Phenotypic Convergence Is Stronger and More Frequent in Herbivorous Fishes.

Constraints on phenotypic evolution can lead to patterns of convergent evolution, by limiting the "pool" of potential phenotypes in the face of endogenous (functional, developmental) or exogenous (competition, predation) selective pressures. Evaluation of convergence depends on integrating ecological and morphological data within a robust, comparative phylogenetic context. The staggering diversity of teleost fishes offers a multitude of lineages adapted for similar ecological roles and, therefore, offers numerous replicated evolutionary experiments for exploring phenotypic convergence. However, our understanding of fish feeding systems has been primarily shaped by marine species, with the monolithic exception of freshwater cichlids. Here we use piranhas and pacus (Serrasalmidae) to explore the evolution of different feeding ecologies and their morphological proxies in Neotropical freshwater environments. Specifically, we explore whether convergence is more widespread among plant-eating fishes, arising from strong constraints on phenotypic evolution in herbivores. Using osteological micro-computed tomographic imaging (μCT), we describe the major axes of morphological variation in pacus and piranhas, regarding their diet and feeding behaviors. Next, we evaluated whether herbivorous niches are less labile than other dietary guilds and whether herbivorous species' phenotypes evolve at a slower evolutionary rate than other taxa. We then assess how convergent herbivorous taxa are, using three different suites of morphological characters (dental, jaw, and abdominal morphometrics). Ecologically, herbivory is not a dead end, exhibiting similar observed transition rates as those between carnivores and omnivores. However, we documented widespread convergence in herbivores and that herbivores have slower rates of phenotypic evolution than carnivores. Most instances of convergence are found in herbivorous taxa, specifically in frugivores and folivores. Moreover, instances of "complete" convergence, indicated by positive convergence metrics observed in more than one morphometric dataset, were only found in herbivores. Herbivores do appear to evolve under constrained circumstances, but this has not limited their ecological ability.