Aquatic Invasions (2006) Volume 1, Issue 3: 148-153 URL: http://www.aquaticinvasions.ru ' 2006 European Research Network on Aquatic Invasive Species 148 Research article The Ponto-Caspian amphipod Obesogammarus obesus (Sars, 1894) arrived the Rhine River via the Main-Danube Canal Stefan Nehring AeT umweltplanung, Bismarckstrae 19, 56068 Koblenz, Germany E-mail: nehring@aet-umweltplanung.de Received 20 June 2006; accepted in revised form 7 August 2006 Abstract The Ponto-Caspian amphipod Obesogammarus obesus (Sars, 1894) was first recorded in the Rhine River near Koblenz, Germany in October 2004. Additional records of small numbers of O. obesus from the same location in 2005 and 2006 indicate that this alien species may have become established in the central section of the Rhine River. Ship transport via the Main-Danube Canal, connecting the Danube basin (Black Sea catchment area) and the Rhine basin (North Sea catchment area), is the plausible mode of introduction of O. obesus to the site at Koblenz. The possible ecological effects of O. obesus in the Rhine River remain uncertain and require further study. Based on the invasion history of other Ponto- Caspian amphipods, it can be speculated that O. obesus will extend its distribution within the German and European river and canal systems in the near future. Key words: Obesogammarus obesus, Crustacea, Danube, Rhine, first record Introduction The introduction and spreading of numerous alien macroinvetebrates has recently accelerated in European waters, mainly a consequence of the removal of natural geographic barriers and rapid increases in the volume of international trade. Following the opening of the Main-Danube Canal in 1992, connecting the Danube and Rhine river basins, this southern corridor proved to be the most important dispersal route into western Europe for Ponto-Caspian species (Tittizer et al. 2000, Bij de Vaate et al. 2002, Nehring 2002). More than 20% of the species and more than 90 % of the macroinvertebrate biomass are represented by alien species in the Rhine River, making this river a truly international waterway (Galil et al. in press). There is consistent evidence that the number of aquatic alien species is still increasing (Gebhardt et al. 1999, Leppkoski et al. 2002, Nentwig et al. 2005, Bernauer and Jansen 2006), with potentially severe consequences for the river ecosystem. The amphipod Obesogammarus obesus is native to the Caspian, Azovs and Black seas as well as to rivers flowing into these basins (Sars 1894, Carausu et al. 1955, Jazdzewski and Konopacka 1988). The species also occurs naturally in the lower parts of the Danube River (Dudich 1967) (Figure 1). In the last few decades, O. obesus has extended its distribution upstream and has successively established populations in the Hungarian, the Slovakian, the Austrian and German sections of the river (Nesemann et al. 1995, Weinzierl et al. 1996, Pckl 2002). In 1996, Weinzierl et al. (1996) predicted that O. obesus soon would penetrate into the Rhine basin via the Main-Danube Canal. Here, we present the first record of O. obsesus from the Rhine River.