Median fin function during the escape response of bluegill sunfish (Lepomis macrochirus). II: Fin-ray curvature.

Although kinematic analysis of individual fin rays provides valuable insight into the contribution of median fins to C-start performance, it paints an incomplete picture of the complex movements and deformation of the flexible fin surface. To expand our analysis of median fin function during the escape response of bluegill sunfish (Lepomis macrochirus), patterns of spanwise and chordwise curvature of the soft dorsal and anal fin surfaces were examined from the same video sequences previously used in analysis of fin-ray movement and orientation. We found that both the span and chord undergo undulation, starting in the anterior region of either fin. Initiated early in Stage 1 of the C-start, the undulation travels in a postero-distal direction, reaching the trailing edge of the fins during early Stage 2. Maximum spanwise curvature typically occurred among the more flexible posterior fin rays, though there was no consistent correlation between maximum curvature and fin-ray position. Undulatory patterns suggest different mechanisms of action for the fin regions. In the anterior fin region, where the fin rays are oriented dorsoventrally, undulation is directed primarily chordwise, initiating a transfer of momentum into the water to overcome the inertia of the flow and direct the water posteriorly. Within the posterior region, where the fin rays are oriented caudally, undulation is predominantly directed spanwise; thus, the posterior fin region acts to ultimately accelerate this water towards the tail to increase thrust forces. Treatment of median fins as appendages with uniform properties does not do justice to their complexity and effectiveness as control surfaces.