Thanks to Peter Scharf for the photo of this dwarf sea hare, Aplysia parvula, taken while he was snorkelling off the reef at Woody Head in early January 2017.
The Sea Slug Forum gives information about this species, plus a photo of a juvenile with the shell visible. The mantle grows over the shell as the animal matures. These sea hares, vegetarians all, are small, up to 6 cm (just over 2 inches) in length.
Another goody at Woody!