Sessilia

Sessilia is a suborder of barnacles (Cirripedia) that comprises the majority of sessile, shelled crustaceans attached directly to hard substrates. Members are typically stalkless and permanently cemented to rock, ship hulls, shells, or other surfaces. The shell, or capitulum, is formed from calcareous plates (usually four to six) that create a rigid enclosure around the body. The openings are protected by opercular plates, and the animals feed by extending feather-like thoracic appendages called cirri to filter plankton from the water. This sessile lifestyle contrasts with stalked barnacles, which possess a long peduncle.

Sessilia species inhabit marine environments worldwide, from intertidal zones to subtidal depths. They are particularly common on rocky shores, pilings, and artificial structures, where they can form dense fouling communities. Reproduction is sexual; most species are hermaphroditic, and fertilization often involves cross-fertilization with nearby individuals. Larval development proceeds through free-swimming nauplius and cypris stages before settlement as juvenile barnacles. Adults are sessile and grow by adding plates and continual calcification of the capitulum.

Taxonomically, Sessilia encompasses several families within Cirripedia, including notable groups in Balanidae (for example Balanus and Semibalanus) and Chthamalidae (Chthamalus). The suborder has a rich fossil record dating back to the Mesozoic, contributing to studies of marine biofouling and crustacean evolution.