Sea squirtsAscidiacea

Sea squirts are sessile marine invertebrate filter feeders, sac-like in form. They vary from individual sacs (solitary species) to clustered communities (social ascidians), or large encrusting colonies formed by numerous small individuals, called zooids, embedded in a common tunic (compound ascidians). Sizes range from a few millimetres to over 30 cm (1 ft) across. While many species are drab, others display vivid colours such as reds, yellows, blues, and purples.

Sea squirts firmly attach themselves to substrates like rocks, coral rubble, and artificial structures across all reef zones, from the intertidal to depths of at least 60 m (200 ft). They feed by drawing in seawater through an oral siphon and expelling filtered water through an atrial siphon, extracting phytoplankton and suspended particulate matter. When disturbed, many species rapidly contract, expelling jets of water from their siphons. They are commonly encountered by divers.