Cayenne Keyhole Limpet
Diodora cayenensis Lamarck, 1822
The Cayenne Keyhole Limpet belongs to a large family of conically-shaped shells found mostly in temperate or tropical seas. The family of shells is known as Fissurellidae. The apex of these oval shells almost always has a slit or notch in it, hence the name “Keyhole.” — True Limpets (Acmaeidae) are similar in shape (though the margin may be spikey) but lack the opening.
Most Keyholes live in the intertidal or littoral zone.
A limpet has a large, fleshy foot with which it can sluggishly creep about on shore rocks where it likes to feed on algae. This feeding is accomplished by the radula, a file-like organ in a sac just inside the mouth. The gastropod moves the rasping dental apparatus/“tongue” forward and back over the algae growing on rocks. The food is scraped up and macerated. This “grazing” eventually wears down the radula and its teeth. To compensate for this deterioration, more teeth are constantly being formed.
The Cayenne Keyhole Limpet shell is highly elevated, relatively solid, and has an elongated slit at the summit. The surface of the gastropod is ribbed with numerous strong, radiating lines, interspersed with weaker lines (approximately every fourth rib is the more distinct). There are also concentric growth lines. The shell may be white, tan, gray, or pink. The porcelain-like interior is whitish or blue-gray. The margin of the shell is faintly and irregularly scalloped or notched. The gastropod lacks an operculum (the “door” or plate which closes off the aperture/opening of many snail shells). The specimen seldom extends a length of 1-1/2 inches. (The Giant Keyhole Limpet of the Pacific may be 4-5 inches long.)
As indicated, limpets are widely distributed. It is said that in the early 1800's many inhabitants of the Isle of Scilly, off the coast of England, “were subsisting on [eating] limpets.” Those gastropods were likely no longer than 2½ inches, so obviously it would take a lot of them to live on.
The Cayenne is found on beaches from Maryland south, on the Gulf coast, in the West Indies, and down to Brazil. It is the most commonly found representative of its genus Diodora.