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

Phenotypic plasticity of the penis and cirri of acorn barnacles is well documented for intertidal species. We investigated whether these phenomena also occur in the epizoic sea-turtle barnacle, Chelonibia testudinaria (Linnaeus, 1758). We compared the morphologies of the penis and cirri, and fertilization rates for adult, hermaphroditic barnacles on different regions of the carapace of sea turtles, and at different levels of crowding on the carapace. Barnacles were more abundant in the posterior position of loggerhead sea turtles, Caretta caretta (Linnaeus, 1758), followed by the central, central costal, and anterior positions. Barnacles in clustered groups were fertilized 68% more often than solitary individuals. Penis size increased with barnacle size and among individual sea turtles. Cirrus length and width also varied with barnacle size, among individual sea turtles, and among positions. Larger barnacles had longer and wider feeding cirri, and cirri were longer and wider in the central and posterior positions of the carapace than in the anterior and central costal positions. The phenotypic plasticity of the morphologies of the penis and cirrus is far less dramatic in Ch. testudinaria than in intertidal species. This may be explained by the epizoic barnacles not being permanently fixed and able to move to preferred positions on the host shell.