Urochordates

Urochordates, commonly known as tunicates, are a subphylum of the phylum Chordata. The group includes three main classes: Ascidiacea (sea squirts), Thaliacea (salps and doliolids), and Appendicularia (larvaceans). They are exclusively marine and occupy a range of habitats from coastal to open ocean.

Adults of most tunicates are sac-like and enclosed in a tough, gelatinous tunic made of tunicin, a cellulose-like substance. They feed by filtering seawater through a perforated pharyngeal basket with numerous gill slits, drawing water in through an oral (incurrent) siphon and expelling it through an atrial (excurrent) siphon. In many species, the characteristic chordate features are reduced in the adult; the pharynx and its slits form the basis of feeding, while the notochord and dorsal nerve cord are present only in the larval stage of ascidians and are retained in the adult body plan of the larvaceans.

Reproduction is often sexual, with many species being hermaphroditic and releasing sperm and eggs into the water. Some ascidians reproduce asexually by budding, and several thaliaceans form colonies. The larval stage in ascidians is free-swimming and tadpole-like, bearing a notochord and dorsal nerve cord before metamorphosis into a sessile adult.

Ecologically, tunicates are important filter feeders in marine ecosystems and can influence nutrient cycling and water clarity. Some pelagic thaliaceans contribute significantly to planktonic populations, while colonial ascidians can dominate local substrates.

Evolutionarily, tunicates are of particular interest because their larval stages display chordate characteristics. Molecular data generally place tunicates as the closest living relatives of vertebrates, forming a clade with vertebrates, though the exact relationships within Chordata continue to be refined. The fossil record of urochordates is sparse due to their soft bodies.