tunicate

[too-ni-kit, -keyt, tyoo-]

tunicate, marine animal of the phylum Chordata, which also includes the vertebrates. The adult form of most tunicates (also called urochordates) shows no resemblance to vertebrate animals, but such a resemblance is evident in the larva. The most familiar tunicates are the sea squirts, or ascidians (class Ascidiacea). Adult sea squirts are sedentary, filter-feeding, cylindrical or globular animals, usually found attached to rocks, shells, pilings, or boat bottoms. The soft body is surrounded by a thick test, or tunic, often transparent or translucent and varying in consistency from gelatinous to leathery. The tunic (for which the tunicates are named) is secreted by the body wall of the adult animal. It is composed of cellulose, an almost unique occurrence of that material in the animal kingdom. Two siphons project from the animal's body; water enters the incurrent siphon at the top of the body and leaves the excurrent siphon at the side. Food particles are filtered from the water by the pharynx, which occupies most of the body, and are then passed into the digestive system. Some species reproduce by budding, resulting in the formation of colonies of sea squirts, joined at their bases by slender stalks or embedded in a slab of common tunic material. In addition, nearly all species reproduce sexually and are hermaphroditic. The free-swimming larva, called a tadpole, has a muscular tail and is similar in appearance to a frog tadpole. The larva has the characteristic chordate features also found in the embryos of vertebrates: a dorsal, hollow nerve cord; a stiffening rod, or notochord; and gill slits leading into the pharynx. The tadpole eventually settles and undergoes a drastic metamorphosis into the adult form. A common solitary sea squirt of both coasts of North America is the slender, yellow, transparent Ciona intestinalis, about 2 in. (5 cm) tall. The sea peach, Tethyum pyriforme, is a round, peach-colored sea squirt found from Maine north. Sea grapes are clusters of the greenish colonial squirt, Molgula manhattensis, common from Massachusetts south. Golden stars are colonies of various Botryllus species; the bright yellow individual animals are grouped in starlike clusters in a flat, encrusting, greenish tunic. Amaroucium species form colonies of minute animals embedded in a grayish, gristly tunic; chunks of such colonies, often washed ashore, are known as sea pork. There are two other groups of tunicates, both found in the plankton of open oceans. The salps (class Thaliacea) are barrel-shaped tunicates, open at both ends; they swim by muscular contractions that force water through the body. The larvaceans (class Larvacea) retain the larval form, with a tail and a notochord, as adults. A commonly held theory maintains that vertebrates evolved from animals like the larvaceans. Larvaceans have no tunics, but secrete gelatinous containers, called houses; these are used to filter food from the water and are continuously discarded and replaced. Tunicates are classified in the phylum Chordata, subphylum Urochordata.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

tunicate

Any of some 2,000 species (chordate subphylum Tunicata, or Urochordata) of small marine invertebrates that are abundant worldwide. Tunicates are either sessile (permanently attached), free-swimming, or planktonic (floating). The name tunicate derives from a secreted protective covering (the tunic) containing cellulose. Floating species often form colonies that may be 13 ft (4 m) long. Some free-swimming species are too small to see. Adults filter feed on microorganisms. Sessile forms growing on ships' hulls may be a nuisance, but some species are pharmaceutically useful. Seealso sea squirt.

Learn more about tunicate with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Tunicate

Tunicate, also known as urochordata, tunicata (and by the common names of urochordates, sea squirts, and sea pork) is the subphylum of a group of underwater saclike filter feeders with incurrent and excurrent siphons, that are members of the phylum Chordata. Most tunicates feed by filtering sea water through pharyngeal slits, but some are sub-marine predators such as the Megalodicopia hians. Like other chordates, tunicates have a notochord during their early development, but lack myomeric segmentation throughout the body and tail as adults. Tunicates lack the kidney-like metanephridial organs, and the original coelom body-cavity develops into a pericardial cavity and gonads. Except for the pharynx, heart and gonads, 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 stages that resembles a tadpole, later developing into a barrel-like, sedentary adult form.

A few kinds of tunicates remain in the water column, as pelagic organisms, for their entire lives. These include the salps, doliolids, and pyrosomes.

Life cycle

Most tunicates are hermaphrodites. The eggs are kept inside their body until they hatch, while is released into the water where it fertilizes other individuals when brought in with incoming water.

Some larval forms appear very much like primitive chordates or hemichordates with a notochord (primitive spinal cord). Superficially the larva resemble small tadpoles. Some forms have a calcereous spicule that may be preserved as a fossil. They have appeared from the Jurassic to the present, with one proposed Neoproterozoic form, Yarnemia.

The larval stage ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion previously used to control movement. 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.

Once grown, adults can develop a thick covering, called a tunic, to protect their barrel-shaped bodies from enemies.

Feeding

Tunicates are suspension feeders. They have two openings in their body cavity: an in-current and an ex-current siphon. The in-current siphon is used to intake food and water, and the ex-current siphon expels waste and water. The tunicate's primary food source is plankton. Plankton gets entangled in the mucus secreted from the endostyle. The tunicate's pharynx is covered by miniature hairs called ciliated cells which allow the consumed plankton to pass down through to the esophagus. Their guts are U-shaped, and their anuses empty directly to the outside environment. Tunicates are also the only animals able to create cellulose.

Tunicate blood is particularly interesting. It contains high concentrations of the transition metal vanadium and vanadium-associated proteins. Some Tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.

Classification

Sea squirts are more closely related to fish, birds, and humans than worms, sea stars, or other invertebrates.

The Tunicata contains about 3,000 species, usually divided into the following classes:

Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia)
Thaliacea (Pyrosomida, Doliolida, and Salpida)
Appendicularia (Larvacea)
Sorberacea.

Although the traditional classification is followed for now, newer evidence suggests that the Ascidiacea is an artificial group. The new classification would be:

Stolidobranchia,
Phlebobranchia and Thaliacea,
Aplousobranchia and Appendicularia,
Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own.

The species Ciona intestinalis and Ciona savignyi have attracted interest in biology for developmental studies. Both species' genomes have been sequenced.

Fossil record

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 of a tunicate (Catellocaula vallata) found in Upper Ordovician bryozoan skeletons of the upper midwestern United States.

Invasive species

Over the past few years, urochordates (notably of the genera Didemnum and Styela) have been invading coastal waters in many countries, and are spreading quickly. These mat-like organisms can smother other sea life, have very few natural predators, and are causing much concern. Transportation of invasive tunicates is usually in the ballast water or on the hulls of ships. 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.

Medical uses

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
Esteinascidin 743, 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.