Taxonomic resolution of the genus Bythotrephes Leydig using molecular markers and re-evaluation of its global distribution

Bythotrephes Leydig is a predatory, onychopod cladoceran native to Eurasia that typically inhabits oligo- and mesotrophic water bodies of the Palaearctic region. It recently invaded 70 North American lakes, prompting a re-evaluation of the taxonomic status, global distribution, and determinants of local occurrence and abundance. European studies have reported two distinct species, B. longimanus, which lacks a kink on the caudal process, and B. cederstroemi, which possesses one. We employed sequencing of the mitochondrial gene cytochrome c oxidase subunit I (COI) in addition to Directed Amplification of Minisatellite-region DNA (DAMD) using polymerase chain reactions (PCRs) to assay the genetic nature of these 'species'. These analyses revealed that the two taxa are consistent with a single, common species, Bythotrephes longimanus Leydig 1860, thus resolving the nomenclatural issue. Furthermore, a common haplotype was identified between Lake Ontario and Lake Puruvesi, Finland, suggesting a potential invasion corridor via the nearby Baltic Sea. Statistical analysis revealed that the two forms also occur in similar habitats in Eurasia and North America. Habitat characteristics of water bodies in Northern Europe, where both forms are found and occasionally co-occur, do not differ statistically. Similarly, no significant differences were detected between characteristics of Eurasian habitats that support longimanus 'forms' and those of inland lakes in North America that support cederstroemi 'forms'. Human activities have had a strong effect on the distribution of Bythotrephes in both Europe and North America. Global and local distributions have been affected by ballast water transfer and by boating and fishing activities, respectively. Cultural eutrophication, oligotrophication and acidification alter the suitability of habitats for fish predators, and may indirectly influence Bythotrephes' local occurrence and abundance.