TUNICATE

also called Urochordate, any member of the subphylum Tunicata (Urochordata) of the phylum Chordata. Small marine animals found in great numbers throughout the seas of the world, they are either sessile (permanently attached to the seafloor or other surfaces) or pelagic (floating). Tunicates show their chordate relationship in the possession of a muscular tail, gill slits, a notochord, and a nerve cord in the larval stage, which are resorbed at metamorphosis. Their name derives from their characteristic tunic, a secreted protective covering containing cellulose, a glucose polysaccharide not normally found in animals. Tunicates include more than 2,000 species grouped in three classes: Ascidiacea, Thaliacea, and Appendicularia. The Ascidiacea, or sea squirts, are sessile; the Thaliacea are pelagic and often form colonies; and the Appendicularia are free-swimming. The sea squirts attach themselves to the bottoms of ships and the sides of pilings, to seaweed, coral reefs and shellfish, as well as the seafloor. They range in size from a few millimetres (a fraction of an inch) to 30 cm (1 foot). The Thaliacea usually form floating colonies that may reach 4 m (13 feet) in length. The Appendicularia are often too small for the human eye to see unaided. Tunicates typically have two apertures, or siphons, in the tunic. Water, carrying food and oxygen, enters the body through the branchial aperture; it is filtered and passes out through the atrial aperture. In colonial species of ascidians, the individuals, called zooids, retain individual branchial apertures, but they may share a common atrial aperture or retain individual ones, depending on the structure of the colony. Some colonies of Thaliacea assume a thimble shape; water discharged into the thimble from the atrial apertures serves to move the colony forward. Most tunicates are hermaphroditic, but asexual reproduction also takes place in colonial forms. In cooler regions spawning takes place in spring and summer, while in the tropics it occurs year-round. An embryo may develop into a larva in just a few hours. The larvae have notochords and nerve cords, as well as muscular tails twice as long as their bodies. The sessile forms are equipped with sticky protuberances, called papillae, by which they fasten themselves to a surface at the end of the larval stage, which may range from six hours to several days. After attachment, the body resorbs the tail, using it as a food supply during metamorphosis into its adult form. Pelagic species achieve metamorphosis, which takes only a short time, without attaching themselves. Larvae do not feed during their extremely brief life; adults feed on microorganisms. Asexual reproduction in colonial forms is effected through budding, that is, by the formation of new individuals by the development of outgrowths on the parent, which then break off. Budding may be achieved by the growth of rootlike stolons on the posterior end of the zooid, or of buds on the pharynx or on the body wall. In some pelagic groups sexually differentiated individuals (gonozooids) produce a larva that becomes an oozoid, or asexually reproducing form, which in turn produces another gonozooid generation. The tunic, also called the test, consists of cellulose. It is composed of living tissues sustained by blood vessels. Within the tunic is the muscular body wall, which controls the opening of the siphons and encloses the pharynx and the atrial cavity. The pharynx, a sac into which the branchial siphon opens, is separated from the body wall by the atrial cavity, which leads to the outside through the atrial siphon. The branchial sac is perforated; each of the many perforations (stigmas) has small, hairlike processes (cilia) that drive water into the body. The branchial sac excretes mucus, to which food adheres before it enters the esophagus, where it is carried to the digestive tract. Waste is passed from the anus into the atrial cavity near the atrial aperture, from which it is discharged with the water passing out of the body. The openings of the genital ducts are situated near the anus. In the simpler forms of ascidians, the viscera are enclosed by membranes called epicardia. In more advanced species, the epicardia assume other functions such as storage. At the end of every 50 to 100 beats the tunicate heart reverses the flow of blood; it stops and then resumes functioning in the opposite direction. The blood includes several different types of cells. Tunicates are important links in food chains. They become a nuisance when they grow on ships' hulls, but some species are useful in making pharmaceuticals. Because tunicates are soft-bodied, there is no fossil record of early tunicate forms. also called urochordate, any member of the subphylum Tunicata of the phylum Chordata. Small marine animals, they are found in great numbers throughout the seas of the world. Adult members are commonly embedded in a tough secreted tunic containing cellulose (a glucose polysaccharide not normally found in animals). The less modified forms are benthic (bottom-dwelling and sessile), while the more advanced forms are pelagic (floating and swimming in open water). A characteristic tadpole larva develops in the life cycle, and in one group the adult closely resembles this larva, which has many features in common with other chordates. Michael T. Ghiselin Additional reading E.J.W. Barrington, The Biology of Hemichordata and Protochordata (1965), is an account of the lower chordates and their evolution; N.J. Berrill, The Origin of Vertebrates (1955), argues the thesis that the urochordate larva represents the prototype from which cephalochordates and vertebrates are derived; A. Willey, Amphioxus and the Ancestry of the Vertebrates (1894), an early but good comprehensive account, presents the orthodox theory of chordate relationships; R.P.S. Jefferies, The Ancestry of the Vertebrates (1986), expounds an alternate theory of chordate origin; and Libbie H. Hyman, The Invertebrates, vol. 5, Smaller Coelomate Groups (1959), is a classic work treating the hemichordates in extensive detail. Later works include Charles K. Weichert and William Presch, Elements of Chordate Anatomy, 4th ed. (1975); and R. McNeill Alexander, The Chordates, 2nd ed. (1981); supplemented by Brian Bracegirdle and Patricia H. Miles, An Atlas of Chordate Structure (1978). N.J. Berrill, The Tunicata with an Account of the British Species (1950, reprinted 1968), a taxonomic survey with a useful section on tunicate biology; Pierre P. Grasse (ed.), Trait de zoologie: anatomie, systmatique, biologie, vol. 11, Echinodermes, stomocords, procords (1966), an advanced zoological treatise devoted to protochordates, with good illustrations; W.A. Herdman, Tunicata (Ascidians and Their Allies) and Cephalochordata, in The Cambridge Natural History, vol. 7, pp. 35138 (1904), an important general account; R.N. Millar, The Marine Fauna of New Zealand: Ascidiacea (1982), a morphological account of a single species of ascidian; Willard G. Van Name, The North and South American Ascidians, Bulletin of the American Museum of Natural History, vol. 84 (1945). For a later treatment, see Invertebrate Chordates: Tunicates and Lancelets, in Vicki Pearse et al., Living Invertebrates (1987). Michael T. Ghiselin