Gould's Bubble Snail - shell damage

Dear Kevin,
If you have by chance a photo showing a whole 'herd' of Bulla it would be nice to add to the Forum. Species of Bulla seem to one of the opisthobranch groups which either are present in large numbers or not at all.

The shell damage in your photo is 'natural' in the sense of  Lord Tennyson's " Nature, red in tooth and claw ..." I suspect a fish or more likely, a crab, has been trying to get at the animal by breaking away the lip of the shell. Fortunately Bulla has still retained a shell big enough and strong enough for the animal to be able to retract inside far enough to escape hungry predators. The reason the new shell, which has grown to replace the old damage, is a different colour pattern, is that the edge of the mantle must have been slightly damaged as well. The mantle is a layer of tissue which lines the inside of the shell wall. The outer edge of this tissue secretes the calcium salts and organic matrix which builds the shell. It also contains the pigment cells which produce the shell colour pattern. In nudibranchs, which have lost the shell, you will often see the dorsal skin of the animal called the mantle. This is because we think this is what became of the old mantle when the shell was  lost.

Best wishes,
Bill Rudman