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. ophiophilus, has remained unclear. Here, we reassess the systematics of the genus through an integrative approach, combining morphological examinations with molecular phylogenetic analyses of mitochondrial COI and nuclear H3 genes from specimens collected from sea turtles, sea snakes, and a dugong. Our results unequivocally demonstrate that P. ophiophilus is a valid species, phylogenetically and morphologically distinct from the widespread turtle barnacle, P. hexastylos. Furthermore, we report the first record of P. ophiophilus 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 strongly suggests that the ancestor of Platylepas was associated with sea turtles, a conclusion supported by the ichnofossil record. The subsequent divergence of the P. ophiophilus clade represents a major macroevolutionary event: an ancient host shift from sea turtles to the taxonomically disparate but ecologically linked inhabitants, sea snakes and sirenians, of seagrass ecosystems. This study not only resolves a long-standing taxonomic problem but also provides a compelling narrative of host-switching that reconciles deep molecular divergence estimates with the sparse body fossil record for this fascinating group of marine symbionts.