Phylogenetic relationships and the evolution of fin and body shape in the surgeonfishes

Patterns of evolutionary change in the fin and body shapes of fishes are strongly related to dietary ecology and locomotor mechanisms, and associations between shapes affects ecomorphological relationships between functional groups and evolutionary rates across taxa. The surgeonfishes (Acanthuridae), a group of 83 species across 6 genera, show high diversity in head, body, and propulsive fin (caudal and pectoral) shape, suggesting that morphometric analysis of these functionally relevant traits may provide insight into the ecomorphology and evolutionary history of this important fish family. Here, we develop a new multi-locus, time-calibrated phylogenetic hypothesis for the surgeonfishes based on 19 loci for 80 acanthurid species and 45 outgroups. Detailed head, body, and fin geometric morphometric data were collected for all 80 surgeonfish species in the tree, and patterns of shape evolution across the family were visualized using phylomorphospaces. We used a three-state feeding trait axis across the herbivore to planktivore continuum to examine how morphometric shape relates to diet. Head and body shape were significantly associated with diet. Additionally, species exhibited two alternative locomotor strategies: high aspect ratio (AR) tails were associated with low AR pectoral fins, slender bodies, and pelagic planktivory, whereas species with low AR tail fins had wing-like, high AR pectoral fins, deeper bodies, and were mostly benthic grazers. Using a novel phylogenetic comparative method to investigate evolutionary correlations between shapes, we found that the caudal fin had the highest evolutionary rates and covaried with both the body and pectoral fin, likely due to dietary and locomotor demands, respectively.