Ascidian habits and interactions with the environment

Both colonial and solitary ascidians can be compared to the occupants of a house without doors that do all

Figure 27.3 Aspects of ascidian morphology (semidiagrammatic). A-B, phlebobranch ascidian: A, from left side; B, oblique section through atrial aperture and gonads. C, inner wall of branchial sac showing: q, transverse vessel; r, para-stigmatic vessel; s1-3 (secondary, bifid and simple, respectively); v, stigmata. D, ascidian larva showing: a, excurrent aperture; b, incurrent aperture; j 1-3 oesophagus, stomach and rectum, respectively; l, trunk epithelium; o, endostyle; u, larval haemocoele; v, stigmata; w 1-2, ocellus and otolith, respectively; x, adhesive organ; y, larval tail; z, larval test. (Figure: Kott 1993.)

Figure 27.3 Aspects of ascidian morphology (semidiagrammatic). A-B, phlebobranch ascidian: A, from left side; B, oblique section through atrial aperture and gonads. C, inner wall of branchial sac showing: q, transverse vessel; r, para-stigmatic vessel; s1-3 (secondary, bifid and simple, respectively); v, stigmata. D, ascidian larva showing: a, excurrent aperture; b, incurrent aperture; j 1-3 oesophagus, stomach and rectum, respectively; l, trunk epithelium; o, endostyle; u, larval haemocoele; v, stigmata; w 1-2, ocellus and otolith, respectively; x, adhesive organ; y, larval tail; z, larval test. (Figure: Kott 1993.)

their business with the outside world that flows past their house through a front and a back window. Adult ascidians are fixed to a substrate, or are rooted in, or lie immobile on, sea floor sediments. Each individual has an incurrent and excurrent opening. Colonial zooids sometimes (like solitary species) have both apertures opening separately to the exterior and are either joined to one another by strands of test or are partially or completely embedded in common test. More integrated colonies with embedded zooids have excurrent openings into common cloacal cavities or canals inside the colonial test. In these integrated colonies, individuals are most isolated from the exterior, their incurrent (branchial) apertures being the only zooid openings with direct access to the external environment.

Ascidians squirt water out of the pharynx as the muscular body contracts, hence their common name 'sea squirt'. This activity has been found to be rhythmic in some species, probably ensuring that the branchial cavity is cleaned and irrigated. Also, in a response known as the 'cross reflex', stimulation of the inside of either aperture (to emulate an intrusive organism or irritant) will cause the other to close while water is ejected to wash away the intruder. However, in life, both solitary and colonial ascidians are almost continuously engaged in filtering water. Although cryptic and often obsured by epibionts, they can be detected by their open apertures. Pharyngeal perforations, the anus, and gonoducts all open into the peri-branchial cavity and faeces and either gametes or larvae are released in the excurrent stream of filtered water propelled away by the positive pressure built up inside the organism. Thus, the excurent water is separated from, and does not pollute, the incurrent water. Pressure gradients in individuals with both openings direct to the exterior are maintained by the smaller diameter of the separate excurrent openings relative to the incurrent one.

In species with zooids arranged around common cloacal cavities, some of the internal pressure is transferred from the zooid into the common cloacal spaces in the colony. This will be affected by the number and size of the common cloacal openings in relation to the number of zooids and the sum of their individual ex-current flows into the common cloacal cavity. Sometimes separately opening zooids are arranged in circular rudimentary systems with excurrent openings crowded in the centre of the circle of zooids. Excurrent flows from each zooid combine into a strong current ejected away from the incurrent apertures spread around the periphery of the circle. Some of the advantages of true cloacal systems in reinforcing the separation of incurrent and excurrent water thus are available to less integrated colonies. This tends to support the view that, at the depths so far explored, advantages of a colonial habit include the isolation of incurrent and spent water. This is additional to the usual advantages associated with coloniality, viz. insulation of zooids from the environment, internal fertilisation, maintenance of populations by incubation of embryos inside the colony and flexibility in the growth form of colonies compared with solitary organisms. Internal pressure gradients in living solitary individuals as well as in colonies are the principle means by which they maintain their shape.

Colonial ascidian species are about five times as numerous as solitary ones in both temperate and tropical waters, and colonial species with well integrated cloacal systems are more numerous in the tropics than in temperate waters. It is possible that the best integrated colonies have some particular advantage in tropical habitats. These include:

• rapid two-dimensional growth to occupy space and internal incubation of embryos to maintain populations (ascidian colonies grow as a result of replication of zooids and colonies with the most prolific replication tend to have the greatest rate of growth); and

• a decrease in zooid size as a result of the interruption to growth of individuals during replication.

These effects are conspicuous in the family Didem-nidae, where prolific replication and zooid size reduction are at their greatest and colony growth is rapid and two dimensional, covering expanses of substrate and excluding other sedentary organisms.

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