Ponto-Caspian gobiids have expanded their ranges throughout Europe since the 1990s. While genetic studies have been widely used to assess the invasion history of gobiids in North America, complex genetic studies involving multiple sites and species have been less common in Europe, severely limiting our understanding of invasion processes along navigable rivers and their tributaries. In this study, we used both nuclear and mitochondrial markers to assess genetic diversity and structure in native and non-native Western tubenose goby Proterorhinus semilunaris, round goby Neogobius melanostomus and bighead goby Ponticola kessleri sampled from the main areas of their joint distribution, i.e. the lower Danube, middle Danube and lower Rhine. Additionally, we describe expansion into Danubian tributaries and provide early genetic data for N. melanostomus from the River Elbe. Our data revealed i) a founder effect in non-native P. semilunaris, ii) an increase in genetic diversity in non-native N. melanostomus samples from the Rhine and Elbe, and iii) no genetic structuring in P. kessleri. This suggests greater initial propagule pressure in P. kessleri; strong propagule pressure with introductions from multiple sources followed by admixture for N. melanostomus in the Rhine and Elbe; and one or very few introduction events for P. semilunaris. We provide further support for the Danubian origin of all three goby species in the Rhine and document lower genetic diversity in fish colonising non-navigable tributaries. Our results illustrate how the ranges of invasive species can become sympatric, despite clear differences in their invasion histories.
