A Coadapted Trophic Niche in Two Species of Crustacea (Isopoda): Asellus aquaticus (L.) and Proasellus coxalis Dolff

The potential trophic niches of sympatric and allopatric populations of the isopods Asellus aquaticus and Proasellus coxalis were compared in laboratory. Sixteen microfungi were singly offered as the sole food resource for 50 days to young animals born in the laboratory. Furthermore, six of all tested fungi were again offered to young of the F1 and F2 generations of P. coxalis from populations living sympatrically. Animals and microfungi were collected in three woodland streams that were similar in geographic location and hydrogeologic, physical and chemical features. The microfungi were selected on the basis of their constant presence throughout six months of sampling in each of three habitats. The results indicate first, that A. aquaticus and P. coxalis from populations living sympatrically have a potential trophic niche much more differentiated than do the populations living allopatrically. In particular, progeny of the two species when sympatric showed similar survival on only 25% of the tested fungal resources. Second, the two species when sympatric are specialized for microfungi not very common in nature; more common microfungi are used without distinction by individuals of both species. Third, the exclusiveness of each resource for the sympatric populations increases inversely with the abundance in nature of the resource itself. Fourth, in P. coxalis, the number of survivors eating a particular fungus increases in the F1 and F2 vs. parental generations. The results emphasize a possible effect of co-occurrence on character displacement and support the hypothesis of coevolutionary trophic partitioning between the two species. It appears that the importance of microfungi in the co-occurrence relationships of A. aquaticus and P. coxalis is much more than appears from the low estimates of the biomass of the microfungi.