Hydatina amplustre from Reunion Island

Dear Hugues,
Thanks for these photos. I guess that apart from a few acteonids, such as Acteon eloiseae, this is probably the only species of sea slug with a shell that is more beautiful and colourful than the animal.

Your photos are particularly valuable because they show how the front of the head shield is developed into a pair of tentacles on each side. Hydatina physis has a similar arrangement but it is more difficult to see because the 'tentacles' are much larger and lobe-like. Yes the two black spots are the eyes.

All species of Hydatina can retract back into their shell but they seldom do. The rely on distasteful acid glands in their skin and burrowing in the sand to protect them. There is also a mechanical reason for their poor ability to retract back into the shell. Most snails have a solid shell and a central heavily calcified central 'pole' we call the columella around which the shell, and animal, spirals. There is a huge columella muscle which attaches the animal to the shell and can quickly pull the animal back inside when necessary. In Hydatina, in which the shell is becoming reduced in importance, the columella has become very reduced, as has the associated  columellar muscle.

Best wishes,
Bill Rudman