The alien amphipod crustacean Dikerogammarus villosus in Lake Garda(N-Italy): the invasion continues.

The discovery of Dikerogammarus villosus, an amphipod crustacean from the Ponto-Caspian region, on the shores of Lake Garda and in other watercourses of northern Italy, is a worrying fact that deserves attention. In the littoral environment of this lake, D. villosus interacts with Echinogammarus stammeri, one of four indigenous gammarid species, which shares a similar microhabitat. According to a previous study in 2003-2004, the invasive species had expanded throughout the shallow zone ofthe lake quite rapidly, particularly in the southern basin, competing with the indigenous gammarid species which it may even replace. In order to monitor the situation in time, this study was done in 2006 to investigate the distribution of the two gammarid species at various depths (1, 2, 5, 10 m) in the littoral zone, and to verify the hypothesis suggested by previous work that E. stammeri is spatially segregated from D. villosus. The indigenous species was found to be mostly relegated to greater depths, thus reducing unfavourable interactions with the invader. It is a matter of particular concern that also D. villosus is moving deeper, with possible adverse effects resulting from niche overlaps.