Previous research has investigated the anatomy, mechanics, and active control of fin rays and their flexibility during normal locomotion, and studied the mechanical properties of isolated fin rays (Lauder et al., 2006, 2011; Alben et al., 2007; Taft et al., 2008; Taft and Taft, 2012). However, there has been no detailed investigation into the flexibility of ray-finned fish fin rays under unsteady locomotor conditions such as turning, and no study to our knowledge has investigated the effect of fluid dynamic perturbations on fin ray function. Fish naturally encounter a variety of fluid jets and vortices from wakes, currents, tides, and storm surges during swimming, and a complete understanding of fin ray mechanics should include analysis of the effect of perturbations on fin ray function. Therefore, our research sought to answer two major questions: a) just how flexible are fish pectoral fin rays during normal swimming and turning, and b) what effect do fluid perturbations have on pectoral fin ray dynamics during normal swimming? We hypothesized that fish can actively stiffen their fins to have lower fin ray curvatures during turning than during swimming. We also hypothesized that flexible fin rays would yield to fluid perturbations and that perturbations would, therefore, disrupt the kinematics of the fin beat. To answer these questions, we first examined the structure of bluegill sunfish (Lepomis macrochirus) fin rays using high-resolution computed microtomography (mCT) scans. Then, we investigated the threedimensional curvature of fish fin rays under normal swimming conditions (steady swimming and low-speed turning maneuvers) and with induced passive deformation by a vortex perturbation impacting the fin. Finally, we used computational fluid dynamic models to evaluate the effect of stiffness and segmentation on patterns of fin ray curvature resulting from perturbations.