Barnacles are common on sea turtles, appearing as white, volcano-shaped structures fixed to the reptile’s shell and skin. This phenomenon is known as epibiosis, where one organism lives harmlessly on the surface of another. The barnacle is a type of crustacean, related to crabs and lobsters, that adopts a sedentary lifestyle once it reaches adulthood. These filter-feeders use specialized feathery appendages called cirri to sweep plankton and detritus from the surrounding water.
Why Sea Turtles Are Attractive Hosts
From the barnacle’s perspective, a sea turtle provides an ideal, mobile habitat. The primary advantage is constant transport to nutrient-rich waters, ensuring a continuous supply of food. As the turtle swims, the water flow across its carapace delivers a steady stream of plankton, maximizing the barnacle’s foraging efficiency. This relationship is typically considered commensal, meaning the barnacle benefits while the turtle is neither helped nor harmed.
The turtle’s hard shell, or carapace, offers a large, stable surface for settlement. Barnacles attached to the shell also gain protection from bottom-dwelling predators that might consume them on stationary substrates. Furthermore, the turtle’s migratory routes and long lifespan provide the barnacles with an expanded range and habitat stability.
How Barnacles Anchor Themselves
Barnacles begin life as free-swimming larvae. The final larval stage, known as the cyprid, is non-feeding and exists to find a suitable surface for permanent attachment. The cyprid explores potential settlement sites using specialized antennules equipped with sensors to detect chemical and textural cues. Once a favorable location on the turtle’s keratinous scutes is chosen, the larva attaches itself head-first.
The attachment relies on a powerful bio-adhesive secreted by cement glands near the antennules. This biological cement is a complex, insoluble proteinaceous compound that cures rapidly even underwater. It creates a tenacious bond with the shell. While most barnacles are permanently sessile, some specialized species, like Chelonibia testudinaria, can exhibit slight movements by partially dissolving and re-secreting their cement, leaving a faint trail on the shell.
Health Consequences for the Turtle
While a light load of barnacles is manageable, a heavy infestation imposes significant health costs. The accumulation of barnacle shells dramatically increases the turtle’s hydrodynamic drag. This heightened drag forces the animal to expend much more energy for swimming and foraging, which can lead to exhaustion and a compromised body condition.
Specific species of burrowing barnacles, such as Platylepas hexastylus, can infiltrate soft tissues around the flippers or eyes. This creates open wounds vulnerable to secondary bacterial infections. A heavy bioburden is often considered a sign of a debilitated or sick turtle, as healthy, active turtles are better able to prevent colonization. Barnacles growing near the eyes or mouth can also physically impede the turtle’s vision and ability to feed.
Natural Methods of Barnacle Removal
Sea turtles use several active and passive strategies to manage or shed barnacles. One common active method is physical abrasion, where the turtle intentionally rubs its shell and soft tissues against hard substrates. They scrape against rocks, coral heads, or submerged logs to dislodge the attached crustaceans.
Turtles also engage in basking, either by floating near the surface or hauling out onto rocks or sand in shallow areas. Exposure to the sun and air causes the barnacles to dry out, weakening their adhesive grip and killing the organisms. Once weakened, the barnacles are easier for the turtle to scrape off or are naturally sloughed off with the next molt of the scutes.