Genetic differentiation among G. fossarum Koch, 1835 from different stations in Germany, Switzerland, Belgium and northern France, and the closely related Belgian form G. caparti Pètre-Stroobants, 1980 was investigated by electrophoresis at 20 enzyme loci. Although morphologically variable, geographically distant populations of G. cf. fossarum were hitherto considered conspecific. In the present study, populations of G. cf. fossarum and G. caparti were examined with reference to G. pulex pulex as an estimate for genetic differentiation at the species level.
With G. p. pulex as a standard, genetic differentiation among geographically distant populations of G. cf. fossarum is occasionally observed at species level. The populations of G. caparti studied were shown to be genetically very similar to certain populations of G. fossarum. The taxonomic status of G. caparti and the genetically distinct forms of G. fossarum is discussed.
Genetic differentiation in populations of the freshwater amphipods Gammarus roeseli and Gammarus fossarum