Fig. 7. Digitized outline traces of simultaneous posterior and ventral images of the pectoral fins during swimming at 0.25 L s^-1: thrust cycle and the beginning of the recovery cycle (from left to right). The body is drawn for reference and is not to scale. Hollow arrows indicate the direction of fin motion. (A) During the beginning of adduction, the dorsal edge of the fin is anterior to its ventral edge such that the plane of the fin is oriented obliquely relative to the transverse (z,y) plane. Rotation of the fin chord about its base causes the dorsal-most ray to move posteriorly relative to the ventral edge of the fin, which exposes a large surface area along the transverse plane. (B) At the end of adduction, the dorsal-most fin ray has completed rotation and has been retracted back towards the body wall, revealing its resting position above the dorsal surface of the body. (C) At the beginning of abduction, the dorsal-most ray leads the ventral-most ray in protraction, rotating the fin into the z,x plane so that it exposes the least surface area in the transverse plane to minimize drag. The time from the beginning of adduction to the end of adduction is 160 ms, while the time from the end of adduction to the beginning of abduction is 80 ms.

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