have completed their metamorphosis. As members of the Chordata, they are trueCoelomatawithendoderm,ectoderm,andmesoderm,but they do not develop very clearcoelomicbody cavities, if any at all. Whether they do or not, by the end of their larval development, all that remain are thepericardial,renal, and gonadal cavities of the adults. Except for theheart,gonads, andpharynx(or branchial sac), the organs are enclosed in a membrane called anepicardium,which is surrounded by the jelly-likemesenchyme.

Ascidian tunicates begin life as a lecithotrophic (non-feeding) mobilelarvathat resembles a tadpole,[39]with the exception of some members of the families Styelidae and Molgulidae which has direct development.[40]The latter also have several species with tail-less larval forms.[41][42]The ascidian larvae very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The species in the classAppendiculariaarepelagic,and the general larval form is kept throughout life. Also the classThaliaceais pelagic throughout their lives and may have complex lifecycles. In this class a free living larval stage is absent: Doliolids and pyrosomatids are viviparous–lecithotrophic, and salpids are viviparous–matrotrophic. Only some species of doliolids still have a rudimentary tailed tadpole stage, which is never free-living and lacks a brain.[43][44][45]

Tunicates have a well-developedheartandcirculatory system.The heart is a double U-shaped tube situated just below the gut. The blood vessels are simple connective tissue tubes, and their blood has several types ofcorpuscle.The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen is transported dissolved in theplasma.Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction.[12]

Tunicatebloodhas some unusual features. In some species ofAscidiidaeandPerophoridae,it contains high concentrations of the transitional metalvanadiumandvanadium-associated proteinsinvacuolesin blood cells known asvanadocytes.Some tunicates can concentrate vanadium up to a level ten million times that of the surrounding seawater. It is stored in a +3 oxidation form that requires apHof less than 2 for stability, and this is achieved by the vacuoles also containingsulfuric acid.The vanadocytes are later deposited just below the outer surface of the tunic, where their presence is thought to deterpredation,although it is unclear whether this is due to the presence of the metal or low pH.[46]Other species of tunicates concentratelithium,iron,niobium,andtantalum,which may serve a similar function.[12]Other tunicate species produce distastefulorganic compoundsaschemical defensesagainst predators.[47]

Tunicates lack the kidney-likemetanephridialorgans typical ofdeuterostomes.Most have no excretory structures, but rely on the diffusion ofammoniaacross their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typicalrenalorgan is a mass of large clear-walledvesiclesthat occupy the rectal loop, and the structure has no duct. Each vesicle is a remnant of a part of the primitive coelom, and its cells extract nitrogenous waste matter from circulating blood. They accumulate the wastes inside the vesicles asurate crystals,and do not have any obvious means of disposing of the material during their lifetimes.[44]

Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland. Both originate from the embryonic neural tube and are located between the two siphons. Nerves arise from the two ends of the ganglion; those from the anterior end innervate the buccal siphon and those from the posterior end supply the rest of the body, the atrial siphon, organs, gut and the musculature of the body wall. There are no sense organs but there are sensory cells on the siphons, the buccal tentacles and in the atrium.[12]

Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form ofcellulose.The production in animals of cellulose is so unusual that at first some researchers denied its presence outside of plants, but the tunicates were later found to possess a functional cellulosesynthesizing enzyme,encoded by a gene horizontally transferred from a bacterium.[48]When, in 1845,Carl Schmidtfirst announced the presence in the test of some ascidians of a substance very similar to cellulose, he called it "tunicine", but it is now recognized as cellulose rather than any alternative substance.[49][50][51]

Feeding

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Clavelina robusta(black and white) andPycnoclavella flava(orange) showing siphons.

Nearly all adult tunicates aresuspension feeders(the larval form usually does not feed), capturingplanktonicparticles by filtering sea water through their bodies. Ascidians are typical in their digestive processes, but other tunicates have similar systems. Water is drawn into the body through the buccal siphon by the action ofcilialining the gill slits. To obtain enough food, an average ascidian needs to process one body-volume of water per second.[12]This is drawn through a net lining the pharynx which is being continuously secreted by the endostyle. The net is made of sticky mucus threads with holes about 0.5 μm in diameter which can trap planktonic particles includingbacteria.The net is rolled up on the dorsal side of the pharynx, and it and the trapped particles are drawn into theesophagus.The gut is U-shaped and also ciliated to move the contents along. The stomach is an enlarged region at the lowest part of the U-bend. Here, digestiveenzymesare secreted and apyloricgland (absent in appendicularians)[52]adds further secretions. After digestion, the food is moved on through theintestine,where absorption takes place, and therectum,where undigested remains are formed intofaecalpellets or strings. Theanusopens into the dorsal orcloacalpart of the peribranchial cavity near the atrial siphon. Here, the faeces are caught up by the constant stream of water which carries the waste to the exterior. The animal orientates itself to the current in such a way that the buccal siphon is always upstream and does not draw in contaminated water.[12]

