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.