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Biological invasions are one of the greatest threats to biodiversity, economic development, and human health. Therefore an important challenge is to understand the mechanisms and factors that facilitate the spread of invasive species. The Ponto-Caspian polychaete Hypania invalida is one of the invaders that have been colonizing the river systems in Europe since the 1950s. The research aim was to map the distribution of H. invalida in the upper Oder and associated aquatic environments, to assess the contribution of H. invalida to the benthic fauna, to identify environmental factors linked to the occurrence of H. invalida, and to provide genetic markers that can be used to identify H. invalida and monitor its dispersal. H. invalida was found at two sites in the canalized section of the upper Oder and one site at the initial section of the Gliwice Canal. It was found at low abundance and coexisted with other alien macroinvertebrates. H. invalida was not recorded in any smaller river, whether natural, semi-natural, or anthropogenically modified. Apart from the watercourse width, no significant relationships between this polychaete and habitat drivers were found. Genetic analysis showed that the amplified cytochrome c oxidase subunit I and 18S gene fragments showed no sequence variation across all analyzed specimens. Further research is needed to follow the spread of H. invalida in the upper Oder River catchment, as at high densities it can adversely affect inhabited ecosystems.

Corbicula fluminea is recognised as being one of the worst alien species in Europe and one of the world's most widespread non-indigenous species in freshwater ecosystems. In Poland, it was detected in a heated water channel of the Lower Oder Power Station near Szczecin for the first time in 2003. In the following years, it continued its spread in the Oder River and the Vistula River. Our study uncovered new findings of C. fluminea in the Upper Oder River, which are its southernmost localities in the Oder River. Its record in the Gliwice Canal indicates that this may be a new dispersion route to the east for this species.

Two polychaete species, Hypania invalida and Laonome calida, were found in the Upper Odra River in 2016. Both species were recorded close to a natural river bank down to 1 m depths. They inhabited sandy-gravelly and sandy-muddy sediments. H. invalida is an alien invasive Ponto-Caspian species, previously known in Poland from the Odra River estuary only. Our results may indicate a further rapid dispersal of H. invalida upstream the Odra River or an accidental introduction. This study is the first record of L. calida in the Baltic Sea catchment. This Australian species has been recently introduced into Europe. Prior to this study, it had been reported from Dutch rivers only. The present data suggest accidental introduction of the species to European rivers; however, our findings show an urgent need for a close monitoring of the polychaete in Europe.