Phallusia

Phallusia is a genus of solitary, sessile marine tunicates in the family Ascidiidae, within the subphylum Tunicata. Members are commonly referred to as sea squirts. They inhabit shallow coastal waters and are found on rocks, shells, and artificial substrates in temperate and tropical seas. Like other ascidians, adults have a tough outer tunic made of a cellulose-like polysaccharide called tunicin, enclosing a sac-like body with incurrent and excurrent siphons, a pharyngeal basket with slits, and a stomach and gonads.

Phallusia species are filter feeders that draw seawater through the incurrent siphon, trap food particles in the pharyngeal basket, and expel water via the excurrent siphon. Most species are hermaphroditic; fertilization occurs in the water column, and tadpole-like larvae disperse before settling as sessile adults. The larval stage bears chordate features, such as a notochord and dorsal nerve cord, albeit transiently.

One well-studied representative is Phallusia mammillata, widely used in developmental biology because its embryos are accessible and develop rapidly. Researchers employ it to study early chordate development and cell lineage, often using labeling and gene expression analyses in transparent larvae.

Taxonomy and distribution details vary by species; the genus comprises several described species distributed in various marine habitats. Phallusia, like other tunicates, is an important model for understanding the evolution of chordates and the diversity of life-history strategies among sessile marine invertebrates.