Barnacles are crustaceans, related to crabs and lobsters, that spend their adult lives permanently fixed to a surface. Their life cycle involves a radical transformation from a tiny, free-swimming larva to an armored, sessile adult encased in a shell. Understanding this process requires examining the two distinct larval stages that facilitate both dispersal and the eventual selection of a permanent home. This transition moves the creature from a mobile aquatic form to one fixed head-first to a rock or ship’s hull.
The Dispersal Phase: Nauplius Larvae
The life cycle begins with the nauplius larva, which hatches from the egg mass released by the adult barnacle. This microscopic larva is dedicated to widespread movement and feeding, ensuring the species spreads away from the parent location. It appears as an unsegmented, oval body equipped with a single, simple median eye, known as the naupliar eye, which senses light.
The nauplius possesses three pairs of appendages originating from the future head region of the adult, which it uses for both swimming and gathering food. These appendages (antennules, antennae, and mandibles) are covered in fine setae that help propel the larva through the water and filter plankton. This planktotrophic feeding stage must eat constantly to fuel the six separate molts it undergoes before transitioning to the next larval form. Its purpose is to grow and disperse into new habitats before the irreversible choice of settlement is made.
The Settlement Phase: Cyprid Larvae
After the final naupliar molt, the barnacle enters the cyprid stage, encased in a chitinous bivalved shell resembling a miniature clam. This is the non-feeding, or lecithotrophic, stage, meaning its sole purpose is to find a suitable permanent location, relying on stored energy reserves for the search. The cyprid is equipped with two compound eyes and a suite of sensory organs.
The search involves the cyprid using its modified first pair of antennae, or antennules, to explore potential surfaces. These antennules have sensory hairs and attachment discs that allow the larva to test the substrate for specific characteristics. The cyprid is highly selective, evaluating surface texture, microbial biofilms, and specific chemical cues left by other settled barnacles. This behavior, known as gregarious settlement, is a response to a settlement-inducing protein complex (SIPC) left by existing adults, ensuring the larva settles near potential mates. The search process can last from a few days to a few weeks, and the cyprid must decide before its limited energy reserves are depleted.
Permanent Adhesion and Metamorphosis
Once a suitable location is identified, the cyprid initiates permanent adhesion. The larva attaches itself head-first to the substrate using the antennules, which secrete a specialized, fast-curing biological cement. This multi-protein complex achieves robust underwater adhesion.
The permanent adhesive, or cyprid cement, is a dual-phase bioadhesive often containing protein and lipid components secreted from specialized cement glands. The lipid phase helps displace the thin layer of water on the substrate, creating a conducive environment for the proteinaceous glue to bond. Following fixation, the larva undergoes a rapid and radical metamorphosis, transforming from a mobile crustacean into a sessile filter feeder. During this process, larval features, including the bivalve shell and the compound eyes, are lost, and the body rearranges itself to live permanently “upside down.”
Structure and Function of the Sessile Adult
The sessile adult is defined by a protective, multi-plated calcareous shell, which grows by adding new material to the edges of the plates. The body consists mainly of a head and thorax, with the head region permanently attached to the substrate by the cement. This protective structure, known as the capitulum, includes a hinged lid called the operculum, which the barnacle closes to prevent desiccation or predation.
The primary function of the adult is filter-feeding, accomplished by six pairs of long, feathery thoracic appendages known as cirri. The barnacle rhythmically extends these cirri through the operculum into the water column to capture plankton and detritus. The captured food particles are scraped off the cirri and moved toward the mouth. While the shell plates do not molt, the barnacle periodically sheds its cuticle, a process that allows for the growth of soft tissues inside the fixed housing.