Adaptive speciation and sexual dimorphism contribute to diversity in form and function in the adaptive radiation of Lake Matano's sympatric roundfin sailfin silversides.

The utility of traits involved in resource exploitation is a central criterion for the adaptive character of radiations. Here, we test for differentiation in morphology, jaw mechanics and nutrition among species and sexes of Lake Matano's sympatric 'roundfin' sailfin silversides. The three incipient fish species differ significant in several candidate traits for adaptation following ecological selection pressure, corresponding to contrasting jaw mechanics and distinct patterns in food resource use. These findings are consistent with functional adaptation and suggest divergence following alternative modes of feeding specialization. Further, intersexual resource partitioning and corresponding adaptation in jaw mechanics is evident in two of the three incipient species, demonstrating that sexual dimorphism contributes to the ecomorphological and trophic diversity of the emerging radiation. This is, to the best of our knowledge, the first study reporting interspecific as well as intersexual adaptation by alternative modes of form and function in an evolving fish species flock.