During steady swimming (Fig. 4A, D), spanwise curvature of the pectoral fin was mostly negative (curved away from the flow) during abduction (0–0.25s) and positive (curved into adduction (0.25–0.45 s), and did not exceed 60.05 mm21. In contrast, during a weak side maneuver, the spanwise curvature was close to zero or positive during much of the abduction or holding phase (0–0.8s) as the fin rays were curved forward into the flow (Fig. 4B,E). The upper distal region of the fin (blue line, Fig. 4B), where the fin rays are more flexible and less able to resist fluid forces, had an average negative curvature during the holding phase. During the strong side maneuver, the curvature was negative during abduction and positive during adduction, similar to the curvature during a steady beat