Adductor mandibulae muscle and lower jaw morphology: an approach to determine bite strategies in piranhas and relatives (Serrasalmidae, Characiformes, Teleostei)

Serrasalmidae (98 species) is a large monophyletic group of South American freshwater fishes. They have different dietary preferences ranging from the predominantly herbivorous or frugivorous pacus to the omnivorous or primarily carnivorous piranhas. Flesh-eating piranhas are supposed to have proportionally the most powerful bite forces among vertebrates but it has never been compared with those of plant, fruit and seed-eating species. Moreover, the bite strategy has never been explored in this family: is there a difference between carnivorous piranhas and herbivorous relatives? In the present study, we used the adductor mandibulae muscle and the lower jaw as models to answer this questioning through morphometric methods. We found that flesh-eating piranhas have a more developed adductor mandibulae muscle than seed-eating species which in turn have a larger jaw muscle than plant-eating species. The dorsomedial section of the jaw muscle (pars malaris) is the most developed in carnivorous species whereas it is the ventrolateral section (pars rictalis) in herbivorous species. We also distinguish two characteristic shapes of lower jaw that are related to dietary preferences. Results suggest different bite strategies: flesh-eating species possess a "scissor-like" way of feeding whereas the plant, fruit and seed-eating species show "vise-like" system.