Fishes exhibit a wide range of dietary ecologies, feeding on everything from the tube-feet of echinoderms to cultivated strands of algae, and even include some of the few examples of vertebrate ectoparasites. Ectoparasitic fishes feed on the blood, fins, scales, and slime of other fishes and appear particularly abundant in tropical South American freshwaters. Of these, the best-known fin-feeders (pterygophagous) and scale-feeders (lepidophagous) are piranhas (Serrasalmidae). While scales and fins are made from fundamentally homologous materials, the differences in how these materials are arranged on prey suggests that piranhas need different tools or behaviors to obtain either resource. Therefore, consuming only the scales or fins of other fishes (and associated mucus), seems like a specialized niche, so we might expect these fishes to have distinctive morphologies relative to other generalist carnivores. We tested for phenotypic differences among ectoparasitic, carnivorous, and omnivorous piranhas by examining cranial morphometrics obtained from microCT scans for 80% of the described piranha species diversity. We also explored whether jaw morphologies among ectoparasitic lineages are phenotypically or functionally convergent, by examining trait evolution across the latest piranha phylogeny. Finally, we affixed cutting blades to a mechanical loading frame and determined how much force is required to remove fin rays from prey. While some ecological specialists like Catoprion mento, an obligate lepidophage, were distinct from other piranhas in our analyses, most scale- and fin-feeding piranhas overwhelmingly resembled their carnivorous cousins. Forces required to remove fins fall well within the range of published bites forces for even the smallest piranha species.