Barnacles are marine arthropods belonging to the subclass Cirripedia in the subphylum Crustacea, making them relatives of crabs and lobsters. They are exclusively marine invertebrates, often found in shallow and tidal waters, and there are about 2,100 described species. Adult barnacles are mostly sessile (fixed in one place) and attach themselves to hard surfaces like rocks, ships, or even animals such as whales and turtles. They have hard calcareous shells and feed by extending feathery legs called cirri to filter plankton and other particles from the water. Barnacles reproduce as hermaphrodites, capable of fertilizing neighboring barnacles using an extraordinarily long penis relative to their body size. They develop from free-swimming larvae to permanently attached adults, often secreting a natural cement to adhere firmly to surfaces. Besides their ecological role, barnacles are notable for causing biofouling on ships, which can reduce hydrodynamic efficiency.
