A limpet is a small sea animal with a cone-shaped shell which attaches itself tightly to rocks. a small sea animal with a shell, which holds tightly onto the rock where it lives (lampreda ). Any of various species of snails that have a flattened shell. Most marine species (subclass Prosobranchia) cling to rocks near shore. A common U.S. species is the Atlantic plate limpet (Acmaea testudinalis) of cold waters. Keyhole limpets have a slit or hole at the apex of the shell. Some limpets (subclass Pulmonata) live in brackish water and freshwater. See also mollusk