Many Ways to Build Scale and Fin Feeding Fish: The Functional Morphology of Piranhas

Abstract

Fishes have an incredibly wide range of feeding habits and behaviors and include some of the few vertebrate ectoparasites. Fish ectoparasites typically feed on the fins, scales, and slime coat of other fishes. Ectoparasitic fishes are most abundant in tropical South American freshwaters, and perhaps the best-known fin-feeders (pterygophagous) and scale-feeders (lepidophagous) are piranhas. While scales and fins are made from fundamentally homologous materials (i.e. hydroxyapatite, collagen, & mucus), the differences in how these materials are arranged on prey suggests that piranhas need different tools and/or strategies to obtain these prey resources. Therefore, consuming only the scales or fins of other fishes (and associated mucus), seems like a specialized or narrow niche, such that we might expect these fishes to have distinctive morphologies relative to other generalist carnivores. The goal of this study was to test whether there are differences in the feeding morphology of piranhas based on their particular prey resource (scales, fins, or flesh). This study used microCT scanning of 80% of the described piranha species to contrast feeding morphologies and jaw mechanics among ectoparasitic, carnivorous, and omnivorous taxa. We also examined how jaw mechanics and morphometrics have evolved across the serrasalmid phylogeny to account for whether jaw mechanics reflect convergence or novelty. We found that 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.