Tunicate (Urochordata): sea squirts, life cycle, diversity and significance

Morphology and feeding

Most tunicates are suspension filter feeders. Water enters through an incurrent siphon, passes through a ciliated pharyngeal or branchial basket where food particles are trapped in mucus produced by the endostyle, and exits through an excurrent siphon. The tunic is an extracellular layer that can be gelatinous or leathery and contains a cellulose-like compound that helps support and protect the animal. Adult anatomy and lifestyle vary: many ascidians attach to rocks and other substrates, while thaliaceans and appendicularians remain planktonic.

Life cycle and reproduction

Tunicates frequently undergo a dramatic metamorphosis. A short-lived tadpole-like larva possesses a notochord and dorsal nerve cord, features linking tunicates to other chordates; these are typically lost or reduced at metamorphosis to the adult form. Reproduction varies by group: many ascidians are hermaphroditic and reproduce sexually by releasing gametes or by brooding, while colonial species also reproduce asexually by budding to form interconnected zooids.

Diversity and classification

The subphylum includes the traditional classes Ascidiacea (sea squirts), Thaliacea (salps and pyrosomes) and Appendicularia (larvaceans). Tunicates feed largely on plankton and suspended particles and are found from shallow coastal areas to the open ocean. Estimates suggest roughly three thousand described species, many of which inhabit the littoral zone and benthic communities.

Ecological role and human relevance

As filter feeders, tunicates influence water clarity, nutrient cycling and plankton communities. Some species are important members of fouling communities on boats and aquaculture gear and can become invasive when transported by shipping. Tunicates are also valuable in research: certain species serve as model organisms in developmental and genomic studies because their simple body plans and larval chordate traits illuminate vertebrate origins.

Many tunicates form colonies or semi-colonies with shared tunics and vascular connections; others are solitary. Their biology touches marine ecology, invasion biology and evolutionary developmental biology. For further information see resources on colonial tunicates, general chordate relationships, tunicate feeding strategies, the structure of the tunic, common substrates for attachment, dietary links to plankton and summaries of species diversity and coastal habitats. For classification details consult specialized taxonomic treatments at Urochordata references.