Cayenne keyhole limpetDiodora cayenensis (Larmarck) Description: (1 inch) Shell shaped like a small, low cone or a coolie hat. Many ribs radiating from the small, subcentral keyholelike opening on top of shell. Inside of keyhole opening outlined by a truncate callous with a deep pit on its concave edge. Color: Exterior white and pinkish gray or brown. Interior white to gray. Habitat: Lives in inlets and offshore waters attached to rocks or shells. Occasionally found on sound and ocean beaches. Range: New Jersey to Brazil. Notes: Also called a little keyhole limpet. A herbivore, it uses radula to scrape algae off of rocks. Its powerful foot creates strong suction to keep waves from washing it off the rocks. Water enters under the edge of the shell and exits through the "keyhole" near the peak. Its eggs are yellow and stick to rocks. Hatched young crawl away.
Source: Seashells of North Carolina, North Carolina Sea Grant College Program
These are photographs of the keyhole limpets I've found along the beaches to date, some small and some larger. The keyhole limpet is classified under Gastropods, and comes in the form of this one unusually-shaped shell.
Carl is right. Limpets are an oddly "designed" species. I began studying to learn more about limpets after beginning beachcombing, (yeah, *chuckles* I live near the ocean and love it) started blogging on the shells I found. I could only wonder "how" do these things ever stay attached to the rock in the waves? Further, how can they possibly avoid predators, like the ever-looming threat of moon snails? (Keyhole limpet "design" is really like an open invitation for Polinices duplicatus "come on over for dinner, the hole is already drilled for you!")
it is simply a terrible design! Unlike most mollusks [for the most part] who can close up inside, or withdraw into their shell for protection, not so easy for the limpet. [there's other bad designs, where some species like barnea truncatathe Fallen Angelwing, if I'm not mistaken is another species, almost as bad as limpet design, can not completely close their shell]... which would seem to leave them vulnerable to predators. Notice the curve near the hinge of the shell...
the keyhole limpet is an obscure little shell I've been collecting and I've been asking myself the same questions, Carl Zimmer has addressed.