Barnacles are marine invertebrates that attach permanently to hard surfaces in saltwater environments. Often mistaken for mollusks due to their hard outer covering, barnacles belong to the subclass Cirripedia, making them crustaceans, a group that includes crabs and shrimp. The adult form is sessile, meaning it is fixed in one place for life, encased within a characteristic, conical shell.
The Outer Fortress
The barnacle’s volcano-shaped exterior is a protective shell composed of multiple calcareous plates. These plates are primarily made of calcite, a form of calcium carbonate, providing a rigid and durable structure. The main body of the wall is formed by a series of fused plates known as parietal plates.
These plates sit atop a base that is either membranous or calcified, depending on the species. The top of the shell is covered by two pairs of movable plates called the operculum, which function as a hinged door. These opercular plates are the scuta and the terga. The barnacle can tightly close the operculum to seal itself off, protecting the soft body inside from desiccation, predators, and harsh wave action.
Anatomy of the Resident
Inside the calcified shell resides the living animal, which is essentially a small crustacean lying on its back. The body plan is highly modified for its fixed existence, consisting mainly of a head and thorax with a reduced or absent abdomen. Despite its sedentary nature, the barnacle shares an evolutionary lineage with more mobile arthropods like shrimp and lobsters.
The most prominent features are the six pairs of highly specialized thoracic limbs. These feathery, jointed appendages are called cirri and are regularly extended through the open operculum into the water column. The cirri beat rhythmically to create a current, acting as a net to filter tiny food particles like plankton and detritus. Food collected is passed toward the mouth and into a digestive system that includes a short foregut and a spacious midgut. The barnacle does not possess true gills; instead, gas exchange occurs across the surface of the cirri and the inner mantle lining.
The Secret to Sticking
The barnacle’s ability to permanently anchor itself is facilitated by a unique biological adhesive, recognized as one of the strongest natural glues. This permanent attachment is achieved through the secretion of a specialized cement, a complex, water-resistant protein mixture. The cement is produced by specialized organs called cement glands.
In acorn barnacles, these glands are clustered near the base of the animal, where the shell meets the substrate. The liquid cement travels through a network of ducts to the adhesion interface, where it cures rapidly even in a wet environment. This compound ensures the animal remains firmly fixed, resisting the tremendous forces of waves and currents. The larval stage, known as the cyprid, uses a temporary adhesive to explore surfaces before secreting the permanent cement to initiate metamorphosis. The strength and water-resistance of this natural bioadhesive are a subject of ongoing research for potential medical and industrial applications.