Sympatric coevolution of the trophic niche of two detritivorous isopods, Asellus aquaticus and Proasellus coxalis.

The role of 16 species of microfungi in the potential trophic niche of populations of Asellus aquaticus (L.) and Proasellus coxalis Dolff. collected in both sympatry and allopatry was determined in the laboratory. Animals and fungi were collected in three woodland streams similar as to geographic location and hydrogeologic, physical and chemical features. The fungi were offered as the sole food source to the animals for 50 d. Young animals, born in the laboratory, were tested for their trophic potential. Sympatric A. aquaticus and P. coxalis had a much more differentiated potential trophic niche than allopatric populations. In particular, the sympatric species had a potential overlap in the use of 25% of the tested fungi, while allopatric species had a potential overlap of 75% of the fungi. The two species from sympatric locations were specialized on fungal species not very common in nature, while more common fungi were used without distinction by both species. The exclusiveness of each fungal species for the isopods living in sympatry increased inversely to its frequency in nature. The results suggest a possible effect of co-occurrence on the character displacement shown by sympatric populations and support the hypothesis of coevolutionary trophic partitioning between them. The results also imply that the importance of the fungi in the co-occurrence of the isopod species is greater than suggested by the small biomass of the fungi.