Vortex impact on the pectoral fin during hovering resulted in deformation of the shape of the fin (Fig. 6A) but no change in the adduction pattern of the fin from normal behavior (Fig. 6B). Relative curvature mapped over time (Fig. 6C) showed that vortex impact on the fin ray did not result in a smooth bending of the fin. Instead, the fin ray deformation consisted of small, localized areas of bending, with curvatures greater than 5 mm21 (illustrated by the small areas of high counter-curvature and hence alternating color in Fig. 6C). Maximum curvature observed at time of vortex impact was 9.38 mm21 , which was significantly higher than the curvatures observed under any of the unperturbed swimming conditions (P < 0.0001; one-sample T-test calculated by hand). Flexural stiffness of the fin ray was calculated to be 565 3 1026 N m2.
PIV analysis of vortex impact on the pectoral fin confirmed that deformation in the shape of the fin and increased curvature of the fin ray was a result of impact of the vortex with the fin ray (Fig. 7A). Vortex impact caused the fin ray to bend in compliance from the force of the moving fluid. Curvature of the third fin ray appears to be relatively uniform at large scale (Fig. 7B), but investigation of relative curvature over time at small regions within the fin ray showed regions of localized bending or buckling (curvatures greater than 5 mm21) at the point of vortex impact (Fig. 7C,D).