Integrative taxonomic revision of turtle barnacle genus Platylepas (Cirripedia: Coronuloidea): an ancient host-range expansion to sea snakes and sirenians
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.