Some ascidians that live on soft sediments aredetritivores.A few deepwater species, such asMegalodicopia hians,aresit-and-wait predators,trapping tiny crustacea, nematodes, and other small invertebrates with the muscular lobes which surround their buccal siphons. Certain tropical species in the familyDidemnidaehavesymbioticgreen algae orcyanobacteriain their tunics, and one of these symbionts,Prochloron,is unique to tunicates. Excessphotosyntheticproducts are assumed to be available to thehost.[12]

Life cycle

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Anatomy of a larval tunicate

Ascidians are almost allhermaphroditesand each has a single ovary and testis, either near the gut or on the body wall. In some solitary species, sperm and eggs are shed into the sea and thelarvaeareplanktonic.In others, especially colonial species, sperm is released into the water and drawn into the atria of other individuals with the incoming water current. Fertilization takes place here and the eggs are brooded through their early developmental stages.[44]Some larval forms appear very much like primitivechordateswith anotochord(stiffening rod) and superficially resemble smalltadpoles.These swim by undulations of the tail and may have a simple eye, anocellus,and a balancing organ, astatocyst.[53]

When sufficiently developed, the larva of the sessile species finds a suitable rock and cements itself in place. The larval form is not capable of feeding, though it may have a rudimentary digestive system,[53]and is only a dispersal mechanism. Many physical changes occur to the tunicate's body duringmetamorphosis,one of the most significant being the reduction of the cerebral ganglion, which controls movement and is the equivalent of the vertebrate brain. From this comes the common saying that the sea squirt "eats its own brain".[54]However, the adult does possess a cerebral ganglion adapted to lack of self-locomotion.[55]In the Thaliacea, the larval stage is rudimentary or suppressed, and the adults are pelagic (swimming or drifting in the open sea).[44]Colonial forms also increase the size of the colony by budding off new individuals to share the same tunic.[56]

Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Sexual reproduction starts within a zooid with an internally fertilized egg. This develops directly into an oozooid without any intervening larval form. This buds precociously to form four blastozooids which become detached in a single unit when the oozoid disintegrates. The atrial siphon of the oozoid becomes the exhalent siphon for the new, four-zooid colony.[12]

A 1901 comparison of frog tadpole and a tunicate larva.

Doliolidshave a very complex life cycle that includes various zooids with different functions. The sexually reproducing members of the colony are known as gonozooids. Each one is a hermaphrodite with the eggs being fertilised by sperm from another individual. The gonozooid isviviparous,and at first, the developing embryo feeds on itsyolk sacbefore being released into the sea as a free-swimming, tadpole-like larva. This undergoes metamorphosis in thewater columninto an oozooid. This is known as a "nurse" as it develops a tail of zooids produced by buddingasexually.Some of these are known as trophozooids, have a nutritional function, and are arranged in lateral rows. Others arephorozooids,have a transport function, and are arranged in a single central row. Other zooids link to the phorozooids, which then detach themselves from the nurse. These zooids develop into gonozooids, and when these are mature, they separate from the phorozooids to live independently and start the cycle over again. Meanwhile, the phorozooids have served their purpose and disintegrate. The asexual phase in the lifecycle allows the doliolid to multiply very rapidly when conditions are favourable.[12]

Salps also have a complex lifecycle with analternation of generations.In thesolitarylife history phase, an oozoidreproduces asexually,producing a chain of tens or hundreds of individual zooids by budding along the length of astolon.The chain of salps is the 'aggregate' portion of the lifecycle. The aggregate individuals, known as blastozooids, remain attached together while swimming and feeding and growing larger. The blastozooids aresequential hermaphrodites.An egg in each is fertilized internally by a sperm from another colony. The egg develops in a brood sac inside the blastozooid and has a placental connection to the circulating blood of its "nurse". When it fills the blastozooid's body, it is released to start the independent life of an oozooid.[12]

Larvaceans only reproducesexually.They areprotandrous hermaphrodites,except forOikopleura dioicawhich isgonochoric,and a larva resembles the tadpole larva of ascidians. Once the trunk is fully developed, the larva undergoes "tail shift", in which the tail moves from a rearward position to a ventral orientation and twists through 90° relative to the trunk. The larva consists of a small, fixed number of cells, and grows by enlargement of these rather than cell division. Development is very rapid and only takes seven hours for azygoteto develop into a house-building juvenile starting to feed.[12]

