As suggested recently by a Museum friend, here is a brief comparison between the shells of two local (Southwest Florida) keyhole limpets. The Cayenne Keyhole Limpet, Diodora cayenensis, is on the top row, the Meta Keyhole Limpet, Diodora meta, on the bottom row. The Cayenne may reach about 25 mm (one inch) locally, whereas Meta may grow to be only half as much. Notice, (1) the color, almost completely absent in Meta; (2) the general outline, more “oval” in Cayenne, more elongate in Meta; (3) the shell height, with a taller shell in Cayenne; (4) the “pinched” aspect of the “keyhole” in Cayenne versus a more rounded keyhole in Meta; and (5) the internal callus surrounding the keyhole, complete in Meta, truncated in Cayenne.
The Cayenne Limpet (top) and the Meta Limpet (bottom). Photo of live snail by Amy Tripp, others by José H. Leal.