Sea turtles often glide through the ocean with small, shelled organisms securely fastened to their carapace. This phenomenon, known as epibiosis, describes one organism living on the surface of another. The most common hitchhiker is the barnacle, a crustacean that has evolved a specialized strategy to colonize the turtle’s hard shell. This attachment involves a unique ecological partnership, a complex larval life stage, and a powerful adhesive.
The Commensal Relationship
The relationship between the sea turtle and the barnacle is frequently classified as commensalism, an interaction where one species benefits while the other is generally unaffected. The barnacle, a sessile filter-feeder in its adult form, derives a significant advantage from this association by solving the problem of mobility and food access. By attaching to the turtle’s shell, or carapace, the barnacle gains continuous, slow-speed transport across vast stretches of the ocean.
This constant movement ensures the barnacle is swept through nutrient-rich water, providing a steady supply of plankton and organic particles. The turtle’s hard, non-shedding keratinous shell provides an ideal, stable substrate that is constantly submerged, unlike rocks or other fixed surfaces. Barnacle species, such as those from the genus Chelonibia, have evolved to exclusively colonize sea turtles, relying on the reptile’s body as their permanent habitat and transport mechanism.
Barnacle Larval Settlement and Attachment
The process of a barnacle finding and adhering to a sea turtle is a remarkable feat of marine biology, centered on the final larval stage. Barnacles begin their life as free-swimming nauplius larvae, which eventually molt into the non-feeding cyprid larva. This cyprid stage is the dispersal and settlement phase, actively searching for a suitable, textured surface to colonize.
The cyprid uses specialized attachment organs on its antennae to explore the turtle’s carapace, often seeking out micro-pockets or existing surface textures for better grip. Once a suitable spot is identified, the larva undergoes metamorphosis and secretes a powerful, proteinaceous adhesive known as barnacle cement. This substance is exceptionally strong, allowing for permanent, irreversible bonding to the turtle’s keratinous scutes, even in the high-energy environment of the open ocean.
Some specialized barnacle species embed or even burrow into the host’s skin or shell material. This mechanism is driven by the need for a stable and secure hold. Once cemented, the barnacle remains stationary for life, relying entirely on the turtle for food and transport.
How Barnacles Impact Sea Turtle Health
While a light load of barnacles is benign, heavy colonization can negatively affect the turtle. The accumulation of many barnacles significantly increases the turtle’s overall body weight and hydrodynamic drag. This added resistance forces the turtle to exert more energy for swimming, slowing its movement and reducing efficiency for foraging and migration.
The location of the barnacles also determines the severity of the impact, as growths near the head, eyes, or mouth can directly interfere with the turtle’s vision and feeding abilities. Furthermore, certain species of barnacles that burrow or embed their structure into the shell can cause physical damage, leading to shell erosion or creating open wounds. These penetration points can then become sites for secondary bacterial infections, particularly in turtles with compromised immune systems.
Excessive barnacle coverage is often an indicator of an underlying health problem rather than the primary cause of debilitation. Sick or weakened turtles, which are less active and spend more time floating at the surface, are unable to perform natural self-grooming behaviors, such as scraping their shells against rocks. This reduced activity provides an opportunistic environment for barnacle larvae to settle and rapidly proliferate, resulting in the heavy loads frequently observed on stranded or ill animals.