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

Brad A Chadwell,Emily M Standen,Miriam A Ashley-Ross,George V Lauder

doi:10.1242/jeb.068593

Brad A Chadwell, Emily M Standen + Show 2 more

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https://doi.org/10.1242/jeb.068593

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Abstract

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.

Highlights

Ray-finned fishes (Actinopterygii) derive their name from the bony supports within each membranous paired or median fin
We show that the curvature patterns in both soft dorsal and anal fins conform to our expectations based on morphology
Axial movements during the C-start have been described in the companion paper and were shown to be consistent across the three fish analyzed (Chadwell et al, 2012)

Summary

Introduction

Ray-finned fishes (Actinopterygii) derive their name from the bony supports within each membranous paired or median fin. It is the fin rays that may be actively controlled by the fish, the fin rays by themselves do not define the fin; the membrane connecting the bony supports plays an integral role by providing both surface and tensile resistance that creates the ‘fan’ of the fin. Individual fin rays may curve, or move laterally or sagittally; such movements of the many fin rays within each fin interact with the flexible membrane, throwing the surface of the ‘fan’ into complex configurations. Actinopterygian fins are flexible structures that interact with the surrounding flow as dynamic curved surfaces (Lauder et al, 2006; Walker, 2004; Walker and Westneat, 2002). Understanding the complex interplay between fin shape and motion during locomotion will provide new insight into the hydrodynamic roles of the fins as control surfaces (Lauder and Drucker, 2004). It is essential to quantify fin curvature as well as the motions of individual fin rays (Chadwell et al, 2012) to understand how fins may function

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