| Batoids (skates, stingrays, and their relatives) exhibit a great deal of diversity in terms of disc shape, swimming behavior, and lifestyle. Disc shape ranges from round to diamond-shaped to tapered and wing-like. Swimming behavior is axial-based, pectoral fin-based, or a combination of the two. Most batoids are benthic, but a few have left the bottom for a pelagic lifestyle. Despite this diversity in batoids, we know little about the mechanics of locomotion in this group. The goals of this study are: (1) to compare pectoral fin locomotion and morphology of the pectoral fins in nine species of batoids that differ in disc shape, swimming behavior, and lifestyle, (2) to elucidate relationships within stingray species in the genus Dasyatis , (3) to discover evolutionary patterns of locomotion in batoid fishes using a phylogenetic framework. Batoid fishes increase swimming velocity by changing any of the following parameters: fin beat frequency, wavespeed, wave number, and fin tip velocity. Disc shape correlates significantly with frequency, wave number, and amplitude. A continuum between undulation and oscillation exists for batoid species. Electromyographic data from Taeniura indicate that muscles are active along the entire length of the pectoral fins and are fired sequentially and alternately down the fin to produce the up and down motions of the fin. Results from phylogenetic systematics of 12 Dasyatis species and three outgroups yield two most parsimonious trees (CI = 0.412). The subset of Dasyatis form a monophyletic group, and D. violacea is nested well within. Discrete character states associated with swimming behavior, morphology, and ecology are mapped onto a combined phylogeny for batoids and Dasyatis stingrays to reveal trends in character evolution. A pelagic lifestyle has evolved at least twice in batoids and correlates with more oscillatory-based swimming, low frequency, greater amount of red muscle fibers, and a thick scapulocoracoid. In general, the evolution of locomotion in batoid fishes moves from axial-based to combination of axial and pectoral undulation to strict pectoral undulation and finally to pectoral oscillation. |
