ascidiacei

Ascidiacei, commonly known as ascidians or sea squirts, are a class of sessile marine invertebrates within the subphylum Tunicata (urochordates). They are attached to substrata in coastal waters and range from solitary individuals to branching or encrusting colonies. They feed by filter feeding, drawing water through an incurrent siphon, filtering particles in a pharyngeal basket, and expelling water through an excurrent siphon.

Anatomy and development: The body is enclosed in a tough, gelatinous tunic composed largely of tunicin. Adults are sac-like with two siphons; a pharynx with slits captures food; a dorsal nervous system is reduced, and the heart periodically reverses blood flow. Most larval forms are free-swimming and possess a notochord and dorsal nerve cord, features that are lost during metamorphosis as the organism settles into an adult form.

Life cycle and reproduction: Ascidiacei reproduce sexually, and many are hermaphroditic. Fertilization is external in many species, and the tadpole-like larva develops into a sessile adult. Colonial species can bud to form additional zooids connected by stolons or shared tunic.

Distribution and ecology: They occur worldwide in temperate and tropical seas, from tidal zones to deeper waters. They are important filter feeders, impacting water quality and nutrient cycles. Some species are invasive when introduced to non-native ecosystems (e.g., Styela clava, Didemnum vexillum).

Human interactions: Several ascidians are harvested for consumption in East Asia (for example, Halocynthia roretzi) or studied as model organisms in developmental biology due to their simple chordate features. Conservation concerns include habitat degradation and pollution that affect sessile organisms.