Trophic Specialization on unique resources despite limited niche divergence in a celebrated example of sympatric speciation

Trophic niche partitioning is observed in many adaptive radiations and is hypothesised to be a central process underlying species divergence. However, patterns of dietary niche partitioning are inconsistent across radiations and there are few studies of niche partitioning in putative examples of sympatric speciation. Here, we conducted the first quantitative study of dietary niche partitioning using stomach contents and stable isotope analyses in one of the most celebrated examples of sympatric speciation: the cichlid radiation from crater lake Barombi Mbo, Cameroon. We found little evidence for trophic niche partitioning among cichlids, including the nine species coexisting in the narrow littoral zone. Stable isotope analyses supported these conclusions of substantial dietary overlap. Our data, however, did reveal that five of 11 species consume rare dietary items, including freshwater sponge, terrestrial ants, and nocturnal foraging on shrimp. Stomach contents of the spongivore (Pungu maclareni) were 20% freshwater sponge, notable considering that only 0.04% of all fishes consume sponges. Overall, we conclude that cichlid species in lake Barombi Mbo overlap considerably in broad dietary niches - in part due to the large proportion of detritus in the stomach contents of all species - but there is evidence for divergence among species in their diet specializations on unique resources. We speculate that these species may utilise these additional specialised resources during periods of low resource abundance in support of Liem's paradox.