Meeting Abstract
Herbivorous fishes may feed on any combination of stems, leaves, flowers, seeds, fruits, and nuts of diverse aquatic plants, as well as algae. In the Neotropics, most pacus, the herbivorous cousins of piranhas, eat a combination of these plant constituents, which vary in their accessibility, material, and nutritional quality. Additionally, pacu diets fluctuate across ontogeny, and with seasonality and flowering demography of their prey. Several species of pacus are phytophagous, a curious kind of herbivore that feeds almost exclusively on Podostemaceae, or riverweed plants, which only occur in rapids, a challenging environment for fishes to live in. The degree to which pacus feed on riverweed varies from obligate year-round consumption to strictly seasonal and facultative feeding. Obligate phytophages feed heavily on riverweed and occur in the rapids, while facultative phytophages only consume riverweed during seasons with low flow. Does ecological specialization (diet) beget morphological specialization in the feeding apparatus of phytophages? We used micro-computed tomography (μCT) scanning to compare functional feeding traits among 24 species of serrasalmids, 3 of them obligate phytophages. We also compared body shape between pacus using geometric morphometrics to identify potential locomotor adaptations for rheophily. Obligate phytophages don’t have distinct jaw mechanics from more generalized herbivores, but they do have the dentition and slicing jaw action more suited to shearing fleshy plant material than other pacus, which have jaws built for crushing seeds. Unrelated obligate phytophages are also converging on similar body shapes that are distinct from sympatric herbivores. Phytophagy involves more drastic changes to body shape than to feeding morphology, suggesting that body shape has equally important ties to diet as feeding morphology.
