A filter-feeding sac that is a closer relative of a fish than of a sponge.
A solitary ascidian to about 12 cm, the body a tough leathery sac in cream to yellow marked with irregular purple veins and a purple rim around each of the two openings. Water is drawn in through the larger siphon, strained through a perforated basket for plankton, and pushed out through the smaller one. It is sessile, and looks like a plant or a sponge.
It is neither. Ascidians are chordates: the swimming larva has a notochord, a dorsal nerve cord and a tail, and it settles head-first onto a hard surface and reabsorbs all of that to become the adult. The animal a diver sees is the sessile stage of a relative of the vertebrates. Ascidians are the food of several animals on this list — the Nembrotha sea slugs and the Pseudoceros flatworms among them — so a colony worth searching is often the reason a slug is where it is. Recorded from the shallows to about 30 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.