Limpet? from the Bahamas

Dear Anne,
Yes this is a 'limpet' - in fact a Keyhole Limpet, so named because its shell has a hole in the apex of the shell through which the siphon to the mantle cavity can communicate with the outside world. They are quite closely related to that other 'sluggy' snail, Scutus which has generated quite a few queries.

The white smoky substance you observed is a secretion from the Hypobranchial Gland, which is found by the gill, in the mantle cavity of many snails. The purpose of the secretions is still not really understood. One snippet of interesting information concerning the hypobranchial gland is that the reddish dye called Tyrian Purple which was used to die the clothes of important people in ancient times, was obtained from the hypobranchial gland of muricid snails (Murex, Thais spp). It was first made in Tyre in the eastern Mediterranean 2-3000 years ago in what was probably one of the earliest chemical industries. The purple ink produced by Sea Hares, which is also reddish, is quite different and not permanent.

In your photo you can see the snail-like head with the eye embedded in the skin at the base of the head tentacle. The shell is completely covered by a flap of skin, but the ridge running around the body indicates the edge of the shell.
Best wishes,
Bill Rudman