The name Limpet is used for many kinds of mostly saltwater but also freshwater snails, specifically those that have a simple shell which is more or less broadly conical in shape, and which is either not coiled, or appears not to be coiled, in the adult snail.
Thus the word "limpet" is an inexact term, which is fairly often used as part of the common name of a wide variety of different marine and freshwater gastropod species, some of which have gills and some of which have a lung. The name is given on the basis of a limpet-like or "patelliform" shell, but the several groups of snails which have such a shell are not at all closely related to one another.
The phrase "true limpets" is used only for marine limpets in the ancient clade Patellogastropoda. This article is primarily about the patellogastropods, the true limpets.
Limpets have flattened, cone-shaped shells, and the majority of species are commonly found adhering strongly to rocks or other hard substrates, looking like little bumps on the surface. Many limpet shells are often covered in microscopic growths of green marine algae, which can make them even harder to see, as they can closely resemble the rock surface itself.
The majority of limpet species have shells that are less than 3 in (8 cm) in maximum length and many are much smaller than that.
Various different species live throughout the intertidal zone, from the high zone (upper littoral zone) to the shallow subtidal.
They attach themselves to the substrate using pedal mucus and a muscular "foot". They locomote using wave-like muscular contractions of the foot when conditions are suitable for them to graze. They can also "clamp down" against the rock surface with very considerable force when necessary, and this ability enables them to remain safely attached, despite the dangerous wave action on exposed rocky shores. The ability to clamp down also seals the shell edge against the rock surface, protecting them from desiccation during low tide, despite their being in full sunlight.
When true limpets are fully clamped down, it is impossible to remove them from the rock using brute force alone, and the limpet will allow itself to be destroyed rather than stop clinging to its rock.
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