The patterns of mitochondrial genetic diversity were studied in Chelonibia caretta, commensal barnacles of the endangered hawksbill sea turtle Eretmochelys imbricata from Mona Island of Puerto Rico. Two mitochondrial genes were sequenced: the large subunit of ribosomal RNA (16S rRNA) and the cytochrome oxidase subunit I (COI). Haplotypic diversity (h) for 16S (N = 34) and COI (N = 26) varied from 0.763 to 0.468, respectively. The nucleotide diversity (π) of 16S/COI (0.00284/0.0013) of C. caretta was low compared to all other published sequences from population studies of barnacles attached to fixed substrate. However, the estimated levels of π from C. caretta were comparable to those reported from C. testudinaria, an epibiont barnacle on loggerhead turtles. Analysis of molecular variance and parsimony network analysis of the sampled turtle epibionts from Mona Island indicated that they are genetically homogeneous, irrespective of the host or its gender. Albeit that the sampling size is small, our results are consistent with the general prediction that higher rates of dispersal reduce population divergence and estimates of nucleotide diversity. Conservation concerns are raised about the status of Chelonibia caretta whose host (the hawksbill sea turtle) has been included in the Endangered Species List.
