Barnacles are marine crustaceans, relatives of crabs and lobsters, even though their hard, volcano-shaped shells make them appear more like mollusks. As filter-feeding organisms, adult barnacles are sessile, meaning they permanently attach themselves to surfaces like rocks, ship hulls, and the skin of large marine animals. The common sight of barnacles encrusting slow-moving whales, particularly humpback and gray whales, highlights a unique biological relationship. This association is a specialized strategy that allows the barnacle to thrive in the open ocean, raising questions about the benefits to the barnacle and the cost to the whale.
The Barnacle’s Strategy: Why Whales Are Ideal Hosts
The relationship between the whale and its barnacle is generally understood as commensalism, a form of symbiosis where one organism benefits while the other is largely unaffected. Since the barnacle is fixed in place once it settles, the whale represents a highly mobile, living substrate. This solves the challenge of finding food and dispersing larvae, as the whale constantly transports the barnacles through nutrient-rich waters during migration.
The constant movement ensures an abundant supply of plankton, which the barnacles filter from the water using their feathery, leg-like appendages called cirri. The whale’s movement also creates a consistent flow over the barnacle, enhancing its ability to filter-feed. Attachment to the whale also protects the barnacles from many of their own predators, such as carnivorous sea snails or gulls found on stationary surfaces.
The distribution of whale barnacles, such as Coronula diadema on humpback whales, is highly specialized and host-specific. This relationship helps ensure that when barnacles reproduce and release larvae, the offspring are released in geographic areas where other potential hosts are congregating. The barnacle life cycle is closely synchronized with the migration and breeding patterns of their specific whale species, maximizing the chance of finding a new host.
Mechanics of Attachment and Growth
The process of colonization begins with the barnacle’s final larval stage, known as the cyprid, which is free-swimming and highly selective about where it settles. This microscopic larva actively explores the whale’s skin, using specialized sensory antennules to detect chemical cues from the host and assess the surface texture. Once a suitable location is identified, the cyprid secretes a temporary adhesive protein to anchor itself while it undergoes metamorphosis.
The larva then transforms into a juvenile barnacle, developing its permanent calcified shell and secreting a specialized biological cement. This powerful adhesive is a protein-based substance that cures underwater, forming an incredibly strong, cross-linked matrix to bond the barnacle to the living tissue. The attachment of whale barnacles from the family Coronulidae is robust because their shells are designed to embed deeply into the whale’s thick outer layer of skin.
Species like Coronula form crown-shaped shells with chambers or prongs that grow into the skin. The whale’s skin then grows up around the barnacle’s base, effectively locking it in place. This embedding strategy provides the stability required to withstand the immense hydrodynamic forces exerted by a fast-moving, multi-ton host in the ocean.
The Whale’s Perspective: Is it Harmful?
While the relationship is often defined as commensal, the presence of barnacles introduces physiological costs for the host. The most significant consequence is the increase in hydrodynamic drag caused by the rough, textured colonies on the whale’s otherwise streamlined body. A single humpback whale can carry up to 450 kilograms (nearly 1,000 pounds) of barnacles, which requires the whale to expend more energy for routine activities like swimming, feeding, and long-distance migration.
The physical presence of the barnacles can also cause superficial skin damage where the shell embeds itself. The whale’s immune system typically manages the intrusion, and the continuous shedding of the outer skin layer helps prevent severe infection or naturally sloughs off some barnacles. Some whales, like the gray whale, have been observed rubbing themselves against the seafloor or rocks, a behavior thought to actively dislodge the encrusting organisms.
Colonization varies greatly among whale species, correlating strongly with their swimming speed. Faster, more pelagic species like blue whales generally have smooth skin and very few barnacles, as their speed makes settlement difficult for the larvae. Conversely, slower-moving species such as humpback and right whales often carry the heaviest loads, suggesting their lifestyle makes them more susceptible to colonization.