DAY, R.; WESTNEAT, M. W.; THORSEN, D. H.; HALE, M. E.; Univ. of Chicago; Field Museum; Univ. of Chicago; Univ. of Chicago: Alternative patterns of fin-based locomotion: Fin coordination and gait transitions in juvenile fishes

Locomotion is often categorized into gaits that demonstrate characteristic limb or axial movement patterns. Swimming in many fish species can be subdivided into a slow swimming gait in which the pectoral fins are the primary propulsors and a fast swimming gait in which the fins are tucked and axial bending generates thrust. The labroid fishes are pectoral fin locomotor specialists and a model for examining fin-based swimming. In adults of this group, slow swimming is driven by synchronous adduction and abduction of the fins. The transition to axial locomotion occurs at near maximum sustained swimming speed. In order to extend previous work to earlier developmental stages, we filmed pectoral fin locomotion in juveniles of two coral reef species, Pomacentrus pavo and Halichoeres chrysus, across a range of speeds in a flow tank in the field. We found that these species have very different patterns of fin coordination and gait transition from each other and from adult labroids. P. pavo alternated the fins at slow swimming speeds. At a critical speed, around 3 bl/s, P. pavo switches from alternating to synchronous fin movements. The transition between these modes was discrete with one fin delaying a beat to become in phase with the other. H. chrysus, like P. pavo, swam with alternating fin strokes at low speeds. However, as speed increased, fin strokes gradually moved toward synchrony with no discrete transition point. Patterns of fin abduction angle, angular velocity, and fin area were also analyzed for patterns of coordination and gait. This work demonstrates that there is considerable diversity in the neural control of the fins across labroid species and indicates that there have been major evolutionary changes in control and coordination of the fins in this group.