Ascidians live exclusively in marine habitats, and most families are represented at all latitudes. They occupy sandy, rocky, and biotic substrata and are notable for their presence as fouling organisms on boats, jetties, and oil rigs. Many species vary seasonally in abundance and at the peak of their growth period may occupy all available ground cover. Simple ascidians may occur as abutting clumps, but the key feature is that each has originated from a separate larva. Individuals of some species grow to be many centimetres long.
Individuals thus formed are clones and together form a colony.
The test is usually thin and delicate and may be encr usted lightly with sand. Individuals are usually less than a centimetre or two in the longest dimension, but colonies may occupy several square metres on rocky, vertical, subtidal walls. The individuals may or may not be visible from the exterior of the colony and are generally less than a centimetre long. Colonies of compound ascidian s pecies occur as gelatinous footballs, thin leathery sheets, small knobs or cushions, or sprawling fleshy masses. Many species are heavily impregnated and encrusted with sand, a characteristic that may make them difficult to identify initially as ascidians.