cirripede

Cirripedes, commonly known as barnacles, are a group of crustaceans that live as sessile adults attached to substrates in marine environments. They have a hard shell composed of calcareous plates that protect the soft body inside. The body bears a short thorax and abdomen, with six pairs of feathery legs, called cirri, used to filter plankton from the water.

Most cirripedes begin life as free-swimming larvae. They pass through free-swimming nauplius stages, then a non-feeding cyprid larva that searches for a suitable surface to settle. After settlement, they secrete their shell plates and become sessile. In many species, adults are hermaphroditic, or have colonies with dwarf males attached to females; fertilization is usually internal, and juveniles may release free-swimming larvae to continue the life cycle.

Cirripedes inhabit a wide range of marine substrates, including rocks, ship hulls, docks, whales, and kelp. Acrothoracica are smaller, often parasitic barnacles that live inside the shells or cavities of other invertebrates, while the more familiar Thoracica include the large, rock- and ship-attached barnacles. Feeding occurs by extending the cirri into the current and sweeping plankton and detritus into the mouth.

Economically, barnacles are notable for biofouling on ships and submerged structures, as well as for their roles in marine ecosystems and as model organisms in developmental biology.