The variability of the dimensions of the merus of the fifth pereiopod in the amphipod Gammarus duebeni Liljeborg, 1852

The variability of the morphological character on which the distinction between a brackish water and a freshwater subspecies of Gammarus duebeni is based, viz. the ratio merus width/merus length in the fifth walking leg, is studied. Evidence is given, that the merus dimensions only very partially can be correlated with the distribution and the ecology of G. duebeni. No clear clinal variation can be found in the merus dimensions in Europe. The subspecies G. d. celticus is found only in inland waters in Ireland and Brittany; the subspecies G. d. duebeni is found in western and northwestern Europe, and in Canada, in brackish water habitats and in some freshwater localities. Evidence is given, that G. d. duebeni has invaded the latter freshwater localities only recently.