Tunicate

The term was coined in 1760 by nurseryman James Lee, meaning "coated or covered with integuments", fancifully anthropomorphizing the creatures' unique outer coverings as being "enclosed in a tunic." The word derives from Latin tunicātus, meaning "clothed with a tunic only (without a toga)." This "tunic", which is formed from proteins and carbohydrates, acts as an exoskeleton. In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff. == Taxonomy ==

'', the bluebell tunicate '' showing 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 the ascidians; fewer than 100 species of these are found at depths greater than . 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 but not by the Integrated Taxonomic Information System. Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks. Sea squirts are so named because of their habit of contracting their bodies sharply and squirting out water when disturbed. Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat. Classification Tunicates are more closely related to craniates (including hagfish, lampreys, and jawed vertebrates) than to lancelets, echinoderms, hemichordates, Xenoturbella or other invertebrates. The clade consisting of tunicates and vertebrates is called Olfactores. The Tunicata contain roughly 3,051 described species, Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group of paraphyletic status. 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. The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues. Fossil record '') in this Upper Ordovician bryozoan may represent a tunicate preserved by bioimmuration in the bryozoan skeleton. Undisputed fossils of tunicates are rare. The well-preserved Middle Cambrian fossil, Megasiphon thylakos, shows that the tunicate basic body design had already been established 500 million years ago. Two earlier putative tunicate species, Shankouclava shankouense and Cheungkongella ancestralis are known from the Lower Cambrian Maotianshan Shale. There is also a common bioimmuration, (Catellocaula vallata), of a possible tunicate found in Upper Ordovician bryozoan skeletons of the upper midwestern United States. Three enigmatic species were also found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia ascidiformis, and one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians. Ausia and Burykhia lived 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. In the Permian and the Triassic, there were also forms with a calcareous exoskeleton. At first, they were mistaken for corals. Hybridization studies A multi-taxon molecular study in 2010 proposed that sea squirts are descended from a hybrid between a chordate and a protostome ancestor (before the divergence of panarthropods and nematodes). This study was based on a quartet partitioning approach designed to reveal horizontal gene transfer events among metazoan phyla. ==Anatomy==

Body form Colonies of tunicates occur in a range of forms, and vary in the degree to which individual organisms, known as zooids, integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called stolons, 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. and complex carbohydrates, and includes tunicin, 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 of connective tissue, muscle fibres, blood vessels, and nerves. 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 an endostyle on 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-like mesenteries which extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs. with the exception of some members of the families Styelidae and Molgulidae which has direct development. The latter also have several species with tail-less larval forms. 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 class Appendicularia are pelagic, and the general larval form is kept throughout life. Also the class Thaliacea is 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. Tunicates have a well-developed heart and circulatory 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 of corpuscle. The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen is transported dissolved in the plasma. 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. Other species of tunicates concentrate lithium, iron, niobium, and tantalum, which may serve a similar function. Other tunicate species produce distasteful organic compounds as chemical defenses against predators. Tunicates lack the kidney-like metanephridial organs typical of deuterostomes. Most have no excretory structures, but rely on the diffusion of ammonia across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typical renal organ is a mass of large clear-walled vesicles that 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 as urate crystals, and do not have any obvious means of disposing of the material during their lifetimes. When, in 1845, Carl Schmidt first 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. File:Oikopleura (Vexillaria) cophocerca 001.png|Oikopleura cophocerca in its "house". Arrows indicate water movement and (x) the lateral reticulated parts of the house. File:Ascidians.jpg|Blue sea squirts from the genus Rhopalaea. File:Ascidian (Rhopalaea Crassa) (4 cm).png| Fluorescent-colored sea squirts, Rhopalaea crassa. File:Sea Squirts Didemnum molle.jpg| Didemnum molle. ==Feeding==

(black and white) and Pycnoclavella flava'' (orange) showing siphons. Nearly all adult tunicates are suspension feeders (the larval form usually does not feed), capturing planktonic particles 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 of cilia lining the gill slits. To obtain enough food, an average ascidian needs to process one body-volume of water per second. adds further secretions. After digestion, the food is moved on through the intestine, where absorption takes place, and the rectum, where undigested remains are formed into faecal pellets or strings. The anus opens into the dorsal or cloacal part 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. Some ascidians that live on soft sediments are detritivores. A few deepwater species, such as Megalodicopia hians, are sit-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 family Didemnidae have symbiotic green algae or cyanobacteria in their tunics, and one of these symbionts, Prochloron, is unique to tunicates. Excess photosynthetic products are assumed to be available to the host. ==Life cycle==

Tunicates are almost all hermaphrodites (Oikopleura dioica being a rare exception), and each has a single ovary and testis, either near the gut or on the body wall. All larvaceans are semelparous and have external fertilization, and all thaliaceans have internal fertilization. In solitary ascidians, sperm and eggs are generally shed into the sea and the larvae are planktonic. In colonial ascidians, fertilization is internal; 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. 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, However, the adult does possess a cerebral ganglion adapted to lack of self-locomotion. In the Thaliacea, the larval stage is rudimentary or suppressed, and the adults are pelagic (swimming or drifting in the open sea). 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. Promotion of out-crossing 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. 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 as C. intestinalis is mechanistically similar to self-incompatibility systems in flowering plants. and the avoidance of inbreeding 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. schlosseri is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission. Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in B. schlosseri. Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%). O. dioica can be maintained in laboratory culture, and is of growing interest as a model organism because of its phylogenetic position within the closest sister group to vertebrates. ==Invasive species==

Over the past few decades, tunicates (notably of the genera Didemnum and Styela) have been invading coastal waters in many countries. The carpet tunicate (Didemnum vexillum) has taken over a area of the seabed on the Georges Bank off the northeast coast of North America, covering stones, molluscs, and other stationary objects in a dense mat. D. vexillum, Styela clava and Ciona savignyi have appeared and are thriving in Puget Sound and Hood Canal in the Pacific Northwest. Invasive tunicates usually arrive as fouling organisms on the hulls of ships, but may also be introduced as larvae in ballast water. Another possible means of introduction is on the shells of molluscs brought in for marine cultivation. ==Use by humans==

Medical uses Tunicates contain a host of potentially useful chemical 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 • Trabectedin, an FDA approved anticancer drug. Tunicates are able to correct their own cellular abnormalities over a series of generations, and a similar regenerative process 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. As food '' tunicates for sale at a market, Busan, South Korea|thumb Various Ascidiacea species are consumed as food around the world. The piure (Pyura chilensis) is used in the cuisine of Chile, both raw and in seafood stews. In Japan and Korea, the sea pineapple (Halocynthia roretzi) is the main species eaten. It is cultivated on dangling cords made of palm 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. Other uses The use of tunicates as a source of biofuel is being researched. The cellulose body wall can be broken down and converted into ethanol, 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. Some tunicates are used as model organisms. Ciona intestinalis and Ciona savignyi have been used for developmental studies. Both species' mitochondrial and nuclear genomes have been sequenced. The nuclear genome of the appendicularian Oikopleura dioica appears to be one of the smallest among metazoans and this species has been used to study gene regulation and the evolution and development of chordates. ==See also==