Recent studies show that populations of the freshwater isopod Asellus aquaticus L. can rapidly become locally differentiated when submerged stonewort (-Chara spp.) vegetation expands in lakes. In the novel Chara habitat, isopods become lighter pigmented and smaller than in the ancestral reed stands. In this study, I used laboratory experiments to investigate if selective predation by fish could be a possible explanation for these phenotypic changes. Predation from fish is generally considered to be a strong selective force on macroinvertebrate traits. In the experiments I let perch (Perca fluviatilis L.) feed on a mixture of Asellus phenotypes in aquaria manipulated to mimic the substrates in either the Chara or the reed habitats. Remaining isopods were significantly smaller and lighter pigmented in the fish aquaria than in the controls, showing that the perch preferred to feed on large and dark individuals. The perch had also eaten significantly more isopods in the Chara habitat than in the reed habitat. In the Chara habitat, selection on isopod pigmentation was according to what could be expected from background matching, but in the reed habitat selection was quite the opposite. These results support the hypothesis that predation from fish is a strong selective force behind the rapid local adaptation seen in Asellus populations in the novel Chara habitat. The results also indicate that fish predation is less important in the reed habitat, as this habitat is dominated by large dark isopods.

Magnus Andersson

Tfn: 46(0)706884487

email: magan464@student.liu.se