Barnacle

Barnacles are a group of sessile crustaceans in the infraclass Cirripedia. Most barnacles attach permanently to hard substrates in marine environments, including rocks, ship hulls, piers, and whale skin. They range from a few millimeters to several centimeters in diameter. Barnacles are divided into stalked forms, which attach by a flexible stalk to a surface, and sessile forms, which cement the body directly to the substrate.

The body is enclosed in a calcareous shell composed of plates. In stalked barnacles a slender stalk elevates the animal from the surface, while in sessile barnacles the body is directly anchored. Feeding occurs with feathery legs called cirri that extend from the shell to filter plankton from seawater; the cirri trap particles as water is drawn in through the shell opening.

Life cycle and reproduction: Barnacles begin life as free-swimming larvae, progressing through nauplius and cypris stages before settling as adults. After settlement, they metamorphose into sessile individuals. Most species are simultaneous hermaphrodites, though some have separate sexes or dwarf males that attach to larger individuals. Fertilization is usually internal, with sperm transferred during mating, and fertilized eggs develop into free-swimming larvae before reaching maturity.

Ecology and significance: Barnacles are important filter feeders in coastal ecosystems and are notorious for biofouling, attaching to ships, docks, and submerged structures and increasing maintenance costs. Their cementing mechanism has attracted interest as a model for strong, underwater adhesives, and some species have become invasive when transported by human activities.