Ascidiacea is a paraphyletic class, so I am also including Thaliacea here. Ascidiacea, commonly called “Sea Squirts”, and Thaliacea, commonly called “salps”, “pyrosomes”, and “doliolids”, are tunicates: invertebrate chordates whose larvae begin life as free-swimming, tadpole-like larvae with rudimentary brains, light sensors, a dorsal nerve chord, and a notochord, before they metamorphize into their adult forms.
Adult Ascidians are sac-like, sessile, filter-feeding animals. They feed using two tubular openings, called siphons, through which they draw in and expel water. Some species live as solitary individuals, some are social, while others replicate by budding and become large colonies of zooids.
Thaliaceans are thought to have evolved from ascidians. Rather than settling down, they are free-floating throughout their entire lives. Salps move by contracting, similarly to jellyfish, straining phytoplankton from the water. They have a complex life cycle, in which one generation of solitary individuals reproduces asexually by producing a chain of tens to hundreds of individuals, which are released from the parent at a small size. The next generation consists of a colony of salps (called blastozooids) remaining attached together while swimming, feeding, and growing (see gif below). This generation reproduces sexually, first maturing as females and later transforming into males. Older chains of male blastozooids will fertilize the eggs of younger female chains. Growing embryos are called oozooids, and eventually detach from their parent blastozoids, to feed and grow as the next solitary, asexual generation.
Due to their soft bodies, the ascidian fossil record is scant. The earliest confirmed ascidian is Shankouclava shankouense from the Lower Cambrian, but they are likely older than that.
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Propaganda under the cut:
Tunicates are “Olfactores”, meaning they have a more developed sense of smell and are more closely related to vertebrates than they are to lancelets. These are our closest invertebrate relatives.
The family Pyrosomatidae (image 3) has cylindrical or cone-shaped colonies of hundreds to thousands of individual zooids reaching up to 18 m (60 ft) long!
Due to their filtering capacity, some species are sensitive indicators of polluted water.
Sea squirts are the natural prey of many animals, including nudibranchs, flatworms, molluscs, rock crabs, sea stars, fish, birds, sea otters, and humans! They are eaten in Japan, Korea, Chile, and Europe (where they are sold under the name "sea violet").
