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'Urochordata' (sometimes known as 'tunicata' and commonly called 'urochordates', 'tunicates', 'sea squirts') is the subphylum of saclike filter feeders with incurrent and excurrent siphons. There are also a few exceptions to this plan, like the predatory ''Megalodicopia hians'', looking something like a cross between a jellyfish and a Venus Flytrap. They are members of the phylum Chordata, which also includes lancelets and all vertebrates including humans. They are now considered as the closest relatives to craniates (i.e. hagfish and vertebrates), having dethroned lancelets from that position.^[1] As with other chordates, tunicates possess a notochord during their early stages of development.
They lack segmentation, even in the tail.
Metanephridia are absent. The original coelom is degenerated to 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 a jelly-like matrix known as mesenchyme.
The motile larval stages may have the appearance of a tadpole, whereas the adult stage has a barrel-like, sedentary form. They feed by filtering sea water through a gill basket.

Contents

Description

Invasive species

Chemical derivatives

Regenerative Medicine

References

External links

Description

Most tunicates are hermaphroditic. 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.
Tunicates are suspension feeders. They have two openings in their body cavity: an incurrent and an excurrent siphon. The incurrent siphon is used to intake food and water and the excurrent 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.
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".^[2]
Since the larval form has a nervous system and is capable of active locomotion as opposed to the adult form which seems more primitive in comparison, it perhaps offers a real life example of the famous quote of Samuel Butler that "the hen is an egg's way of producing another egg". Here, the egg is analogous to the adult form, and the hen to the larval form.
Once grown, adults can develop a thick covering, called a ''tunic'', to protect their barrel-shaped bodies from enemies.
Sea squirts are more closely related to fish, birds, and people than worms, sea stars, or other invertebrates.
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.
Tunicate blood is particularly interesting. It contains high concentrations of the rare metal vanadium and vanadium-associated proteins. Some Tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. It is still unknown how they do this or why.
The Tunicata contains about 3,000 species, usually divided into the following classes.

★ Ascidiacea, (Aplousobranchia, Phlebobranchia, and Stolidobranchia)

★ Thaliacea

★ Appendicularia (Larvacea)

★ Sorberacea.
Newer evidence indicates that the Ascidiacea is an artificial group. The new classification would then look like this.

★ Stolidobranchia,

★ Phlebobranchia and Thaliacea,

★ Aplousobranchia and Appendicularia,

★ Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own.
For the moment, the traditional classification is followed.
A species that has attracted interest in biology is Ciona intestinalis which is used for developmental studies.

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.^[3] 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 nuisance in aquaculture operations.^[4][5]

Chemical derivatives

Tunicates have been found to contain a host of potentially significant compounds, among which are:

★ Didemnins (effective against various types of cancer, antivirals, immunosuppressants)

★ Aplidine (effective against various types of cancer)

★ Esteinascidin 743 (effective against various types of cancer)

Regenerative Medicine

In the May 2007 issue of The FASEB Journal, researchers from Stanford University show that the tunicate can correct abnormalities over a series of generations, and they suggest that a similar regenerative process might be possible for humans. Ayelet Voskoboynik of Stanford University and author of the study says "We hope the mechanisms underlying this phenomenon will ultimately lead to new insights regarding the potential of cells and tissues to be reprogrammed and regenerate compromised organs in humans." Gerald Weissman, editor-in-chief of The FASEB 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."^[6]

References

★ Dennis. 2003. pers. comm. Marine Science Dept. Orange Coast College.

★ Solomon, E., L. Berg, D. Martin. 2002. ''Biology''. Brooks/Cole.
1. Error in the Genealogy of Human
2. Brainless Fish in Topless Bar - Fast Company
3. Squirt Alert
4. Marine Nuisance Species
5. Woods Hole Oceanographic Institution
6. Sea Squirt, Heal Thyself: Scientists Make Major Breakthrough In Regenerative Medicine

External links

★ Error in the Genealogy of Human

★ washington.edu page on tunicates

★ Ascidians.com has pictures of species around the world

★ Squirt Alert: Science News Online, [December 24], [2005]

★ Ode to the Cunji, in Macquarie University ''Biology Bytes'' June 1998