We used frame-by-frame analysis of high-speed videotapes to quantify midline kinematics during steady swimming in largemouth bass at five standardized speeds (0.7, 1.2, 1.6, 2.0 and 2.4 L s-1, where L is total length). By combining morphological data from X-ray photographs with mathematical reconstructions of the midline of each fish, we determined the amplitude and timing of lateral displacement (zmax), lateral flexion (ssmax) and the angle between the midline and the axis of forward travel (thetamax) for each vertebral joint, the hypural bones and four equally spaced segments of the caudal fin rays. Analysis of variance revealed pervasive significant effects of both swimming speed and longitudinal location on variables describing amplitude, phase and wavelength. The amplitudes of zmax, ssmax and thetamax generally increased in a non-linear fashion from approximately 25 %L to the tip of the caudal fin, and the greatest speed-related increases occurred between 0.7 and 1.6 L s-1. For the snout, the first caudal vertebra and the trailing edge of the caudal fin, mean values of zmax increased with speed from 0.004 to 0.012 L, from 0.005 to 0.012 L and from 0.053 to 0.066 L, respectively. For joints between the skull and the first vertebra, between the trunk and the tail vertebrae, and among the most posterior caudal vertebrae, mean values of ssmax increased with speed from 1.2 to 1.7 degrees, from 0.6 to 0.9 degrees and from 1.4 to 2.2 degrees, respectively. Within each swimming speed, values of ssmax of the distal caudal fin commonly exceeded twice those of the proximal caudal fin. Surprisingly, at a given longitudinal location, the times of maximum lateral displacement and bending did not occur simultaneously. Instead, the phase of zmax relative to ssmax was commonly shifted by more than one-sixth of a cycle. Furthermore, the phase shift between zmax and ssmax changed significantly with increased swimming speed. Angles of attack of the tail structures changed periodically from negative to positive values. Maximum angles of attack of the distal caudal fin ranged from 5 to 17 degrees, changed significantly with swimming speed and were less than those of the hypural bones of the tail. Mean tail-beat frequency increased significantly from 2.0 to 4.2 Hz with increased swimming speed. Estimated speeds of wave propagation showed considerable longitudinal variation, and the ratio of swimming speed to posterior wave speed increased from 0.59 to 0.83 with increased swimming speed.

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