The sight of large, textured patches covering the skin of migrating whales is a common image for many ocean observers. These growths are colonies of barnacles, specialized marine organisms that have formed one of the most remarkable relationships in the sea. This biological partnership involves one species permanently relying on the other for survival. Understanding this unique arrangement requires examining the barnacle’s biology, the mechanics of its adhesion to a living host, and the ecological classification of their long-term association.
The Barnacle: A Unique Crustacean
Barnacles are not mollusks, as their hard, shell-like exterior might suggest, but are arthropods belonging to the subphylum Crustacea, making them relatives of crabs and shrimp. As adults, these animals are sessile, meaning they permanently affix themselves to a substrate, a life strategy that contrasts sharply with their mobile relatives. The barnacle’s body is encased in a protective shell composed of calcareous plates constructed from calcium carbonate.
When submerged and feeding, the organism opens a small aperture at the top of its shell, extending feathery appendages called cirri into the water column. These six pairs of modified thoracic legs beat rhythmically to filter plankton and minute food particles from the surrounding water. This mechanism necessitates a constant flow of water, making a large, mobile host like a whale an attractive home for these stationary organisms.
The Mechanism of Attachment and Transport
The permanent settlement of a barnacle onto a whale begins during its brief, free-swimming larval phase, specifically the cyprid stage. This microscopic larva uses specialized antennules to explore and “test” potential surfaces. For whale barnacles, the cyprid is often prompted to settle by a chemical cue released from the host’s skin, signaling an appropriate location for permanent attachment.
Once a suitable spot is identified, the larva secretes a specialized biological cement, one of the strongest natural adhesives known, to anchor itself head-first to the whale. This cement is a complex, protein-based compound that hardens into an insoluble matrix, creating a tenacious bond against powerful hydrodynamic forces. As the barnacle grows, its basal plate develops structures that embed deeply into the whale’s dead, outer layer of skin, ensuring a secure hold. Attachment secures perpetual, passive transport across vast ocean distances, carrying the barnacle to nutrient-rich waters where food is abundant.
Defining the Commensal Relationship
The long-term association between the whale and its barnacle passengers is categorized as commensalism, a specific type of symbiotic relationship. Commensalism occurs when one organism benefits from the interaction while the other is neither helped nor harmed. For the barnacle, the benefits are clear: consistent access to food, protection from benthic predators, and free transport.
The whale experiences minimal negative impact from carrying a colony of barnacles. The barnacles only embed themselves into the dead, non-innervated epidermal layers of the skin, not the living tissue beneath. While a large infestation can weigh up to 450 kilograms, this mass is negligible for an animal that weighs multiple tons, causing only a minor increase in drag.
Species Specificity and Host Adaptation
Not all barnacles attach to whales; those that do belong to the specialized family Coronulidae, which has evolved to live exclusively on cetaceans. These whale barnacles exhibit a high degree of host specificity, meaning certain species are only found on specific types of whales. For example, Coronula diadema is commonly associated with humpback whales, while other genera are endemic to right whales or gray whales.
The barnacles demonstrate distinct morphological adaptations that differ from typical coastal barnacles. Their shells are often star-shaped or feature ridged, crown-like plates designed to interlock with the grooves and folds of the whale’s skin. This shape maximizes the surface area for the adhesive, providing superior anchorage to withstand the immense water pressure and speed generated by the moving host. Furthermore, barnacles that embed into the skin of right whales can stimulate the growth of callosities, which are areas of roughened, calcified skin that aid in securing the barnacle’s position.