Morphological variation of the cirri and penis in the hermaphroditic sea-turtle barnacle Chelonibia testudinaria (Linnaeus, 1758) (Cirripedia: Balanomorpha: Chelonibiidae).

This article investigates the phenotypic plasticity of the penis and cirri morphologies, as well as fertilization rates, in the epizoic sea-turtle barnacle Chelonibia testudinaria (Linnaeus, 1758) across different positions and crowding levels on the carapace of loggerhead sea turtles (Caretta caretta). The study found that barnacles were most abundant on the posterior carapace and that penis size and cirri dimensions increased with barnacle body size and varied among individual turtles and carapace positions, though penis morphology showed less plasticity than in intertidal barnacles. Crowding did not significantly affect penis or cirri morphology, nor fertilization rates, although barnacles in clusters had a higher fertilization probability than solitary individuals. The authors suggest that the ability of C. testudinaria to move on the host and the presence of complemental males may reduce the need for pronounced morphological plasticity seen in intertidal species.