Tunicate

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts or cunji) 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, hagfish and all vertebrates including man. 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.

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

Tunicates 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.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). 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.

Once grown, adults can develop a covering, called a tunic, to protect themselves from enemies. 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.

They are 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.

Invasive species

Over the past few years, sea squirts of the genus Didemnum 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. ^*

References

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

External links

Tunicates

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Invasive species

References

External links