Mechanics of pectoral fin locomotion in the surfperches (Perciformes)

A mechanistic analysis of pectoral fin swimming (labriform locomotion) was undertaken in the surfperches (Perciformes: Embiotocidae). The family was selected for comparative study of musculoskeletal function on the basis of its reliance on labriform propulsion across a wide range of swimming speeds, throughout ontogeny, and in varied ecological settings.; With increasing speed, surfperches undergo a distinct transition from purely pectoral fin swimming to combined pectoral and caudal fin propulsion. At the pectoral-caudal gait transition speed ({dollar}Usb{lcub}rm p{lcub}-{rcub}c{rcub}){dollar}, pectoral fin beat frequency and amplitude plateau. Striped surfperch (Embiotoca lateralis) varying 100-fold in body mass (M{dollar}sb{lcub}rm b{rcub}){dollar} exhibit kinematically similar fin movements with size-independent propulsive efficiency at {dollar}Usb{lcub}rm p{lcub}-{rcub}c{rcub}.{dollar} On this basis, the speed is considered 'biomechanically equivalent' for swimmers of different size. Stride frequency measured at gait transition scales in proportion to {dollar}rm Msb{lcub}b{rcub}sp{lcub}-0.12pm0.03{rcub},{dollar} a size dependence consistent with that observed for running and other swimming vertebrates. An underlying similarity in the allometry of muscle shortening velocity is suggested.; When the performance limit {dollar}Usb{lcub}rm p{lcub}-{rcub}c{rcub}{dollar} is expressed in terms of body lengths traveled per second, a traditional normalization of swimming speed, it varies markedly across size and habitat in the Embiotocidae. This result has the important implication that length-specific speeds may not induce comparable levels of activity from different fishes. Kinematic and physiological comparisons at such speeds yield misleading results. An alternative approach which successfully controls for variation in animal activity level involves standardizing speed to a percentage of {dollar}Usb{lcub}rm p{lcub}-{rcub}c{rcub}.{dollar}; The phyletically and ecologically divergent surfperches Embiotoca lateralis and Amphistichus rhodoterus exhibit a generally conserved musculoskeletal mechanism of steady labriform swimming. Similarities in the timing of kinematic and electromyographic events during the fin stroke are presumed to be characteristic of the family, although may be more widely distributed in the Perciformes. Mechanistic differences (e.g. in propulsor range of motion and intensity of muscle activity) may correspond to the species' respective maneuvering and cruising habits in nature. Synchronized profiles of fin movement and muscle activity reveal that the pectoral musculature serves a more complex function during the stride than was predicted from inspection of anatomical lines of action.