Combining DNA variation data and risk assessment procedures offers important diagnostic and monitoring tools for evaluating the relative success of exotic species invasions. Risk assessment may allow us to understand how the numbers of founding individuals, genetic variants, population sources, and introduction events affect successful establishment and spread. This is particularly important in habitats that are "hotbeds" for invasive species--such as the North American Great Lakes. This study compares genetic variability and its application to risk assessment within and among three Eurasian groups and five species that successfully invaded the Great Lakes during the mid 1980s through early 1990s; including zebra and quagga mussels, round and tubenose gobies, and the ruffe. DNA sequences are compared from exotic and native populations in order to evaluate the role of genetic diversity in invasions. Close relatives are also examined, since they often invade in concert and several are saline tolerant and are likely to spread to North American estuaries. Results show that very high genetic diversity characterizes the invasions of all five species, indicating that they were founded by very large numbers of propagules and underwent no founder effects. Genetic evidence points to multiple invasion sources for both dreissenid and goby species, which appears related to especially rapid spread and widespread colonization success in a variety of habitats. In contrast, results show that the ruffe population in the Great Lakes originated from a single founding population source from the Elbe River drainage. Both the Great Lakes and the Elbe River populations of ruffe have similar genetic diversity levels--showing no founder effect, as in the other invasive species. In conclusion, high genetic variability, large numbers of founders, and multiple founding sources likely significantly contribute to the risk of an exotic species introduction's success and persistence.

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Cryptic taxa present unique difficulties in the description of biological diversity, which DNA sequencing approaches often readily resolve. The tubenose goby Proterorhinus, along with other Ponto-Caspian fauna, has undergone recent Eurasian range expansion, as well as colonized the North American Great Lakes in 1990. We analysed mitochondrial (cytochrome b and cytochrome c oxidase subunit I) and nuclear (recombination activating gene 1; RAG1) DNA sequences and morphological characters from exotic Great Lakes as well as introduced and native Eurasian populations of Proterorhinus marmoratus (Pallas) sensu lato to assess their species identity and biogeographic patterns. The results obtained show marked genetic and morphological divergence that indicates species-level separation between fresh water and marine/brackish lineages, dating back approximately 3.82-4.30 million years. In addition, freshwater lineages within the Black and Caspian Sea basins show significant genetic and morphological differentiation, corresponding to an estimated 0.92-1.03 million years. We describe new evidence to support at least three separate species: the original P. marmoratus in marine and estuarine habitats within the Black Sea, a freshwater species in the Black Sea basin that was introduced to the North American Great Lakes, and another freshwater species inhabiting the Caspian Sea/Volga River basin. The freshwater tubenose goby in the Black Sea basin originally was described as Proterorhinus semilunaris (Heckel), and this is confirmed to be a valid taxon. The Caspian basin taxon may correspond with Proterorhinus semipellucidus (Kessler), a putative freshwater species in the Caspian basin that was originally described from a single specimen.

Aim  Hidden diversity within an invasive ‘species’ can mask both invasion pathways and confound management goals. We assessed taxonomic status and population structure of the monkey goby Neogobius fluviatilis across Eurasia, comparing genetic variation across its native and invasive ranges.Location  Native populations were analysed within the Black and Caspian Sea basins, including major river drainages (Dnieper, Dniester, Danube, Don and Volga rivers), along with introduced locations within the upper Danube and Vistula river systems.Methods  DNA sequences and 10 nuclear microsatellite loci were analysed to test genetic diversity and divergence patterns of native and introduced populations; phylogenetic analysis of mtDNA cytochrome b and nuclear RAG‐1 sequences assessed taxonomic status of Black and Caspian Sea lineages. Multivariate analysis of morphology was used to corroborate phylogenetic patterns. Population genetic structure within each basin was evaluated with mtDNA and microsatellite data using FST analogues and Bayesian assignment tests.Results  Phylogenetic analysis of mitochondrial and nuclear sequences discerned a pronounced genetic break between monkey gobies in the Black and Caspian Seas, indicating a long‐term species‐level separation dating to c. 3 million years. This pronounced separation further was confirmed from morphological and population genetic divergence. Bayesian inference showed congruent patterns of population structure within the Black Sea basin. Introduced populations in the Danube and Vistula River basins traced to north‐west Black Sea origins, a genetic expansion pattern matching that of other introduced Ponto‐Caspian gobiids.Main conclusions  Both genetic and morphological data strongly supported two species of monkey gobies that were formerly identified as subspecies: N. fluviatilis in the Black Sea basin, Don and Volga Rivers, and the Kumo‐Manych Depression, and Neogobius pallasi in the Caspian Sea and Volga River delta. Genetic origins of introduced N. fluviatilis populations indicated a common invasion pathway shared with other introduced Ponto‐Caspian fishes and invertebrates.

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