New evidence supporting upstream pathways of Hypania invalida (Grube, 1860) invasion.
The freshwater polychaete Hypania invalida is indigenous to the Ponto-Caspian region. The transfer of the species from the Black Sea to most of Europe was mainly via the Danube River, and its distribution increased rapidly after the opening of the Rhine-Main-Danube Canal in 1992. The presence of this species may influence the relationships of other species within the benthic community and thus has significant implications for the food webs of aquatic ecosystems. Our aim was to provide detailed records of the distribution of this potentially invasive species in Croatia, with evidence of upstream invasion and a description of environmental conditions at the sites, together with species co-occurrence, as a basis for assessing the future risk of upstream invasion and for monitoring purposes. Hypania invalida individuals were recently found at five different sites in Croatia in the Drava and Sava River systems and at one additional site in the Danube. Most of the new records support the idea of upstream invasion predominantly via large rivers. We found that the species inhabits both semi-natural river sections, and smaller canals, contradicting the idea that it inhabits only hydromorphologically heavily impacted main river channels.