The role of soft tissues in prey processing or prey manipulation are critical to vertebrate feeding performance, extending beyond the musculature to ligaments, tendons, and even less-conspicuous composite organs like lips and tongues. The usefulness of soft tissues during feeding is best captured from studies on terrestrial mammal foraging, where prehensile lips are used to defoliate branches or tongues used to reposition a bolus during feeding. Here, we used histological sectioning to compare lip morphologies among seed-eating and grazing pacus, as well as carnivorous piranhas (Serrasalmidae). We suspect that the pacu’s hypertrophied lower lip could behave actively during feeding, manipulating plant parts like a rhino’s prehensile tongue. Alternatively, the lower lip could act passively, with rugosity lending better grip on fruits and seeds. If the former is the case, our histological methods should find extensive musculature around the circumference of the lip, while the latter hypothesis implies little, if any muscle will be present. The lips of pacus are composites of collagen, fat, putative sensory nerves, and macrophages. Grazing pacus such as Tometes and Myloplus have slick lips filled with goblet cells that may resist abrasion and shear from caustic plant material, whereas the lips of fruit-eating pacus like Piaractus have a dense, highly folded epithelium followed by a heavily regionalized dermis that likely increases surface area during prey manipulation to increase grip. In contrast, the lips of carnivorous piranhas are interspersed with loose connective tissue and fat, with no regionalization of tissues. Lips of grazing and fruit-eating pacus are both passive but the arrangement of collagen and connective tissues will affect the deformability of the lips and how the lips work to distribute stresses while stretching to handle prey.