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We examine the evolving concept of what constitutes a nonnative (or alien) freshwater fish. In an attempt to distinguish between biogeographical and socio-political perspectives, we review the patterns in the introduction and dispersal of nonnative fishes in Europe and North America, and especially the recent expansion of Ponto-Caspian gobies in Europe. We assess patterns in the development of national policy and legislation in response to the perceived threat of non-native fish introductions to native species and ecosystems. We review, and provide a glossary of, the terms and definitions associated with non-native species. Finally, we discuss perspectives as regards the future treatment of naturalized species.

BackgroundMolecular taxonomy studies and barcoding projects can provide rapid means of detecting cryptic diversity. Nevertheless, the use of molecular data for species delimitation should be undertaken with caution. Especially the single-gene approaches are linked with certain pitfalls for taxonomical inference. In the present study, recent and historical species descriptions based upon morphology were used as primary species hypotheses, which were then evaluated with molecular data (including in type and historical museum material) to form secondary species hypotheses. As an example of cryptic diversity and taxonomic controversy, the European Phoxinus phoxinus species complex was used.ResultsThe results of the revision showed that of the fourteen primary species hypotheses, three were rejected, namely P. ketmaieri, P. likai, and P. apollonicus. For three species (P. strandjae, P. strymonicus, P. morella), further investigation with increased data sampling was suggested, while two primary hypotheses, P. bigerri and P. colchicus, were supported as secondary species hypotheses. Finally, six of the primary species hypotheses (P. phoxinus, P. lumaireul, P. karsticus, P. septimanae, P. marsilii and P. csikii) were well supported by mitochondrial but only limitedly corroborated by nuclear data analysis.ConclusionThe approach has proven useful for revision of species complexes, and the study can serve as an overview of the Phoxinus genus in Europe, as well as a solid basis for further work.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-017-1032-x) contains supplementary material, which is available to authorized users.

Nonparasitic Lethenteron reissneri (Dybowski) is redescribed based on four syntypes (two adults and two ammocoetes) from the Onon and Ingoda rivers, Russia, and 15 topotypic specimens (seven metamorphosing ammocoetes and eight ammocoetes) from the Onon River system, Russia and Mongolia. Posterial teeth were not mentioned in the original description, but Berg (1931) stated that they were sometimes absent, which he later (Berg 1948) changed to usually absent, based on material (some of which we have re-identified as parasitic Lethenteron camtschaticum) from far outside of the type locality. The latter view has been widely accepted by subsequent authors. Unfortunately, the poor condition of the two adult syntypes did not permit verification of this character. However, a row of posterials was clearly visible in six of the seven topotypic metamorphosing ammocoetes and indicates their usual presence in the species. The first full description of the ammocoetes, including pigmentation, is provided. The present study restricts the distribution of Lethenteron reissneri to the Shilka and Songhua river systems within the Amur River basin, until a more geographically comprehensive study is undertaken. Additionally, in this study, feeding versus non-feeding at the adult stage, are considered to be valid taxonomic characters at the species level.