Barnacles, common marine crustaceans, frequently attach to various underwater surfaces, including the shells and skin of marine animals. This association raises questions about the nature of these interactions and their effects on the host animals.
Barnacle Basics: Anatomy and Attachment
Barnacles are sessile crustaceans, typically encased within a hard, protective shell. They attach to a substrate by their “forehead,” using modified antennae that secrete a powerful natural adhesive known as barnacle cement. This cement, primarily protein-based, allows for strong and permanent underwater attachment, even to moving surfaces like animals.
Once attached, most barnacles are filter feeders. They extend feathery, jointed appendages called cirri through an opening in their shell to sweep plankton and other particles from the water. This feeding strategy necessitates a stable attachment point, which host animals can provide, allowing the barnacles access to currents rich in food.
Physical and Physiological Impacts on Hosts
Barnacles can cause various physical and physiological impacts on hosts. A common physical effect, particularly for fast-moving marine creatures like whales and sea turtles, is increased hydrodynamic drag. A heavy covering of barnacles can significantly increase a host’s surface area and weight, requiring more energy for movement. For example, substantial barnacle growth on sea turtles can hinder their swimming ability and potentially affect their capacity to surface for air or escape predators.
Beyond drag, barnacles can cause physical irritation and tissue damage at attachment sites. Some species of barnacles can burrow into a host’s skin or shell, creating entry points for infections or causing discomfort. While less common, a large number of barnacles on sea turtles might even obstruct their vision or nostrils, leading to more severe problems. The extent of these impacts often depends on the barnacle species, the number of barnacles, and the host’s overall health, with weakened animals often experiencing greater burdens.
Commensalism vs. Parasitism: Defining the Relationship
The relationship between barnacles and their animal hosts can vary, falling into categories of commensalism or, less frequently, parasitism. Commensalism describes an interaction where one organism benefits, and the other is neither helped nor harmed. Many barnacle-animal associations, like those with whales, are considered commensal. The barnacles benefit from a stable place to live, transportation through nutrient-rich waters for feeding, and protection from some predators, while the whale is minimally affected.
However, not all barnacle-animal relationships are commensal; some are parasitic, where one organism benefits at the expense of the other. A parasitic example is Sacculina barnacles, which infect crabs. Adult Sacculina bear little resemblance to typical barnacles, having a highly reduced body. The female Sacculina larva injects itself into a crab, developing a root-like system to absorb nutrients. This parasitic invasion castrates the crab, prevents molting, and can even manipulate the crab’s behavior, causing male crabs to develop female characteristics and care for the barnacle’s reproductive sac as if it were their own eggs.
Host Adaptations and Countermeasures
Marine animals cope with barnacle attachments through various strategies. Behavioral adaptations are common, such as whales engaging in breaching or rubbing against the seafloor, rocks, or even other whales to dislodge barnacles. Sea turtles also exhibit self-grooming behaviors, rubbing against sessile structures to remove epibiota.
Physical adaptations also play a role in managing barnacle loads. Whales, for instance, naturally shed their outer layer of skin, which dislodges barnacles and other parasites. Similarly, sea turtles can shed their scutes, the outer layers of their shells, which aids in removing attached barnacles. These natural processes, combined with behavioral efforts, help hosts maintain their health and reduce the negative impacts.