Ascidiacea (commonly known as the ascidians or sea squirts) is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide cellulose.
Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea (Appendicularia) swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to their 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 a tube, 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.
Underwater photos at Akrotiri by Kostas Aristeidou