Comparative morphology of shark pectoral fins

Sharks vary greatly in morphology, physiology, and ecology. Differences in whole body shape, swimming style, and physiological parameters have previously been linked to varied habitat uses. Pectoral fin morphology has been used to taxonomically classify species and hypotheses on the functional differences in shape are noted throughout the literature; however, there are limited comparative datasets that quantify external and skeletal morphology. Further, fins were previously categorized into two discrete groups based on the amount of skeletal support present: (a) aplesodic, where less than half of the fin is supported and (b) plesodic where greater than half of the fin is supported. These discrete classifications have been used to phylogenetically place species, though the methodology of classification is infrequently described. In this study, we sampled fins from 18 species, 6 families, and 3 orders, which were also grouped into five ecomorphotype classifications. We examined the external morphology, extent of skeletal support, and cross-sectional shape of individual cartilaginous elements. Using phylogenetic comparative methods, we show that fin shape does not differ significantly between ecomorphotypes, suggesting there may be some mechanical constraint. However, we find that the internal anatomy of the fin does vary significantly between ecomorphotypes, especially the extent and distribution of calcification of skeletal support, suggesting that the superficial similarity of fin shapes across ecomorphotypes may belie differences in function. Finally, we find that a number of morphological variables such as number of radials, radial calcification and shape, and fin taper all correlate with the extent of skeletal support. Within these morphospaces, we also describe that some orders/families tend to occupy certain areas with limited overlap. While we demonstrate that there is some mechanical constraint limiting external variations in shark pectoral fin morphology, there are compounding differences in skeletal anatomy that occur within ecomorphotypes which we propose may affect function.