Ascidians

Ascidians, commonly known as sea squirts, are sessile marine invertebrates that belong to the class Ascidiacea within the subphylum Tunicata (urochordates) of the phylum Chordata. They are mostly filter feeders enclosed in a tough outer tunic composed largely of cellulose. Ascidians range from solitary individuals to colonial forms that can form large mats on submerged surfaces.

Anatomy and life cycle: Adults possess two siphons—the incurrent and excurrent—through which water is drawn in and expelled, passing food-laden water through a pharyngeal basket with slits. The free-swimming larva resembles a tadpole and contains features typical of chordates, including a notochord and dorsal nerve cord. Metamorphosis transforms the larva into a sessile adult; the notochord and much of the tail regress. Most ascidians are sexual and hermaphroditic, releasing eggs and sperm into the water; several have short lifespans. Colonial species can reproduce asexually by budding, producing genetically identical zooids.

Habitat and ecology: Ascidians are found in seas worldwide, from the intertidal zone to the deep ocean, on rocks, ships' hulls, and undersea structures. They are important filter feeders, contributing to water clarity and nutrient cycling. Some species are invasive and form dense fouling communities. Predation and microbial symbionts influence their ecology and chemical defenses.

Taxonomy and significance: Ascidians are of interest in evolutionary biology because their larvae possess chordate features that link vertebrates to invertebrates. The model species Ciona intestinalis and Ciona savignyi have been studied extensively to understand development and genomics. Ascidians also produce bioactive compounds with potential pharmaceutical applications, and colonial species can exhibit remarkable regenerative abilities.