ascidian

Ascidians, also known as sea squirts, are marine invertebrates belonging to the subphylum Tunicata. They are characterized by a leathery or gelatinous outer covering called a tunic, which is secreted by the animal's epidermis. Ascidians are filter feeders, drawing water into their bodies through an incurrent siphon and expelling it through an excurrent siphon after extracting food particles. Their body plan is typically sac-like, with a pharynx perforated by slits, an esophagus, stomach, intestine, and anus.

The life cycle of ascidians involves a free-swimming larval stage that possesses a notochord, a characteristic of chordates, which is later lost during metamorphosis into the sessile adult form. This larval stage is often tadpole-like and exhibits a distinct head and tail. Adult ascidians are often found attached to hard substrates in marine environments, ranging from intertidal zones to the deep sea. They can be solitary or colonial, with colonial forms developing from a single larva that buds repeatedly.

Ascidians play important roles in marine ecosystems, serving as food sources for various predators and contributing to biofouling on submerged structures. Some species are also of scientific interest due to their unique biological properties, including the production of novel compounds with potential pharmaceutical applications. They are found in all oceans worldwide, occupying a diverse array of ecological niches.