Ecophenotypic variation or genetic differentiation? Ambiguity of morphological and molecular relationships presents uncertainty in host-specific plasticity of Chelonibia barnacles

Abstract

The genus Chelonibia, informally referred to as the turtle barnacles, was historically subdivided into four species by both morphological differentiation and host affinity. More recent studies, however, hypothesized that three of these barnacle species – C. testudinaria, C. manati, and C. patula – comprise a single species, C. testudinaria, that exhibits host-specific ecophenotypic plasticity. In this study, we examined both morphological and molecular datasets to assess whether host attachment or genetic differentiation best explains the morphotypes assigned to C. testudinaria and C. manati. For morphology, we compared 11 mensural test and cirral characters from 71 C. testudinaria and 25 C. manati specimens identified by the previously established absence or presence of a ridged external dorsal surface. The two morphotypes overlapped in overall body size, although C. testudinaria averaged significantly larger and C. manati exhibited relatively longer cirri. Discriminant function analyses (DFA) showed strong but incomplete differentiation (92.6% classification success), with one C. manati morphotype collected from a sea turtle classified with high confidence as C. manati. Thus, the morphological data contradict the host-specificity hypothesis. Genetic analyses using three target loci (16s, 28s, COI) from the mitochondrial and nuclear genomes, as well as a single concatenated sequence of >1600 bp, likewise failed to support genetic differentiation between the two morphotypes within the Atlantic sample. With inclusion of a broader sample of Atlantic C. testudinaria and C. manati and Pacific C. testudinaria barnacles, we found strong population distinction between the ocean basins without morphotype separation. Results of the present study suggest that Balanomorph barnacle taxonomy, especially of endosymbiont species, requires further study, as neither phenotypic plasticity nor genetic differentiation accounted for existence of the two distinct morphotypes.