A heavy yellow-orange slug with a small circular shell sitting on its back like a cap.
A large heavy-bodied sea slug, to about 15 cm, with a broad muscular foot and a body surface divided into coarse polygonal tubercles, cream to yellow or deep orange, that looks more like a piece of encrusting sponge than an animal. Sitting on top is a flattened circular shell, white and cap-shaped, far too small for the animal to withdraw into and usually so overgrown with algae, sponge and other encrusting growth that it reads as a stone resting on a mound of tissue.
The head end carries a pair of rolled rhinophores and a short broad oral veil, and the gill is a plume on the right side under the shell margin. The animal is a gastropod that has kept an external shell while losing any use for it as shelter, and the combination of the exposed shell and the tuberculate body is not shared by anything else on this ground.
It feeds on sponges and moves very slowly, so the difficulty is entirely in finding one rather than in photographing it. Records come from rubble and sand on the lower parts of a slope, and they are thin: this is an animal that turns up a few times a season on well-worked ground. Recorded from about 5 to 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.
