Barnacles are marine arthropods belonging to the subclass Cirripedia, making them cousins to crabs and lobsters, despite their hard, shell-like appearance. As adults, they are sessile, meaning they permanently attach themselves to a surface where they spend their lives filter-feeding from the water column. The relationships barnacles form with other animals are complex, ranging from a simple, one-sided benefit to a profoundly destructive form of biological manipulation for the host. These interactions depend entirely on the specific barnacle species and the host animal it encounters.
How Barnacles Anchor Themselves
The permanent attachment of a barnacle begins with the cyprid, the second and final free-swimming larval stage, whose sole purpose is to find a suitable settlement location. This larva uses specialized, modified antennules to explore surfaces, assessing factors like texture, chemistry, and the presence of surface biofilms. Once a favorable spot is located, the cyprid executes a head-first attachment to the substrate.
The larva then secretes a complex proteinaceous compound, known as barnacle cement, which is one of the strongest natural adhesives known. This biological glue is made of polyproteins and trace components like calcium, allowing the larva to permanently fix itself to the surface. The strength of this cement prevents the adult barnacle from being dislodged by strong currents or the rapid movement of a host animal. After cementing its position, the larva undergoes metamorphosis into the shelled juvenile form.
External Relationships: The Burden of Commensalism
For large, mobile marine animals like whales and sea turtles, the relationship with barnacles is a classic example of commensalism, where the barnacle benefits, and the host is largely unaffected, though often inconvenienced. The barnacle gains a stable, mobile habitat that transports it through nutrient-rich waters, enhancing its ability to filter-feed. Gray whales and humpback whales commonly carry dense clusters of barnacles, which are firmly embedded in the host’s outer skin layers.
While the host’s tissue is not consumed, the sheer biomass of these external passengers creates a substantial physical burden. A heavy accumulation of barnacles significantly increases the host’s hydrodynamic drag, forcing the animal to expend more energy to maintain its swimming speed. For migrating species like whales, this added resistance can negatively impact the efficiency of long-distance journeys, potentially affecting feeding and breeding success.
Sea turtles, such as the loggerhead, also serve as mobile substrates, often hosting over a hundred other species, including numerous barnacles, on their carapace. The weight and friction from the attached organisms can cause chronic skin irritation. In severe cases, the constant presence of barnacles may lead to abrasions or minor injuries on the softer skin around fins. Even without direct consumption of host tissue, the accumulated energy cost and physical discomfort transform the theoretically neutral commensal relationship into a notable physiological drain on the host animal.
Internal Relationships: Parasitic Tissue Invasion
A far more destructive interaction occurs with highly specialized barnacles in the infraclass Rhizocephala, such as the genus Sacculina, which are true parasites of crabs and other crustaceans. Unlike their shelled relatives, the adult Sacculina bears no resemblance to a typical barnacle, existing instead as a mass of tissue inside the host. The parasitic relationship begins when a female larva finds a host crab and injects its soft body into a joint, shedding its outer shell in the process.
This injected tissue develops into an intricate, root-like network called the interna, which spreads throughout the crab’s body cavity, wrapping around internal organs, including the hepatopancreas and digestive tract. The interna functions as a nutrient-absorbing system, drawing sustenance directly from the crab’s hemolymph, or blood. This process diverts energy away from the host’s normal biological functions. The parasite eventually forms an external reproductive sac, known as the externa, which emerges from the underside of the crab’s abdomen where the crab’s own eggs would typically be carried.
The most severe consequence of this invasion is parasitic castration, where the Sacculina completely destroys the host’s ability to reproduce, redirecting all reproductive energy to the parasite’s own growth. The parasite also manipulates the host’s hormonal balance, resulting in the feminization of male crabs.
Infected males develop broader abdomens and smaller claws, adopting the body language and maternal behaviors of an uninfected female. This includes the instinct to clean, aerate, and protect the parasite’s externa as if it were their own egg clutch. This profound alteration ensures the survival and reproductive success of the invading barnacle.