AskDefine | Define tunicate

Dictionary Definition

tunicate n : primitive marine animal having a saclike unsegmented body and a urochord (a notochord) conspicuous in the larva [syn: urochordate, urochord]

User Contributed Dictionary



  1. Any of very many chordate marine animals, of the subphylum Tunicata or Urochordata, including the sea squirts


  1. Of or pertaining to these animals.
  2. Enclosed in a tunic

Extensive Definition

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.

Life cycle

Most tunicates are hermaphrodites. The eggs are kept inside their body until they hatch, while sperm 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.


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.


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:
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:
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."


  • Dennis. 2003. pers. comm. Marine Science Dept. Orange Coast College.
  • Solomon, E., L. Berg, D. Martin. 2002. Biology. Brooks/Cole.

External links

tunicate in Belarusian (Tarashkevitsa): Абалоньнікі
tunicate in Catalan: Urocordat
tunicate in Czech: Pláštěnci
tunicate in German: Manteltiere
tunicate in Spanish: Urochordata
tunicate in Esperanto: Tunikulo
tunicate in French: Urochordata
tunicate in Irish: Urochordata
tunicate in Korean: 피낭동물
tunicate in Croatian: Plaštenjaci
tunicate in Italian: Tunicata
tunicate in Hebrew: מיתרני זנב
tunicate in Georgian: გარსიანები
tunicate in Latvian: Tunikāti
tunicate in Lithuanian: Gaubtagyviai
tunicate in Hungarian: Előgerinchúrosok
tunicate in Macedonian: Туникати
tunicate in Dutch: Manteldieren
tunicate in Japanese: 尾索動物
tunicate in Norwegian: Kappedyr
tunicate in Occitan (post 1500): Urochordata
tunicate in Polish: Osłonice
tunicate in Portuguese: Urochordata
tunicate in Russian: Оболочники
tunicate in Slovak: Plášťovce (taxón)
tunicate in Slovenian: Plaščarji
tunicate in Serbian: Плашташи
tunicate in Finnish: Vaippaeläimet
tunicate in Swedish: Manteldjur
tunicate in Turkish: Tulumlular
tunicate in Ukrainian: Покривники
tunicate in Chinese: 尾索动物亚门
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