Barnacles are marine crustaceans, related to crabs and lobsters, that spend their adult lives attached to surfaces. Most species are filter feeders, extending feathery appendages called cirri to capture plankton and detritus from the surrounding water. They are marine invertebrates, with over 2,100 described species found predominantly in shallow and tidal waters. While many attach to rocks or man-made structures, a significant number also settle on various marine animals.
How Barnacles Attach
Barnacles possess an adhesive system, allowing them to anchor to diverse substrates. After a free-swimming larval stage, known as a cyprid, the barnacle searches for a suitable location. Once chosen, it attaches head-first using specialized antennules and secretes a powerful, fast-curing glycoproteinous cement.
This natural glue, produced by cement glands, is considered one of the strongest known biological adhesives, exhibiting properties superior to many synthetic products. The cement forms a robust bond, allowing the sessile adult barnacle to remain in place for its entire lifespan. This ensures they maintain position for feeding and reproduction.
Animals Affected and Types of Interactions
Barnacles commonly attach to various marine animals, including large mammals like whales, reptiles such as sea turtles, and other crustaceans like crabs. Their relationship with hosts can vary. Many barnacle species engage in commensal relationships, where the barnacle benefits by gaining a stable surface, access to nutrient-rich water as the host moves, and protection from predators, while the host is largely unaffected. Whale barnacles, for instance, attach to baleen whales and are considered commensal, simply hitching a ride.
However, not all barnacle-host interactions are benign. Some species are parasitic, harming their hosts. An example is the Sacculina barnacle, a parasitic castrator of crabs that alters the host’s biology and behavior by forming a root-like system throughout the crab’s body to absorb nutrients. Burrowing barnacles (order Acrothoracica) also represent a parasitic form, boring into calcareous material like mollusk shells or coral, and can damage the host’s shell or tissue.
Mechanisms of Harm
When barnacles attach to animals, they can cause several forms of harm, particularly with heavy infestations or specific parasitic species. One impact is increased hydrodynamic drag, forcing the host animal to expend more energy for movement. For whales, a large number of barnacles can increase drag by as much as 60%, requiring up to 40% more fuel to swim efficiently. This added resistance can hinder a whale’s ability to hunt, migrate, and other activities.
Physical damage is another mechanism of harm. While many barnacles attach superficially, some species, especially burrowing types, can cause skin irritation, lesions, or even penetrate the host’s tissue. Sea turtles are susceptible to burrowing barnacles, which can create open wounds in their skin and shells, leading to secondary infections. These wounds weaken shell integrity and provide entry points for bacteria and fungi.
Barnacles can also obstruct sensory organs or orifices. In sea turtles, barnacle growth around the eyes or nostrils can impede vision, making it difficult to locate food or avoid dangers, and can hinder breathing. Heavy barnacle loads can also add considerable weight, further interfering with a sea turtle’s swimming ability and range of motion.
How Animals Cope
Marine animals have developed strategies to manage barnacle accumulation. Whales, for instance, rub against rough surfaces like rocks, buoys, or the gravelly seafloor to dislodge barnacles and shed dead skin. Breaching, where whales jump out of the water and crash back down, can also help dislodge barnacles. While these behaviors help, complete removal, especially of larger barnacles, can be difficult for whales due to their size and inability to reach all parts of their bodies.
Sea turtles, particularly green sea turtles, cope with barnacles by shedding layers of their skin. This exfoliation helps dislodge attached barnacles. Loggerhead sea turtles, however, are more prone to barnacle infestations than green sea turtles, possibly due to differences in skin shedding or shell characteristics.
Crabs, as crustaceans, undergo molting, shedding their entire exoskeleton. This shedding allows them to discard attached barnacles, providing a clean surface for growth. Since molting occurs periodically, it effectively manages external biofouling.