Temporal range: Cambrian "Stage 3"–recent
|Sea tulips (Pyura spinifera)|
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata which is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata and the term urochordates is still sometimes used for these animals. Some tunicates live as solitary individuals but others replicate by budding and become colonies, each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Most adult tunicates are sessile and are permanently attached to rocks or other hard surfaces on the ocean floor; others such as salps, doliolids and pyrosomes swim in the pelagic zone of the sea as adults. Various species are commonly known as sea squirts, sea pork, sea liver or sea tulips.
The Tunicata first appear in the fossil record in the early Cambrian period. Despite their simple appearance and very different adult form, their close relationship to the vertebrates is shown by the fact that during their mobile larval stage, they possess a notochord or stiffening rod and resemble a tadpole. Their name derives from their unique outer covering or "tunic" which is formed from proteins and carbohydrates and acts as an exoskeleton. In some species it is thin, translucent and gelatinous while in others it is thick, tough and stiff.
There are about 2,150 species of tunicate in the world's oceans, mostly living in shallow water. The most numerous group are the ascidians and fewer than 100 species of these are found at depths greater than 200 metres (660 ft). Some are solitary animals leading a sessile existence attached to the seabed, but others are colonial and a few are pelagic. Some are supported by a stalk but most are attached directly to the substrate, which may be a rock, shell, coral, seaweed, mangrove root, dock, piling or ship's hull. They come 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 a metre (yard) tall.
The Tunicata was established by Jean-Baptiste Lamarck in 1816. In 1881, Francis Maitland Balfour introduced a second name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates. No doubt largely because of his influence, various authors supported the term, either as such, or as "Urochordata", but the usage is invalid because "Tunicata" has precedence and there never were grounds for superseding the name. 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 and by ITIS, 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.
Tunicata contains about 2,150 described species, traditionally divided into the following classes:
- Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia)
- Thaliacea (Pyrosomida, Doliolida, and Salpida)
- Appendicularia (Larvacea)
Members of the Sorberacea were included in Ascidiacea in 2011 as a result of rDNA sequencing studies. Although the traditional classification is provisionally accepted, newer evidence suggests that the Ascidiacea is an artificial group of paraphyletic status.
Sea squirts have been involved in a controversy about the extent to which cross-species gene transfer and hybridization have influenced animal evolution. In 1990, Donald I. Williamson of the University of Liverpool (U.K.) fertilised sea squirt (Ascidia mentula) eggs with sea urchin (Echinus esculentus) sperm resulting in fertile adults that resembled urchins, but Michael W. Hart of Simon Fraser University failed to find sea-squirt DNA in tissue samples from the supposed hybrids. Williamson claims to have repeated the experiment with sea urchin eggs and sea squirt sperm, producing sea urchin larvae which developed into squirt-like juveniles. These findings have been disputed. A multi-taxon molecular study in 2010 suggested that sea squirts are descended from a hybrid between a chordate and a likely extinct protostome ancestor which took place at a time before the diversification of nematodes and arthropods. This study also examined whether there was any evidence of a sea urchin and tunicate hybridization event that could possibly explain the distribution of genes in modern sea squirts. None could be found.
Undisputed fossils of tunicates are rare. The best known (and earliest) is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming (South China). There is also a common bioimmuration, (Catellocaula vallata), of a possible tunicate found in Upper Ordovician bryozoan skeletons of the upper midwestern United States.
There are also two enigmatic species from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia and a second new Ausia-like genus from the Onega Peninsula, White Sea of northern Russia. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians. These two organisms lived in the shallow waters of a sea, slightly more than 555–548 million years ago and are believed to be the oldest evidence of the chordate lineage of metazoans.
A Precambrian fossil known as Yarnemia is thought to be a tunicate however this has been disputed. 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 for sponge spicules.
Colonies of tunicates come in a range of forms and 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 grows across 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.
By far the largest class of tunicates is the Ascidiacea. The body of an ascidiacean is surrounded by a test or tunic, from which the subphylum derives 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 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. This is composed of connective tissue, muscle fibres, blood vessels and nerves. There are two openings 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 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.
Thaliacea, 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. The pyrosomes are bioluminous colonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. There are about ten species, and all are found in the tropics. The twenty three species of doliolids are small, mostly under 2 centimetres (0.8 in) long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies and swim by jet propulsion. The forty species of salps are also small, under 4 centimetres (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.
A third class, Larvacea (or Appendicularia), is the only group of tunicates to retain its chordate characteristics in the adult state. There are seventy species of larvacean which superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. The notochord is retained and the animals, mostly under a centimetre 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.
Physiology and internal anatomy
Like other chordates, tunicates have a notochord during their early development, but by the time they have completed their larval stages they have lost all myomeric segmentation throughout the body. As members of the Chordata they are true Coelomata with endoderm, ectoderm and mesoderm, but they do not develop very clear coelomic body-cavities if any at all. Whether they do or not, by the end of their larval development all that remain are the pericardial, renal, and gonadal cavities of the adults. Except for the heart, gonads, and pharynx (or branchial sac), the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole. A minority of species, those in the Larvacea, retain the general larval form throughout life, but most Tunicata very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The Thaliacea however, are pelagic throughout their lives and may have complex life cycles.
