|Solitary sea squirts||Social sea squirts||Compound sea squirts||Pelagic sea squirts|
Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the Urochordata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.
Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to substratum such as rocks and shells.
There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter.
Sea squirts feed by taking in water through the oral siphon. The water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon
Sea squirts are rounded or cylindrical animals ranging from about 0.5 to 10 centimetres in size. One end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of a cellulose-like substance called tunicin along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure.
The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx, while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonads. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly.
As its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth. The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. The beating of the cilia sucks water through the siphon, and then through the stigmata. A long ciliated groove, or endostyle, runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous with the thyroid gland of vertebrates, despite its differing function.
The Southern African coast is a diverse region including a number of different habitats each with a wide variety of fauna and flora and it is estimated that there is over 10 000 species of marine organisms (Branch et al 1994). A number of books, although small is relationship to those published on the better know terrestrial fauna and flora of this biologically rich and diverse tip of Africa, are available on the marine habitats and the organisms found within this unique area
The diversity of fauna en flora present along the southern African coast is a reflection of the different water masses that influence the climate, sea temperature and other factors associated with the different geographical (Branch et al 1994). The narrow continental shelf on the East Coast North of East London bathes the coastal area in warm water brought down by the Agulhas current from the subtropics. As the continental self widens to the south the near shore temperature becomes cooler and more temperate. The west coast is however chilled by northward drifting cold water from the Benguela current. This and the added occurrence of up-welling where cold nutrient rich water from the deep replaces surface water blown off shore set the scene for an coastal region with a higher productivity but lower diversity that the eastern coast (Branch et al 1994).
The Southern African coast can thus be divided into three zones. The highly productive but cold west coast, north of Cape point. The diverse temperate south coast, between Cape point and just north of East London and the more tropical coastal area to the north of East London. The area north of Durban is characterized by coral reefs otherwise not found along the southern African coast.
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