Fig. 6. Kinematic patterns for the soft dorsal fin (D) and tail fin (T) oscillating in tandem during steady swimming at 1.1Ls^-1, where L is total body length. A–D show video images of the two fins moving within a frontal-plane laser sheet (Fig.2D, position 2) over the course of one complete stroke cycle. Digitizing such images allowed measurement of temporal and spatial patterns of fin tip excursion. In E, left–right movements of the trailing edges of the soft dorsal fin and the dorsal lobe of the tail are plotted against time for two consecutive stroke cycles. Although both fins oscillate at a frequency of 2.5Hz, motion of the tail lags behind that of the dorsal fin by an average of 121ms, or 30% of the stroke cycle. The abduction amplitude of the tail on each side of the body exceeds that of the dorsal fin by 0.51cm on average, a difference corresponding to 19% of the tail’s total sweep amplitude.