Evolution of the feeding morphology in serrasalmid fishes (pacus and piranhas)

Serrasalmid fishes (98 species including pacus and piranhas) have largely diversified their diet in terms of food items. Some species prefer to feed on algae, plants, fruits, seeds or nuts whereas others eat flesh, fishes, fins, scales, parts of vertebrates or invertebrates. Despite this high trophic diversity, all species have in common the use of biting to capture food. How does the morphology of the feeding system evolve to trophically diversify in this monophyletic taxon while maintaining a unique feeding mode? To answer this questioning, we dissected and photographed different elements of the feeding system (cephalic region, adductor mandibulae muscle, lower and upper jaws, and buccal teeth) in 30 serrasalmid species representing a large part of dietary diversity. We quantified shape variation of these elements using landmark-based geometric morphometrics and explored their degree of specialisation using phylogenetic comparative methods. Our results reveal two main groupings of species that are mainly consistent with diet (carnivorous vs. herbivorous) and few intermediate forms (omnivorous?) considering the effect of shared ancestry. Carnivorous species show smaller eyes for a larger adductor mandibulae muscle and a superior mouth position allowing the lengthening of lower and upper jaws. They also have numerous teeth, which are highly triangular and sharp. Herbivorous species display larger variation of the size of the adductor mandibulae muscle, the mouth position and tooth shape. This difference in morphological disparity could be related to the feeding constraints and suggest that herbivorous taxa could exploit more ecological niches than carnivorous species. This study highlights how a set of subtle modifications in the feeding system of serrasalmid fishes has allowed their large trophic diversification.