Barnacles appear as pale, rough patches that dot the dark surfaces of many ocean giants. These organisms, related to crabs and lobsters, spend their adult lives permanently anchored to a surface in the marine environment. The presence of these shelled creatures on a massive, constantly moving host prompts curiosity about how they establish this relationship and what it means for the whale.
Defining the Specialized Whale Barnacle
Whale barnacles are not the same organisms found clinging to coastal rocks or the hulls of ships; they belong to specialized genera, such as Coronula and Cryptolepas, which have evolved exclusively to inhabit cetaceans. These organisms are crustaceans, classified in the subclass Cirripedia, characterized by their sessile, or stationary, adult life. Unlike relatives that attach to inanimate surfaces, whale barnacles are obligate symbionts, meaning they must live on a whale to survive.
The physical structure of these barnacles is highly adapted for their host. They construct a hard outer shell of calcium carbonate and chitin, which protects the soft-bodied animal inside. This shell is deeply embedded into the whale’s skin tissue, securing the barnacle firmly in place against the immense forces of the ocean. The whale’s outer skin layer actually grows around and into the base of the barnacle, providing a stable, interlocking attachment that withstands the whale’s powerful swimming movements.
The attachment begins when the barnacle is in its larval stage, a tiny, free-floating organism known as a cyprid. This larva detects chemical signals from the whale’s skin, often settling on the host when whales are moving slowly in warm breeding waters. Once attached, the larva secretes a powerful natural adhesive, cementing itself to the dead outer layers of the whale’s skin before developing its protective, multi-plated shell.
The Commensal Relationship and Benefits
The relationship between the whale and the barnacle is classified as commensalism, an interaction where one species benefits while the other is generally unaffected. For the barnacle, the benefits of this arrangement are significant, solving the challenges of a sessile filter-feeder in the open ocean. The whale acts as a mobile habitat, transporting the barnacle to nutrient-rich waters where food is abundant.
As the whale moves, it continuously pushes a current of water past the barnacle, creating ideal conditions for filter feeding. The barnacle extends its feathery, modified legs, called cirri, into this flow to strain plankton and other microorganisms. This constant, passive delivery of food means the barnacle expends minimal energy and is always moving toward better feeding grounds.
The whale also provides protection from predators that might otherwise target a stationary organism. Barnacles often congregate in specific areas, such as the head, flippers, and tail flukes, where the whale’s skin is relatively stable. Because the barnacles only anchor into the dead, outer layers of the whale’s skin, the host is not actively harmed or used as a food source, differentiating this from a parasitic relationship.
The Physical Impact on the Whale
While the relationship is generally considered benign, the presence of a large colony of barnacles introduces a physical cost to the whale, primarily through increased drag. The rough, uneven texture of the barnacles disrupts the smooth flow of water over the whale’s body, requiring the animal to expend more energy to swim at the same speed. For migrating species that travel thousands of miles, this cumulative energy expenditure can be significant.
A humpback whale, for example, can carry hundreds of pounds of barnacles, which acts like a constant, heavy backpack. This added weight and turbulence can slow the whale and potentially impact its ability to perform essential activities like hunting. Minor skin irritation and abrasions may also be caused by the continuous friction of barnacle clusters, though the whale’s thick skin offers considerable defense.
Barnacle shells also serve as forensic tracking devices for researchers. As a barnacle grows, its shell incorporates chemical signatures, specifically stable isotopes of oxygen, from the surrounding ocean water. Since the chemical makeup of ocean water varies based on location and temperature, analyzing the isotopic layers in a barnacle’s shell helps reconstruct the precise migration path and feeding history of the whale that carried it.