A barnacle is a type of arthropod constituting the infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile (nonmobile) and most are suspension feeders, but those in superorder Rhizocephala are parasitic. They have four nektonic (active swimming) larval stages. Around 1,000 barnacle species are currently known. The name "Cirripedia" is Latin, meaning "curl-footed". The study of barnacles is called cirripedology.
Barnacles are encrusters, attaching themselves temporarily to a hard substrate. The most common, "acorn barnacles" (Sessilia), are sessile, growing their shells directly onto the substrate. The order Pedunculata (goose barnacles and others) attach themselves by means of a stalk.