| Fins act as control surfaces by which fish can generate and react to hydrodynamic forces during a variety of locomotor behaviors. Within the ray-finned fish, the unique segmented, bilaminar design of their fin rays, for which they are named, provide the fish with the ability to independently control the degree of stiffness and curvature of each ray, enabling them to modulate the fin surface and the resultant hydrodynamic forces. While fin morphology and kinematic properties have been studied extensively, previous researchers have looked at the fins as a whole, overlooking variation between rays within the same fin. This work focused on describing the morphological and kinematic variation among fin-rays within the dorsal and anal fins of the bluegill sunfish, Lepomis macrochirus.;Examining several musculoskeletal features of individual fin-rays within the dorsal and anal fins of bluegill, variation in (1) spine and ray lengths, (2) the proportion of the rays that were segmented or branched and (3) masses of the muscle slips that actuate the fin-rays were found; differences were correlated to longitudinal position within the fin. The quantitative results matched with a qualitative assessment of positional variations in fin-ray flexibility and joint mobility. Based on variation in morphological and biomechanical properties, regional differences in fin-ray kinematics during locomotion, allowing discrete regions of the fins to perform distinct functional roles, were proposed.;Three-dimensional kinematics of selected individual fin-rays were quantified during the escape response of bluegill sunfish, a stereotypical behavior in which the fish undergoes a rapid acceleration and displacement to avoid predation. During this behavior, timing and magnitude of angular displacement and curvatures among fin-rays also differed predictably with longitudinal position.;Most interestingly, a chordwise cupping of all three fins during Stage 1 of the fast-start was consistent with recent findings that median fins contribute to thrust forces generated by the body. Furthermore, a traveling wave along the lengths of the posterior rays of the soft dorsal and anal fins may be integral in determining the final direction of water jet to optimize the fin's thrust component, rather than generating only lateral, stabilizing forces. |
