Cirripedes

Cirripedes, commonly known as barnacles, are a group of crustaceans that attach permanently to hard substrates in marine environments. They include stalked forms that hang from a flexible peduncle and sessile, cone-shaped acorn barnacles that reside inside a calcareous shell. They are found worldwide in oceans from the intertidal zone to depths.

Adults are enclosed by calcareous plates forming a protective capitulum. Stalked barnacles have a muscular peduncle used for attachment; acorn barnacles are anchored directly to the substrate. They feed by extending feathery thoracic appendages called cirri to filter plankton and detritus from the water.

Cirripede life cycles begin with free-swimming larvae, including the nauplius and cypris stages, which settle on a surface and metamorphose into sessile adults. The mature animals are typically hermaphroditic, often with reciprocal or cross-fertilization between nearby individuals, and fertilized eggs develop into free-swimming larvae that continue the life cycle.

Barnacles occupy a variety of ecological roles as sessile filter feeders and epibionts on rocks, ships, and marine organisms. They are significant as biofouling agents in man-made environments and, in some regions, include species harvested for food, such as certain goose barnacles. Their diverse forms and life histories make them important for studies in crustacean evolution, development, and adaptation to sessile life.