Ascidians, Ascidies (French), Ascídia (Portuguese), Sessile tunicates, Tuniqués sessiles (French)
Ascidians are sessile marine invertebrates, occurring throughout the world, usually in shallow waters and attached to hard surfaces. The family includes over 2000 described species, many of which are colonial -- along with some of the Thaliacea, the only colonial chordates. When disturbed, ascidians often forcibly eject water from their siphons -- the reason for their common name, "sea squirts."
The epidermis of Ascidians is covered by a characteristic tissue called the “tunic,” which is typically thick and fibrous but in some colonial species has the consistency of jelly. The tunic is composed primarily of proteins and carbohydrates as well as “tunicin,” a cellulose that forms sheets of parallel structural fibers. These sheets are often layered like plywood, with fibers oriented in different directions to produce a strong and tough outer layer. The tunic’s functions include support, protection, and attachment of the animal to the substratum. For additional protection and support, some species secrete calcareous plates or spicules within the tunic, while others incorporate sand or even their own feces. Unlike other animals with thick exoskeletons, ascidians do not molt as they grow. Instead the tunic grows along with the growing zoid.
Ascidians vary in size from a few millimeters to over a meter in length.
Sea squirts may have up to three general body regions, the thorax, abdomen, and post-abdomen. The abdomenal and post-abdomenal regions can form a long stalk, holding the thorax, with the siphons and pharynx, above/away from the surface to which the animal is attached. But in many species, the abdomen and post-abdomen are absent, and all the organs are contained within the thorax. (Ruppert et al 2004)
"Unlike the adult, the tadpole larvae expresses all the key chordate traits, but it differs from the ancestral chordate in that it does not feed" (Ruppert et al 2004). Adults lack the notochord, dorsal hollow nerve cord, and postanal tail.
Ascidians are sessile animals, usually occurring in waters < 200 m deep and attaching to hard substrates, either natural or artificial, though some live on soft sediments and a few species have been found in the interstices of sandy beaches (Ruppert et al 2004).
Many species are highly invasive, spreading rapidly and smothering or out-competing other benthic organisms.
“Sea squirts” are common marine invertebrates with a worldwide distribution (Ruppert et al 2004).
Most ascidians are suspension or deposit feeders, filtering plankton from the water or small organisms from sediments. However, there are deep sea species that capture fish or other larger prey Millar 1970) and shallow water species with mutualist algae within their tissues that provide photosynthate (Hirose et al 1999). Some solitary species cannibalize conspecific eggs and tadpoles (Young 1988).
Predators vary depending on life-stage. Ascidian larvae are eaten by other pelagic organisims, including fish and jellyfish. Among the predators of the sessile adults are fish, gastropods, flatworms, and birds. Many ascidians produce secondary metabolites or organic acids, sequestered in their tunics, which serve as a defense against predation (Pisut & Pawlik 2002).
Most ascidians are hermaphroditic and typically cross-fertilize, but some can self (Ruppert et al 2004).