Barnacles

Barnacles are a group of sessile crustaceans within the infraclass Cirripedia. Most species attach permanently to hard substrates such as rocks, ships, or whales, while a few forms are stalked and float on the surface. Adults live inside a hard shell formed by calcareous plates that shield a soft body, and the shell opening is normally closed by movable plates. The animal extends feathery legs called cirri to filter food from the water.

Life cycle begins when barnacle larvae hatch as free-swimming nauplius stages and molt into a cypris larva, which seeks a surface and settles. After metamorphosis, a juvenile barnacle slowly grows a capitulum of plates and becomes sessile for life. Most barnacles are simultaneous hermaphrodites, but fertilization typically involves mating with neighboring individuals, sometimes via long reproductive extensions, and eggs develop into free-swimming larvae.

Feeding and habitat: Adult barnacles are filter feeders that use cirri to strain plankton from the water. They occur in a range of marine environments, from exposed shores to protected harbors, and include both stationary forms that attach directly to substrates and pedunculated, or stalked, barnacles that anchor to floating objects.

Ecology and human impact: Barnacles are a major component of marine biofouling communities and can increase drag on ships and require frequent maintenance of offshore structures. Control methods include anti-fouling coatings and mechanical cleaning to reduce settlement.

Taxonomy and evolution: Barnacles comprise several groups, notably acorn barnacles (directly attached to substrates) and pedunculate, or stalked, barnacles. The fossil record extends back to the Paleozoic, with extensive diversification in later eras. They are crustaceans, closely related to other mandibulate crustaceans but uniquely adapted to a sessile, filter-feeding lifestyle.