Dikerogammarus villosus (Amphipoda : Gammaridae): another invasive speciesnewly established in the Moselle river and French hydrosystems

Based on field observations performed in 1999 and 2000, the exotic amphipod Dikerogammarus villosus is now well established throughout the navigable s ector of the River Moselle in France. D. villosus co-occurs with Gammarus p ulex, G. tigrinus and D. haemobaphes, another recent exotic species found i n this tributary of the River Rhine. The maximal size of individuals of D. villosus collected (15.7 mm) is lower in the River Moselle than in other ba sins, where this species could reach up to 30 mm (Neseman et al. 1995). Nev ertheless it represents the largest gammarids found in the French sector of the River Moselle. Other particular traits are its wide phenotypic plastic ity, varying from a melanic to a striped form, and its predatory behavior. These particular traits may favour the establishment of this species on a r ecipient ecosystem. The occurrence of D. villosus was noted in the Alsace C anal and in the River Rhone, showing that its immigration across French hyd rosystems is still in progress.