A sap-sucking slug of shallow algae, with the parapodia held up like folded wings.
A sacoglossan to about 3 cm — a sap-sucking slug rather than a nudibranch, though it is looked for in the same places and filed with them by most divers. The body is dark, near-black to deep olive, scattered with small pale blue-white spots, and the two lateral flaps are held folded up over the back so the animal reads as narrow and high rather than flat. The flap margins carry a band of orange with a paler line inside it. The head has two rolled rhinophores. Several Thuridilla occur here and they are separated chiefly on those margin bands and on whether the body pattern is spotted or lined — T. lineolata is lined — so a photograph square to the side, showing the edge of the parapodia, is what an identification turns on.
Feeds by piercing the cells of green algae and sucking out the contents. Whether it keeps the chloroplasts working afterwards, as Elysia and Plakobranchus do, is not established for this species; where retention has been measured in Thuridilla it has been short-lived, so the “solar-powered” label that gets applied across the sacoglossans as a group does not obviously belong to this one.
Found in the top few metres wherever the food algae grows, including on rubble, rope and rubbish, and active by day. It moves steadily and does not hide, which makes it a straightforward subject once found; the difficulty is the size and the fact that it sits among algae of much the same colour.
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.
