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The Seashore

Star Ascidians

Star Ascidians

Star Ascidians are a compound (colonial) form of Sea Squirt. These encrust on to rocks and seaweeds. They are fairly rigid gelatinous lumps, flattened on to the substrate in sometimes large lumps of 10 cm or more. "Set" into this gelatinous lump are coloured individuals, sometimes in a star-shape like Botryllus (above) or in rows, like Botrylloides. These individuals are only a millimetre or two wide and cluster around a single exhalent siphon.

What makes the Star Ascidians (Sea Squirts) particularly fascinating is the fact that they are closely related to the true Chordates containing fish and mammals. The link is in the presence of a notochord but they lack the backbone. Looking at the sessile, tiny star-like individuals on a shore it is difficult to see the link but the larvae are more like tadpoles. They have a muscular tail for swimming and use the notochord for support. When they land on the beach they settle head first on to the substrate then metamorphose into the adult, by absorbing the tail. The larva is mobile and chordate-like. The adult colony has siphons to draw water in and out by the action of cilia beating, producing a current of water. From this stream of water oxygen is extracted by the gills. Plankton and detritus is filtered from the water and passed to the mouth. They are hermaphrodite, having both male and female organs. The sperm and eggs are released through the siphons. They are eaten by cowrie shells.

Star Ascidians

Star Ascidians are found throughout the European shores including the Mediterranean. They encrust on to the rocks and plants of the lower shore and can be very abundant.

See the Sea Squirts - the Ascidians

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