During embryonic development, tunicates exhibitdeterminate cleavage,where the fate of the cells is set early on with reduced cell numbers andgenomesthat are rapidly evolving. In contrast, theamphioxusand vertebrates showcell determinationrelatively late in development and cell cleavage is indeterminate. Thegenome evolutionof amphioxus and vertebrates is also relatively slow.[57]

Promotion of out-crossing

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Ciona intestinalis(class Ascidiacea) is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously. It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility.[58]Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of the egg. It appears that self/non-self recognition in ascidians such asC. intestinalisis mechanistically similar to self-incompatibility systems in flowering plants.[58]Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of the masking of deleterious recessive mutations (i.e. genetic complementation)[59]and the avoidance ofinbreeding depression.

Botryllus schlosseri(class Ascidiacea) is a colonial tunicate, a member of the only group of chordates that are able to reproduce both sexually and asexually.B. schlosseriis a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission.[60]Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible inB. schlosseri.Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%).[60]Also a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower. These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.[59]

A model tunicate

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Oikopleura dioica(classAppendicularia) is asemelparousorganism, reproducing only once in its lifetime. It employs an originalreproductive strategyin which the entire femalegerm-lineis contained within an ovary that is a single giantmultinucleatecell termed the "coenocyst".[61]O. dioicacan be maintained in laboratory culture, and is of growing interest as amodel organismbecause of itsphylogeneticposition within the closest sister group tovertebrates.[19]

Invasive species

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Over the past few decades, tunicates (notably of the generaDidemnumandStyela) have beeninvadingcoastal waters in many countries. The carpet tunicate (Didemnum vexillum) has taken over a 6.5 sq mi (17 km2) area of the seabed on theGeorges Bankoff the northeast coast of North America, covering stones, molluscs, and other stationary objects in a dense mat.[62]D. vexillum,Styela clavaandCiona savignyihave appeared and are thriving inPuget SoundandHood Canalin thePacific Northwest.[63]

Invasive tunicates usually arrive asfouling organismson the hulls of ships, but may also be introduced as larvae inballast water.Another possible means of introduction is on the shells of molluscs brought in for marine cultivation.[63]Current research indicates many tunicates previously thought to be indigenous to Europe and the Americas are, in fact, invaders. Some of these invasions may have occurred centuries or even millennia ago. In some areas, tunicates are proving to be a major threat toaquacultureoperations.[64]

Use by humans

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Medical uses

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Tunicates contain a host of potentially usefulchemical compounds,including:

  • Plitidepsin,a didemnin effective against various types of cancer; as of late January 2021 undergoing Phase III trials as a treatment for COVID-19[65]
  • Trabectedin,an FDA approved anticancer drug.

Tunicates are able to correct their own cellular abnormalities over a series of generations, and a similarregenerativeprocess may be possible for humans. The mechanisms underlying the phenomenon may lead to insights about the potential of cells and tissues to be reprogrammed and to regenerate compromised human organs.[66][67][68]

As food

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Main article:Ascidiacea § Culinary
Halocynthiatunicates for sale at a market,Busan,South Korea

VariousAscidiaceaspecies are consumed as food around the world. Thepiure(Pyura chilensis) is used in thecuisine of Chile,both raw and in seafood stews. In Japan and Korea, thesea pineapple(Halocynthia roretzi) is the main species eaten. It is cultivated on dangling cords made ofpalm fronds.In 1994, over 42,000 tons were produced, but since then, mass mortality events have occurred among the farmed sea squirts (the tunics becoming soft), and only 4,500 tons were produced in 2004.[69]

Other uses

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The use of tunicates as a source ofbiofuelis being researched. The cellulose body wall can be broken down and converted intoethanol,and other parts of the animal are protein-rich and can be converted into fish feed. Culturing tunicates on a large scale may be possible and the economics of doing so are attractive. As tunicates have few predators, their removal from the sea may not have profound ecological impacts. Being sea-based, their production does not compete with food production as does the cultivation of land-based crops for biofuel projects.[70]

Some tunicates are used asmodel organisms.Ciona intestinalisandCiona savignyihave been used fordevelopmental studies.Both species' mitochondrial[71][72]and nuclear[73][74]genomes have been sequenced. The nuclear genome of the appendicularianOikopleura dioicaappears to be one of the smallest among metazoans[75]and this species has been used to study gene regulation and the evolution and development of chordates.[76]

See also

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References

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