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Presented at the NABS Annual meeting, Pittsburgh, Pennsylvania, 2002
in Lentic Ecology
LOCAL COLOR ADAPTATION IN THE FRESHWATER ISOPOD ASELLUS AQUATICUS.
J. Johansson1, A. Hargeby1, and J. Ahnesjö2. 1Limnology, Dept. of Ecology, University of Lund, S-223 62, Sweden, 2School of Bioscience and Process technology, Växjö University, S-351 95 Växjö, Sweden
Although local adaptation to environmental change is fundamental in microevolution, it is not often documented in natural populations. Here, we show that in two Swedish lakes, the isopod Asellus aquaticus was on average more brightly pigmented in vegetation stands of stoneworts, than in stands of reed. Pigmentation appeared to vary continuously between individuals and was therefore treated as a quantitative trait. In both lakes, average pigmentation decreased with time in recently established stonewort stands. Experimental crossings of bright and dark isopods showed that pigmentation had a genetic basis. Choice of substrate did not differ between bright and dark isopods, which makes phenotype specific movements between habitats unlikely as an explanation to the observed pattern. Predation experiments, in which perch was used as predator, showed that visual predation was a possible selective force behind the habitat differences in A. aquaticus pigmentation. In concert, the results indicate that the habitat difference in pigmentation of A. aquaticus is a result of local adaptation. Both the stonewort and the reed stands constitute large and persistent patches, habitat characteristics that tend to preserve adaptations produced by local natural selection and may be prerequisites for local adaptive coloration to evolve in A. aquaticus.
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