The vast, open ocean is home to countless biological partnerships, known collectively as symbiosis. One of the most common examples involves the great baleen whales and the small, shelled organisms that cling to their skin. The relationship between a massive whale and its barnacle passengers serves as a classic illustration of commensalism.
Defining Commensalism in Ecology
Commensalism describes an interaction between two organisms where one species benefits while the other is neither helped nor harmed. This is often represented in biology as a “+/0” relationship, distinguishing it from the two other main forms of symbiosis. Mutualism, a “+/+” relationship, occurs when both participating species receive a benefit.
In contrast, parasitism is a “+/-” interaction, where one organism benefits at the direct expense of the other. The whale and barnacle relationship fits the definition of commensalism because the barnacle gains advantages from its host, which experiences no measurable detrimental effect.
The Barnacle’s Gain: Locomotion and Resource Access
For the barnacle, being permanently attached to a whale represents an enhancement of its survival chances, providing benefits it could not achieve on its own. Barnacles are sessile invertebrates, meaning they are fixed in one location as adults. Species that live on whales, such as Coronula diadema, gain mobility, receiving transportation across thousands of miles of ocean to nutrient-rich feeding grounds and reproduction areas.
The constant movement of the whale also increases the efficiency of the barnacle’s filter-feeding apparatus. As the host swims, a steady stream of water delivers plankton and other microscopic food particles directly to its extended feathery appendages, called cirri. This passive flow of food allows the barnacle to grow rapidly without expending energy on seeking currents. Furthermore, residing on the massive whale provides protection from benthic predators like carnivorous snails and sea stars.
The Whale’s Neutrality
The whale’s role in this commensal relationship is defined by its lack of a significant reaction, representing the “zero” in the interaction. The vast size and weight of a great whale, such as a Humpback or Gray whale, render the physical burden of the barnacle colonies negligible. Even though a single whale can carry hundreds of kilograms of barnacles, this weight is insignificant compared to the host’s massive size.
The claim that barnacles increase hydrodynamic drag and impede movement is considered a minor factor, especially for slower-moving baleen species. The barnacles do not behave as parasites because they feed exclusively by filtering water and do not penetrate the whale’s living tissue. While barnacles embed themselves into the whale’s thick, non-sensitive outer skin layer for stability, this process does not cause injury or impact the whale’s fitness or survival.
The Mechanics of Attachment and Growth
The process begins when the barnacle, in its final larval stage known as a cyprid, searches for a suitable host. These larvae are attracted to chemical cues released from the whale’s skin, often settling during the breeding season when the animals move slowly in warm, shallow waters. Once a cyprid finds a suitable spot, it secretes a powerful bio-cement to anchor itself permanently to the whale’s skin.
The barnacle then undergoes metamorphosis and begins to build its protective shell, which is made of calcium carbonate and chitin. For species like Coronula diadema, the shell grows in a crown-like shape, featuring chambers that the whale’s epidermal tissue grows into, securing the attachment against strong water flow. Barnacles tend to colonize areas on the whale’s body that experience consistent water movement, such as the head, flippers, and throat, maximizing their filter-feeding opportunities.