Barnacles clinging to the skin of whales are a common and often striking sight in marine environments. These crustacean hitchhikers appear to be an integral part of a whale’s life, prompting curiosity about the nature of their relationship. What compels these small organisms to attach to such colossal, free-roaming hosts, and what are the implications for the whales themselves?
The Commensal Relationship
The interaction between barnacles and whales is a form of symbiosis known as commensalism. In a commensal relationship, one species benefits significantly, while the other is neither notably harmed nor helped. For the barnacle, the whale serves as a mobile habitat, providing advantages without imposing a substantial cost on its host. This arrangement contrasts with mutualism, where both organisms benefit, or parasitism, where one benefits at the expense of the other. Whales carry these attached organisms with minimal impact on their well-being.
Barnacle Advantages
Barnacles gain several advantages by attaching themselves to whales. A primary benefit is a continuous food supply. As filter feeders, barnacles extend feather-like appendages, called cirri, to strain microscopic organisms like plankton from the water. The whale’s constant movement through nutrient-rich waters ensures a steady flow of food past the barnacles, effectively providing a mobile buffet. This arrangement also offers transportation to new feeding grounds, allowing them to cover vast oceanic distances without expending energy.
Beyond nutrition and mobility, attachment to a whale protects barnacles from predators that might otherwise target them if they were fixed to stationary surfaces. The whale’s size deters many threats. Additionally, being on a living host keeps barnacles in consistently submerged, suitable marine environments, avoiding risks like desiccation that barnacles on intertidal rocks might face.
The Attachment Process
Barnacles undergo a specialized process to attach to whales. They begin their lives as tiny, free-swimming larvae called nauplii. After several molting stages, they develop into the cyprid larval stage, a non-feeding, settlement-seeking stage. This cyprid larva finds a suitable surface for permanent attachment.
Upon encountering a whale, the cyprid larva uses specialized sensory antennules to explore the host’s skin, often seeking areas with consistent water flow, such as the head, fins, or around the genital region. Chemical signals from the whale’s skin may also help identify a suitable host. Once a preferred spot is identified, the larva secretes an incredibly strong, protein-based adhesive. This biological glue, known for its strength underwater, firmly anchors the barnacle. As the barnacle matures, it develops hard, protective shell plates. In many species, the whale’s skin tissue grows into cavities within the barnacle’s shell, creating an exceptionally secure bond.
Whale Well-being
For whales, the presence of barnacles is generally considered to have a neutral or minimal impact. Unlike parasites, barnacles do not feed on the whale’s tissues or blood. Whales possess thick skin and a substantial layer of blubber, providing a robust barrier against barnacle attachment and preventing tissue damage. While a heavy infestation of barnacles, sometimes weighing hundreds of kilograms, could theoretically increase drag and affect swimming efficiency, this is typically not a substantial burden for a whale’s size.
Large barnacle clusters might cause minor skin irritation or, in rare cases, localized inflammation. However, whales engage in behaviors like breaching or rubbing against the seafloor, which may help dislodge barnacles and shed old skin. While primarily commensal, some theories suggest barnacles could offer slight protective armor during fights between male whales or deter predators like orcas, though this remains under discussion.