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

Despite considerable recent progress in understanding the functionof the axial muscles and skeleton in fishes, generalizing fromthese results has been hindered by the great phylogenetic diversityof taxa, the lack of quantitative morphometric data on axialmusculoskeletal structure, and limited analysis of the fullrange of locomotor behaviors exhibited within any one taxon.This paper reviews novel results from our studies of two taxawithin a single monophyletic clade, the sunfish family Centrarchidae.Integrated analyses of lateral displacement, lateral bending,and axial muscle activity reveal widespread effects of swimmingspeed both within a particular mode of swimming and among differentbehavioral modes. The longitudinal position along the body ofthe fish also commonly affects kinematics, muscle activity andthe timing of electromyograms (EMGs) relative to kinematics.EMGs and kinematic events propagate from head to tail for bothsteady and kick and glide swimming. In contrast, during theescape response, the onset of EMGs forms a standing wave pattern,whereas kinematic events are propagated. Several novel featuresof the axial motor pattern are summarized for the kick and glidemode of unsteady swimming. For example, the onset of white fiberEMGs lags significantly behind that of the red fibers at thesame longitudinal position, and red fibers are inactivated athigher unsteady swimming speeds. Muscle activity propagatesposteriorly via the sequential activation of myomeres, but thereare statistically significant differences in the timing of EMGsfrom the contractile tissue opposite a single vertebra. Duringrelatively slow kick and glide swimming, the extreme dorsaland ventral portions of myomeres are not active. Estimates ofthe longitudinal extent of the fish with simultaneous muscleactivity indicate that EMGs from an individual myomere usuallyhave temporal overlap with more than 20 neighboring myomereson the same side of the fish. Consequently, the functional unitsfor axial locomotion of fishes do not correspond simply to theanatomical units of individual myomeres. Rather the in vivomotor pattern is a primary determinant of the functional unitsinvolved in swimming.