The ascidian test

Ascidians are surrounded by an inert acellular coat of tunicin (the test). It endows them with the flexibility to exploit different habits and occupy a diversity of habitats. In addition to affording protection from predators and physical forces, it has adhesive properties that attach the organism to the substrate. Its intrinsic strength and protective qualities are often enhanced by the development of hairs and roots that allow attachment to, or incorporation in, a wide range of sediments. It can also develop stalks to raise individuals or colonies above the sediments and allow them to take advantage of currents that flow by. Tests form a framework for colonies, connect zooids to each other, or form a matrix in which they are embedded and in which embryos are incubated. It can be soft and gelatinous, flexible, firm and rigid, delicate, brittle, or tough and leathery, thick or thin. In several taxa calcareous spicules produced by the ectoderm are embedded in it. These are sometimes so crowded that they form an internal skeletal support for large branching colonies. In most solitary stolido-branch species, the test lining the body openings is armed with microscopic overlapping spines that point outwards and appear to prevent other organisms from intruding into the open apertures. Ectodermal vessels projecting from the body wall synthesise and nourish the test and help to anchor the body to it. Otherwise, the usually muscular ascidian body is attached only by

Figure 27.4 Evolution of aplousobranch ascidian colonial systems. A-C, zooids independently opening. A, partially and B, completely embedded; C, rudimentary systems; D-F, zooids arranged around common cloacal cavities; D, simple circular systems (e.g. Polycitoridae); E-F, extensive common cloacal canals and cavities (gonads in gut loop, e.g. Didemnidae; or in a posterior abdomen, e.g. Polyclinidae). (Figure: Kott 1990.)

Figure 27.4 Evolution of aplousobranch ascidian colonial systems. A-C, zooids independently opening. A, partially and B, completely embedded; C, rudimentary systems; D-F, zooids arranged around common cloacal cavities; D, simple circular systems (e.g. Polycitoridae); E-F, extensive common cloacal canals and cavities (gonads in gut loop, e.g. Didemnidae; or in a posterior abdomen, e.g. Polyclinidae). (Figure: Kott 1990.)

invaginations of the test that line its siphons. The test often shelters other organisms such as crustaceans and molluscs that bore into it. In some didemnid species, symbionts embedded in the test or attached to the lining of common cloacal cavities contribute to the nutrition of their hosts. The test also contributes to ascidian interactions with the environment by differential growth that affects the orientation of apertures, offering incurrent openings to oncoming (food laden) currents and away from falling sediments and from the excur-rent (spent) water from the same or adjacent individuals or colonies.

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