Abstract :
[en] In jawed vertebrates, a diet transition between herbivory and carnivory is predicted to result in a change in functional constraints on the feeding system. Curiously, in this context, few studies have explored the phenotypic evolution of the feeding system as an integrated functional system including teeth, bones and muscles. The Neotropical family Serrasalmidae (pacus, piranhas and relatives) is a highly successful group of South American characiform fishes having a large diversity of feeding habits and tooth morphologies. Feeding habits vary from the predominantly frugivorous and granivorous pacus having molariform teeth to the primarily piscivorous piranhas having caniniform teeth. More importantly, serrasalmids are atypical among teleosts because they feed only by biting on various types of plant and animal foods while using only the oral jaw system. All these specificities make serrasalmids the ideal study group to understand how the biting system evolves under a trophic shift, and in particular, how oral teeth, oral jaws and associated jaw muscles evolve in relation to each other. The main objectives of this research thesis were to determine, in a phylogenetic context, the impact of a diet transition between herbivory and carnivory on the feeding performance and functional morphology of the biting system, along with evolutionary patterns of phenotypic integration and modularity between the different dental and musculoskeletal structures of this system in serrasalmids. To this end, we recorded for the first time bite forces in many species of the family, we generated functional and morphological (geometric morphometric) datasets for each structure of the biting system, and we performed the first comparative analysis of function and microstructure of oral teeth between herbivorous and carnivorous serrasalmids. Our results demonstrate that the evolutionary history of serrasalmids is characterized by a main trophic shift from herbivory to carnivory, which is associated with a major phenotypic shift in the biting system enhancing bite performance. Sharper teeth, longer jaws, a jaw joint more in line with teeth and a larger jaw muscle changed the entire configuration of the biting system to generate more efficient and stronger bite forces for animal foods in a scissor-like strategy. In addition to these phenotypic adaptations, our results provide evidence of a shift in tooth attachment and microscopic adaptations at the level of oral teeth. Teeth of piranhas attach by their base to the jawbone to better withstand extreme bite forces and have an increased mineral content that most likely improves their stiffness. Our results also reveal that the biting system evolves as a modular system by having a certain degree of independence between oral teeth, oral jaws and associated jaw muscles. Our results also highlight, for the first time, a sequence of phenotypic adaptations in which oral teeth, oral jaws and then the adductor mandibulae muscle shift in morphology and function. Altogether, our findings suggest that oral teeth might be a key structure in the adaptive evolution of the biting system in serrasalmids in relation to trophic shift. However, their stronger phenotypic integration with oral jaws compared to adductor mandibulae muscle suggest that oral teeth and oral jaws, together, might enable the shift between the herbivorous and carnivorous adaptive zones and ecological diversification within these zones.
