Herbivorous fishes feed on stems, leaves, flowers, seeds, fruits, and nuts of diverse aquatic plants, as well as algae. Pacus are the herbivorous cousins of piranhas and consume a myriad of diets comprised of these plant products, but a few species are phytophages, herbivores that feed almost exclusively on rapids-dwelling (rheophilic) riverweed plants from the family Podostemaceae. The degree to which pacus feed on riverweed varies from obligate year-round consumption to strictly seasonal, facultative feeding. Obligate phytophages feed heavily on riverweed and strictly occur in river rapids, while facultative phytophages only consume riverweed during seasons with low flow. Does ecological specialization (diet) beget morphological specialization in the feeding apparatus and/or body shape of phytophages? Under a phylogenetic framework, we used micro-computed tomography (µCT) scanning to compare functional feeding traits among 26 species of serrasalmids, four of which are obligate phytophages. We also compared body shape between pacus using geometric morphometrics to identify potential locomotor adaptations for rheophily. Obligate phytophages have dentitions and slicing jaws well-suited for shearing fleshy plant material relative to other pacus, which are equipped with fruit and seed crushing morphologies. Unrelated obligate phytophages have also converged on a similar body shape that is distinct from sympatric congeneric herbivores. Phytophagy involves more consistent changes to body shape than to feeding morphology, suggesting that body shape has more important ties to diet.