An examination of subspecific differences in the merus of the fifth walking leg of the amphipod Gammarus duebeni Lilljeborg

Stock SL Pinkster (1970) erected two subspecies of Gammarus duebeni primarily on the basis of a difference in the relative dimensions of the merus on the fifth walking leg. The diagnostic feature was given as the ratio of merus length/width, with ratio < 2 in G. duebeni duebeni and ratio > 2 in G. duebeni celticus. It is shown here that the ratio varies with the size of the animal, and this particular ratio is therefore not valid as a taxonomic character. Regressions of log merus width on log merus length were made on large samples of G. duebeni assumed to belong to subspecies celticus and duebeni respectively. The regressions were significantly diiferent and were used to predict that the ratio log merus width/log merus length is relatively constant in both subspecies. Diagnostic values for this ratio were 0.71–0.73 in celticus and 0.74–0.77 in duebeni. The ratio was also determined in small samples of G. duebeni from other localities in western Britain. The morphological difference which characterizes celticus and duebeni was not associated with habitat salinity or with previously determined physiological characteristics. It may be an instance of cHnal variation reinforced by geographical isolation. Regressions of merus length against cephalic length suggest that the relative growth rates of the merus may also differ in the two subspecies.