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Tunicata (Urochordata)

Contributed by: Ivan Goodbody Publication year: 2014

A subphylum of marine animals within the phylum Chordata. Members of the Tunicata (also referred to as Urochordata) are characterized by a perforated pharynx or branchial sac used for food collection, a dorsal notochord restricted to the tail of the larva (and the adult in one class), absence of mesodermal segmentation or a recognizable coelom, and secretion of an outer covering (the test or tunic) that contains large amounts of polysaccharides related to cellulose (see illustration). See also: CHORDATA.

Four classes of tunicates are usually recognized: the sessile Ascidiacea (sea squirts or ascidians); the planktonic Thaliacea (salps, doliolids, and pyrosomids); Appendicularia (Larvacea), minute planktonic forms (with tails) living inside a specialized test or house that is adapted for filtering and food gathering; and Sorbacea, a small group of abyssal carnivorous tunicates. More than 3000 species of Tunicata are recognizable. The group is found in all parts of the oceans. See also: APPENDICULARIA (LARVACEA); ASCIDIACEA; THALIACEA; ZOOPLANKTON.

Except for the few deep-sea carnivorous forms, tunicates feed on minute plankton and finely divided organic detritus. Food is drawn into the pharynx in water currents created by ciliary or muscular activity and filtered on a mucous sheet that is secreted in the floor of the pharynx and rolled into a cord on the dorsal side. The food cord is passed into the esophagus and from there to the rest of the alimentary canal. Digestion is extracellular; feces are discharged into the outgoing current of water. There are no excretory organs, and most nitrogenous wastes are removed in soluble form. Concentrations of urates may also accumulate in closed vesicles throughout life. The heart is tubular and reverses the direction of its beat at intervals, alternately driving blood in opposite directions through the body organs and through the pharyngeal wall. Heavy metals, usually vanadium or iron, accumulate in specialized cells in the blood; these are probably concerned with test formation or chemical defense and are not respiratory carriers. The nervous system is simple and consists of a solid dorsal ganglion and a few peripheral nerves. The ganglion is closely associated with a neural gland that discharges into the entrance of the pharynx.

Most tunicates are hermaphroditic and may be oviparous or viviparous. The larva is a minute [approximately 1 mm (0.04 in.)] tailed tadpole, with a notochord, dorsal nerve cord, and sense organs. Larvae are free swimming for a short period; except in Appendicularia, they lose the tail and notochord at metamorphosis into an adult. Ascidians may be solitary or form colonies. Thaliaceans form aggregates or colonies by asexual budding and may alternate between solitary and aggregate forms. Appendicularians are always solitary. AccessScience from McGraw-Hill Education Page 2 of 3 www.accessscience.com

WIDTH:DRepresentative members of Tunicata. (a) Botryllus, a compound ascidian; (b) Doliolum, a planktonic thaliacean (after P. A. Meglitsch, Invertebrate Zoology, Oxford University Press, New York, 1967); (c) Oikopleura,a planktonic larvacean, removed from its house (after A. Alldredge, Appendicularians, Sci Amer., 235:94–102, 1976).

Tunicates have little economic importance, except as fouling organisms. A few species have pharmacological properties, and a few larger ascidians are used for food. Ivan Goodbody

Keywords

Tunicata; Urochordata; tunicates; pharynx; branchial sac; Ascidiacea; Thaliacea; Appendicularia (Larvacea); Sorbacea AccessScience from McGraw-Hill Education Page 3 of 3 www.accessscience.com

Bibliography

Q. Bone (ed.), The Biology of Pelagic Tunicates, Oxford University Press, Oxford∕New York, 1998

F. Delsuc et al., Tunicates and not cephalochordates are the closest living relatives of vertebrates, Nature, 439:965–968, 2006 DOI: http://doi.org/10.1038/nature04336

T. Stach and J. M. Turbeville, Phylogeny of Tunicata inferred from molecular and morphological characters, Mol. Phylogenet. Evol., 25(3):408–428, 2002 DOI: http://doi.org/10.1016/S1055-7903(02)00305-6

G. Tsagkogeorga et al., An updated 18S rRNA phylogeny of tunicates based on mixture and secondary structure models, BMC Evol. Biol., 9:187, 2009 DOI: http://doi.org/10.1186/1471-2148-9-187

Additional Readings

L. L. Cole, Diversity and distribution of Tunicata (Urochordata) in Tobago, J. Life Sci., 6(2):221–232, 2012

D. L. Felder, Gulf of Mexico Origin, Waters, and Biota: Biodiversity, Texas A&M University Press, College Station, TX, 2009

O. Giere, Meiobenthology: The Microscopic Motile Fauna of Aquatic Sediments, 2d ed., Springer-Verlag, Berlin, Germany, 2009

Tunicata

Tunicates Extraordinaire