The wide flat skirt around this slug is not only camouflage. It is detachable, and losing part of it is a normal event in the life of the animal.
Dorid sea slugs carry a mantle that extends well beyond the foot on all sides, a thin overhanging rim that flattens the outline against the rock and removes any shadow. In this genus that rim is also a sacrificial part. Seized by a fish or a crab, the animal tears the gripped section of the margin away and leaves with the rest, and the detached piece goes on writhing for a while, which is exactly long enough to matter. It is the same trade a lizard makes with its tail, and it explains why so many photographs of these animals show a mantle edge with a bite-shaped gap or a healed notch in it.
The rest of it is worth a look for the pattern. Four centimeters (1.6 in) of brown back broken by purple and cream patches, irregular, with no line or margin to fix on: that patchwork is doing the same job the skirt does, breaking the animal into pieces that do not add up to a slug against sponge-covered rubble.
It eats sponges, it comes out after dark, and the sensible way to find one is to check the underside of loose rubble slabs with a torch and look for something with a torn edge.
- brown back broken up by purple and cream patches
- wide flat mantle skirt standing out well beyond the foot
- the skirt is often ragged or notched, from pieces torn off in an earlier attack
- flattened profile, sitting close to the surface it is on
