A rapid and parallel Late Pleistocene/Holocene morphological radiation in a predaceous planktonic water flea: the case of <i>Bythotrephes</i> (Cladocera: Cercopagididae)

The predatory cladoceran Bythotrephes is one of North America’s most successful and impactive invasive species in freshwater plankton communities. The taxonomic status of the genus Bythotrephes Leydig, 1860 (Crustacea: Cladocera: Cercopagididae) has remained unclear and a subject of intensive debate for over 150 years. We applied an integrative taxonomy approach with multi-gene analysis (mitochondrial COI and 12S genes, and nuclear 18S and 28S genes) on 80 individuals (representing at least four morphospecies) from various regions spanning the genus distribution (North America, Europe, and Asia) to resolve the taxonomic status of species within Bythotrephes. The results of our study strongly support the hypothesis of a single species—Bythotrephes longimanus Leydig, 1860, and ecological morphs should be accepted as junior synonyms. Our work also elucidates the very recent radiation of Bythotrephes, which may have begun rapidly and parallel during the Late Pleistocene, or even after the last glaciation. Finally, we provide a comprehensive biogeographic reconstruction of Bythotrephes dispersal within the Holarctic realm. Europe likely served as a dispersal centre for Bythotrephes from where they spread relatively recently (possibly, less than 10 kyr) and we distinguish five possible dispersal events in its evolutionary history.