Species boundaries, host associations, and possible host shifts in the turtle barnacle genus Platylepas

The barnacle genus Platylepas contains obligate epibionts of marine vertebrates, but the taxonomic status and evolutionary history of the sea-snake specialist, P. ophiophila, have remained unclear. Here, we reassess species boundaries and host associations in Platylepas using shell-morphological comparisons and sequence data from mitochondrial COI and nuclear H3 genes from specimens collected from sea turtles, sea snakes, and a dugong. Our results show that P. ophiophila is a valid species, phylogenetically and morphologically distinct from the widespread turtle barnacles P. decorata and P. hexastylos. Furthermore, we report the first record of P. ophiophila from a sirenian host (Dugong dugon), a discovery that necessitates the re-examination of historical records of barnacles on sirenians worldwide. Our phylogeny places P. decorata, a turtle epibiont, as the earliest diverging lineage within the genus. This is consistent with the hypothesis that the ancestor of Platylepas was associated with sea turtles, a conclusion supported by the ichnofossil record. The subsequent divergence of the P. ophiophila clade may represent an important host-use transition: an ancient host shift from sea turtles to the taxonomically disparate but ecologically linked inhabitants--sea snakes and sirenians--of seagrass ecosystems. This study clarifies species limits within Platylepas and provides a framework for discussing host associations and potential host shifts in light of existing molecular and fossil evidence.