#ascidiacea

Stage-lit Gerard as Star Tunicate Botryllus schlosseri

bonus B. schlosseri photos by me under the cut :)

(Sources - 1, 2, 3, 4)

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.

Propaganda under the cut:

Sea squirts (Rhopalaea sp.) off the coast of the Maldives

Obese ascidian (Phallusia obesa)

Tall urn ascidian (Didemnum molle) lodged in coral at Siquijor Island in the Philippines