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 there are 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.
Tunicate blood has some unusual features. In some species of Ascidiidae and Perophoridae it contains high concentrations of the transitional metal vanadium and vanadium-associated proteins in vacuoles in blood cells known as vanadocytes. 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 a pH of less than 2 for stability and this is achieved by the vacuoles also containing sulphuric acid. The vanadocytes are later deposited just below the outer surface of the tunic where it is thought that their presence deters predation. Other species of tunicate concentrate lithium, iron, niobium and tantalum which may serve a similar function.
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 but 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 lifetime.
Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of cellulose. The production in animals of cellulose is so unusual that at first some researchers denied its presence outside plants, but for some time now it has been accepted that it occurs in the dermis of mammals. However, the tunicates are unique in their scale of application and production of the material, and possibly in having a functional cellulose synthesising enzyme. 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 now is recognised as cellulose rather than any alternative substance.
Nearly all tunicates are suspension feeders, 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. In order to obtain enough food, an average ascidian needs to process one body-volume of water per second. 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 including bacteria. The net is rolled up on the dorsal side of the pharynx and it and the trapped particles are drawn into the oesophagus. The gut is U-shaped and also ciliated in order to move the contents along. The stomach is an enlarged region at the lowest part of the U-bend. Here digestive enzymes are secreted and a pyloric gland 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 deep water 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. It is assumed that excess photosynthetic products are available to the host.
Ascidians are almost all hermaphrodites and 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 the larvae are planktonic. In others, especially colonial species, sperm is released into the water and drawn into the atria of other individuals with the incoming water current. Fertilisation takes place here and the eggs are brooded through their early developmental stages. Some larval forms appear very much like primitive chordates with a notochord (stiffening rod) and superficially resemble small tadpoles. These swim by undulations of the tail and may have a simple eye, an ocellus, and a balancing organ, a statolith.
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, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body during metamorphosis, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree. Colonial forms also increase the size of the colony by budding off new individuals to share the same tunic.
Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Sexual reproduction starts within a zooid with an internally fertilised 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.
Doliolids have 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 hermaphrodite with the eggs being fertilised by sperm from another individual. The gonozooid is viviparous and at first the developing embryo feeds on its yolk sac before being released into the sea as a free-swimming, tadpole-like larva. This undergoes metamorphosis in the water column into an oozooid. This is known as a "nurse" as it develops a tail of zooids produced by budding asexually. Some of these are known as trophozooids, have a nutritional function and are arranged in lateral rows. Others are phorozooids, 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 life cycle allows the doliolid to multiply very rapidly when conditions are favourable.
Salps also have a complex life cycle with an alternation of generations. In the solitary life history phase, an oozoid reproduces asexually, producing a chain of tens or hundreds of individual zooids by budding along the length of a stolon. The chain of salps is the aggregate portion of the life cycle. The aggregate individuals are known as blastozooids and remain attached together while swimming and feeding and growing larger. The blastozooids are sequential 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.
Larvaceans only reproduce sexually. They are protandrous hermaphrodites, except for Oikopleura dioica which is gonochoric, 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 ninety degrees 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 it only takes seven hours for a zygote to develop into a house-building juvenile starting to feed.
During embryonic development, tunicates exhibit determinate cleavage, where the fate of the cells is set early on with reduced cell numbers and genomes that are rapidly evolving. In contrast, the amphioxus and vertebrates show cell determination relatively late in development and cell cleavage is indeterminate. The genome evolution of amphioxus and vertebrates is also relatively slow.
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 6.5 square miles (17 km2) area of the seabed on the Georges Bank off the north east 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. Current research indicates that 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 to aquaculture operations.
Use by humans
Tunicates contain a host of potentially useful chemical compounds, including:
- Didemnins, effective against various types of cancer, as antivirals and immunosuppressants
- Aplidine, effective against various types of cancer
- Trabectedin, effective against various types of cancer
In the May 2007 issue of the FASEB Journal, researchers from Stanford University showed that tunicates can correct abnormalities over a series of generations, and they suggest that 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 regenerate compromised human organs. Gerald Weissman, editor-in-chief of the journal, said "This study is a landmark in regenerative medicine; the Stanford group has accomplished the biological equivalent of turning a sow's ear into a silk purse and back again."
Various Ascidiacea species are consumed as food around the world. In Japan and Korea, the sea pineapple (Halocynthia roretzi) is the main species eaten. There is an aquaculture industry where it is cultivated on dangling cords made of palm fronds. In 1994, over 42,000 tons were produced but since then there have been problems with mass mortality events and only 4,500 tons were produced in 2004.
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. The researchers believe that culturing tunicates on a large scale is possible and that the economics of doing so are attractive. They believe that, 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. The species 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.
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