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Limpet is a common name applied to aquatic snails with shells broadly conical in shape, rather like the familiar conical Asian hat. The term "limpet" is purely informal, a term of convenience; it refers to any gastropod whose shell has no obvious coiling such as one sees in familiar garden snails or in winkles. Although all limpets are Gastropoda, the group is highly polyphyletic, meaning that the various lines that we call limpets have descended independently from different ancestral Gastropoda. This general category of conical shell is technically known as "patelliform", meaning dish-shaped.
Some species of limpet live in fresh water, but by far the majority are marine.
All members of the large and ancient marine clade Patellogastropoda are limpets, and within that clade the family Patellidae in particular often are called the "true limpets". However, other groups, not in the same family, also are called limpets of one type or another because of the general shapes of their shells. Examples include the Fissurellidae; they are the keyhole limpet family, contained in the clade Vetigastropoda, though many of the members of the Vetigastropoda do not have the morphology of limpets at all.
True limpets in the family Patellidae live on hard surfaces in the intertidal zone. Unlike barnacles or mussels, true limpets are capable of locomotion instead of being permanently attached to a single spot. However, when they need to resist strong wave action or other disturbances, limpets cling extremely strongly to the hard surface on which they live, using their muscular foot to apply suction combined with the effect of adhesive mucus. It often is very difficult to remove a true limpet from a rock without injuring or killing it.
All "true" limpets are marine and have gills. However, because the adaptive feature of a simple conical shell has repeatedly arisen independently in gastropod evolution, limpets from many different evolutionary lineages occur in widely different environments. Some saltwater limpets, such as Trimusculidae breathe air, and some freshwater limpets are descendents of air-breathing land snails (e.g. the genus Ancylus) whose ancestors had a pallial cavity serving as a lung. In these small freshwater limpets that "lung" underwent secondary adaptation to absorption of dissolved oxygen from water.
The common name "limpet" also is applied to a number of not very closely related groups of sea snails and freshwater snails (aquatic gastropod mollusks). Thus the common name "limpet" has very little taxonomic significance in and of itself; the name is applied not only to true limpets (the Patellogastropoda), but also to all snails that have a simple shell that is broadly conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snail. In other words the shell of all limpets is "patelliform", which means the shell is shaped more or less like the shell of most true limpets. The term "false limpets" is used for some (but not all) of the other groups that have a conical shell but are not true limpets.
Thus, the name limpet is used to describe various extremely diverse groups of gastropods that have independently evolved a shell of the same basic shape (see convergent evolution). And although the name "limpet" is given on the basis of a limpet-like or "patelliform" shell, the several groups of snails that have a shell of this type are not at all closely related to one another.
Gastropods that have limpet-like or patelliform shells are found in several different clades:
Most marine limpets have gills, whereas all freshwater limpets and a few marine limpets have a mantle cavity adapted to breathe air and function as a lung (and in some cases again adapted to absorb oxygen from water). All these kinds of snail are only very distantly related.
The humorous author Edward Lear wrote "Cheer up, as the limpet said to the weeping willow" in one of his letters. Simon Grindle wrote an illustrated book said to be "in the great tradition of Edward Lear and Lewis Carroll" called The Loving Limpet and Other Peculiarities.