Limpets are a common mollusk of the class Gastropoda. Its shell is generally low, flat, oval, and more bilaterally symmetrical than coiled (like a snail's), and it covers the entire soft body, so that the living animal inside is rarely visible. Limpets adhere strongly to rocks by means of a broad muscular foot. It is important for their survival that they are not dislodged easily, since their shell structure does not permit withdrawal into the shell.
The Atlantic plate limpet is Tectura testudinalis; testudinal means resembling a tortoise shell. The common European limpet is Patella vulgata; the patella in human anatomy is the kneecap; patelliform means saucershaped, like a kneecap or limpet shell.
Limpets are prosobranch snails of the Order Archaeogastropoda, which allies them with the slit shells, abalones, top shells, and nerites. They are sufficiently varied that they are assigned to several families. Species of Tectura and a number of Pacific coast species of the genus Lottia are classified as Family Lottiidae. Patella is in the Family Patellidae. The small white Acmaea mitra of the Pacific coast and a few deep-sea limpets are in the Family Acmaeidae.
Quite different from the families named above are the key-hole limpets, Family Fissurellidae. A common species is Diodora cayenensis, which ranges from New Jersey to Brazil. Members of the Fissurellidae family have an opening near the apex of the shell, giving the appearance of a miniature volcano and permitting the out-flow of fecal matter and water that has already passed over the gills. The hole begins as a slit in the embryonic shell, and becomes closed as the mantle deposits more shell during growth to adult size.
Limpets are intertidal herbivores, and they do not often feed when exposed to the open air. Some species have a radula, a horny edge on the shell with projections resembling teeth, that helps tear up the animal's food and bring it to its mouth. The food then passes through a simple stomach, where it is exposed to enzymes from paired digestive glands, and then into a long, coiled intestine, where it is turned into feces. Some limpets are not stationary, but make forays of up to 5 ft (1.5 m) at night or at high tide, then return to their original position, which is sometimes marked by a "home scar" on the rock surface. The "brain" of limpets consists of a relatively small number of neurons, and it is not clear how they find their way home.
Like other archaeogastropods, male and female limpets look much the same, and can be distinguished only by the color of the gonads and microscopic examination of their sex cells, or gametes. The majority of smaller animals are males and the larger animals are females, so it is probable that individual males, as they age, become females. During reproduction, eggs and sperm are released into the water at the same time. After several days, 10 for Patella, the new larvae settle on some solid substratum, and grow into adults. The larvae of Lottia strigatella prefer to settle on boulders where adults of the same species are established, even though other species may be present. This is known as gregarious settlement.
The largest limpet is Lottia gigantea, a Mexican species 4-8 in (10-20 cm) long. Most species of limpets are 0.4-1 in (1-2.5 cm) long. Some species have been estimated to live 15 years. The larger species are consumed, cooked or raw, in various parts of the world, but limpets are not known to support a commercial fishery.
Abbott, R.T. Seashells of the Northern Hemisphere. New York: Gallery Books, 1991.