A barnacle is a type of arthropod belonging to 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 (non-motile) suspension feeders, and have two nektonic (active swimming) larval stages. Around 1,220 barnacle species are currently known.
The name "Cirripedia" is Latin, meaning "curl-footed". Barnacles are encrusters, attaching themselves permanently 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.
Most barnacles are suspension feeders; they dwell continually in their shell – which is usually constructed of six plates and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption. Other members of the class have quite a different mode of life. For example, members of the genus Sacculina are parasitic, dwelling within crabs.
Although they have been found at water depths up to 600 m (2,000 ft), most barnacles inhabit shallow waters, with 75% of species living in water depths of less than 100 m (300 ft), and 25% inhabiting the intertidal zone. Within the intertidal zone, different species of barnacle live in very tightly constrained locations, allowing the exact height of an assemblage above or below sea level to be precisely determined.
Barnacles are displaced by limpets and mussels, which compete for space. They also have numerous predators. They employ two strategies to overwhelm their competitors: "swamping" and fast growth. In the swamping strategy, vast numbers of barnacles settle in the same place at once, covering a large patch of substrate, allowing at least some to survive in the balance of probabilities.