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

This article focuses on resolving the long-debated taxonomic status and evolutionary history of the predatory cladoceran genus *Bythotrephes* Leydig, 1860, a highly invasive freshwater plankton species in North America and Eurasia. Using an integrative taxonomy approach combining morphological analyses with multi-gene sequencing (mitochondrial COI and 12S, nuclear 18S and 28S genes) of specimens from North America, Europe, and Asia, the study supports the hypothesis that *Bythotrephes* constitutes a single species, *Bythotrephes longimanus*, with multiple ecological morphotypes considered junior synonyms rather than distinct species. Molecular clock and biogeographic reconstructions indicate a recent radiation and dispersal of *Bythotrephes* from Europe within the Holarctic realm, with North American populations resulting from recent anthropogenic introductions. The findings highlight significant phenotypic plasticity within *Bythotrephes* and suggest that morphological variation reflects ecological adaptation rather than speciation.