Barnacles are arthropods belonging to the infraclass Cirripedia of the subphylum Crustacea. They are closely related to similar crustaceans such as crabs and lobsters. Barnacle species are exclusively marine and they live attached to surfaces. Cirripedia comes from Latin, and translates into curl-footed.
There are over 1,200 described species of barnacle, living mostly in shallow marine waters. They attach themselves permanently unto hard surfaces after which they encrust themselves. The common acorn barnacle is sessile, attaching itself unto various hard surfaces, building a shell unto the substrate, while the goose barnacle and many other species use a stalk in order to attach themselves.
While some barnacles have been found to live as deep as 600 meters below sea level, most of them prefer shallow waters of less than 100 meters, with some other species living in intertidal zones. Because intertidal zones have periodical desiccations, most barnacle species living in this area are well adapted against water loss. They have calcite shells that are impermeable, and they also possess two plates they can slide to enclose themselves while they are not feeding. These plates also serve to protect these animals from various forms of predation.
Most barnacle species are suspension feeders. They attach themselves permanently unto a surface, encrust themselves with a shell composed of six plates, and when feeding, they extend their cirri, or feathery legs which they rhythmically beat in order to draw in water along with plankton and detritus into their shell for feeding. However, there are other barnacles that live as parasites. For example, members of the genus Sacculina are parasitic to crabs, living within them.
Most barnacle species are displaced by mussels and limpets in the competition for space. Besides competition, there are also many predators that feed on barnacle species, and the latter have developed several mechanisms to overwhelm both their competitors and predators. The first strategy is swamping, which means that the barnacle species overcomes competition by covering an entire area with individuals, allowing some of them to survive.
Fast growth is another employed strategy that allows them to grow and reach higher areas in the water column that are unreachable by their competitors. Not only that, but the fast growth also allows them to become large enough so as to be difficult or impossible to be displaced by limpets and mussels. Some barnacles such as Megabalanus may grow to 7 centimeters in length.