Gammarus fossarum Koch, 1835: redescription based on neotype material and notes on its local variation (Crustacea, Amphipoda)

Gammarus fossarum A redescription of Koch, 1835, is given, together with some notes on the variability in morphological characters of this species, both within one population and in different populations. It is proved through crosses that five morphologically different populations all belong to the same species, G. fossarum. The geographic variation of the various characters is discussed.