Limpet

Limpet is a common name for a number of different groups of sea snails and freshwater snails (aquatic gastropod mollusks). The common name is applied to those snails that have a simple shell which 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 shaped more or less like that of most true limpets.

As well as being applied to the true limpets (the Patellogastropoda), the common name "limpet" is used for many other, widely different, snails, all of which have a shell that does not appear to be spirally coiled in the adult stage. The term "false limpets" is used for some of the other snails that have a shell that is limpet-like or "patelliform".

All the true limpets are all marine; however, the feature of a simple conical shell has arisen independently many times in gastropod evolution, in many different evolutionary lineages, and as a result, there are freshwater limpets and well as saltwater limpets. Some limpets have a gill or gills, some have a lung, and in some of those cases the lung has been secondarily modified to absorb oxygen from water. In other words, 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).

Thus, 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 form are not at all closely related to one another:

• Clade Patellogastropoda, example Patellidae, the true limpets
• Clade Vetigastropoda, examples Fissurellidae the keyhole limpets and slit limpets, Lepetelloidea
• Clade Neritimorpha, example Phenacolepadidae, small limpets related to nerites
• Clade Heterobranchia, group Opisthobranchia, example Tylodinidae,, the umbrella slugs
• Clade Heterobranchia, group Pulmonata, examples Siphonariidae, Latiidae, Trimusculidae