Barnacles are common organisms found along coastlines, often appearing as small, volcano-shaped bumps covering rocks, piers, and the hulls of ships. Their widespread presence is a major concern for maritime industries, which spend billions each year combating them as biofouling agents. These organisms have evolved a specialized life history to thrive in the harsh intertidal zone, allowing them to be found everywhere from the deepest ocean trenches to the backs of migrating whales. To truly understand these ubiquitous marine hitchhikers.
Defining the Barnacle
Despite their hard, external shells resembling those of mollusks like clams or snails, barnacles are actually crustaceans, making them relatives of crabs and shrimp. They belong to the subclass Cirripedia, a name referencing their feathery feeding appendages. The misconception that they are mollusks arises from the protective, cone-shaped housing they build.
This protective structure, called a carapace, is made of several interlocking calcareous plates. Acorn barnacles, the most common type, are sessile, meaning they attach directly to a surface. Other forms, like goose barnacles, are attached to the substrate by a long, muscular stalk called a peduncle. Inside this rigid shell, the barnacle lies on its back, permanently cemented, with its limbs projecting upward toward the opening.
The Stationary Life Cycle
The life cycle involves a transformation from a free-swimming larva to a permanently attached adult. This begins when fertilized eggs hatch into the first larval stage, the nauplius. The nauplius is a microscopic, single-eyed larva that swims freely and feeds, growing through several molts over weeks.
The final larval form is the cyprid, a non-feeding stage whose purpose is to find a permanent home. This larva possesses a bivalved carapace and specialized antennules, which it uses to explore surfaces for chemical and textural cues. Cyprid larvae often settle near other barnacles, increasing their chances of reproduction since adults cannot move. Once chosen, the cyprid cements itself head-first to the substrate and undergoes metamorphosis into the juvenile form, which begins to build its protective shell.
Attachment and Feeding Mechanisms
The barnacle adheres permanently underwater using a protein-based biological adhesive known as barnacle cement. This cement is secreted from glands at the base of the antennae and cures rapidly in the marine environment. The resulting glue is a multi-protein complex that forms a robust and durable bond, allowing the barnacle to withstand the forces of waves and water currents.
Once cemented, the adult barnacle feeds as a suspension feeder. When submerged, the plates at the top of the shell open, and the animal extends its six pairs of thoracic limbs, called cirri. These cirri are feathery, net-like appendages swept rhythmically through the water to filter out plankton and suspended organic matter. The barnacle can adjust the rate and style of its cirral beating, even creating its own feeding currents in still water to draw food toward its net.