Whales are often seen adorned with crusty, white patches. These are colonies of barnacles, small marine organisms that find a unique home on the skin of these mammals. This common sight raises questions about the relationship between whales and their barnacle hitchhikers.
Barnacle Biology and Attachment
Barnacles are not mollusks, but rather crustaceans, making them relatives of crabs and lobsters. Their life cycle involves distinct larval stages before they become the sessile (non-moving) adults commonly observed.
The first larval stage, called a nauplius, is free-swimming and planktonic, feeding in the water column. After several molts, the nauplius transforms into a cyprid larva. The cyprid larva is a non-feeding stage, primarily locating a suitable surface for permanent attachment. It explores the surface using specialized antennae, then attaches head-first by secreting a powerful, permanent adhesive, known as barnacle cement. This glue, primarily composed of proteins, is strong, enabling the barnacle to bond to almost any underwater surface, including a whale’s skin. As it matures, the barnacle builds a hard shell of calcified plates around its soft body, anchoring itself firmly for life.
Why Whales Are Preferred Hosts
Whales offer an ideal habitat for certain barnacle species. Their large size provides a stable surface area for attachment. Many baleen whales, primary hosts for whale barnacles, are filter feeders that move slowly through plankton-rich waters. This behavior provides barnacles with a constant food supply as the whale swims, allowing them to extend their feathery cirri (feeding appendages) to capture passing food particles.
Some barnacle species are highly specialized, evolving to live exclusively on certain whale species. For instance, Coronula diadema is a species of whale barnacle predominantly found on humpback whales, attaching to areas like the head, flippers, and flukes. Another example is Cryptolepas rhachianecti, which lives only on gray whales. The whale’s migration routes also serve to disperse barnacle larvae over great distances, increasing their chances of finding new hosts and suitable environments.
Impact on Whales and the Nature of the Relationship
The relationship between barnacles and whales is generally considered commensal; barnacles benefit while the whale is largely unaffected. Barnacles gain a stable place to live, protection from predators, and a continuous food supply without relying on the whale for nutrition. While a single humpback whale can carry hundreds of kilograms of barnacles, this weight is negligible compared to the whale’s size, typically causing no significant burden.
Despite the commensal classification, there can be minor impacts on the whale. Barnacles might slightly increase hydrodynamic drag, potentially requiring the whale to expend more energy while swimming. In some cases, they can cause minor skin irritation or discomfort, particularly if numerous or if they detach, leaving small wounds. However, whales typically have strong immune responses that prevent severe infections. Some theories suggest the hard barnacle shells could offer protection against predators like orcas or be used as armor during fights, though these benefits are still debated.