New Data on Axial Locomotion in Fishes: How Speed Affects Diversity of Kinematics and Motor Patterns

Abstract

Synopsis. Despite considerable recent progress in understanding the func? tion of the axial muscles and skeleton in fishes, generalizing from these results has been hindered by the great phylogenetic diversity of taxa, the lack of quantitative morphometric data on axial musculoskeletal structure, and limited analysis of the Ml range of locomotor behaviors exhibited within any one taxon. This paper reviews novel results from our studies of two taxa within a single monophyletic clade, the sunfish family Centrarchidae. Integrated analyses of lateral displacement, lateral bending, and axial muscle activity reveal widespread effects of swimming speed both within a particular mode of swimming and among different behavioral modes. The longitudinal position along the body of the fish also commonly affects kinematics, muscle activity and the timing of electromyograms (EMGs) relative to kinematics. EMGs and kinematic events propagate from head to tail for both steady and kick and glide swimming. In contrast, during the escape response, the onset of EMGs forms a standing wave pattern, whereas kinematic events are propagated. Several novel features of the axial motor pattern are summarized for the kick and glide mode of unsteady swimming. For example, the onset of white fiber EMGs lags significantly behind that of the red fibers at the same longitudinal position, and red fibers are inactivated at higher unsteady swim? ming speeds. Muscle activity propagates posteriorly via the sequential acti? vation of myomeres, but there are statistically significant differences in the timing of EMGs from the contractile tissue opposite a single vertebra. During relatively slow kick and glide swimming, the extreme dorsal and ventral portions of myomeres are not active. Estimates of the longitudinal extent of the fish with simultaneous muscle activity indicate that EMGs from an in? dividual myomere usually have temporal overlap with more than 20 neighboring myomeres on the same side of the fish. Consequently, the functional units for axial locomotion of fishes do not correspond simply to the anatom? ical units of individual myomeres. Rather the in vivo motor pattern is a primary determinant of the functional units involved in swimming.