MADDEN, P.G.A.*; LAUDER, G.V.; Harvard University; Harvard University: Pectoral fin function in fishes: musculoskeletal design, kinematics and hydrodynamics during propulsion and maneuvering

A full understanding of pectoral fin swimming mechanics includes quantifying skeletal and muscular structure, fin ray and fin surface mechanics, and hydrodynamic mechanisms of fin function. We studied pectoral fin function in bluegill sunfish (Lepomis macrochirus), yellow perch, (Perca flavescens) and killifish (Fundulus diaphanus) during steady swimming and maneuvering. In sunfish, the musculature of each pectoral girdle contains up to 60 muscle bundles that attach to 14 fin rays. Pectoral fin skeletal anatomy was imaged at 13 micron resolution using microCT and reconstructed in 3D to measure fin ray moment arms and geometric relationships among the fin ray elements. High speed video and 3D reconstruction of pectoral fin shape during both steady swimming and maneuvering show that fishes can use the dorsal-most ray, the ventral-most ray or both simultaneously as hydrodynamic leading edges. During maneuvering, the strong side fin (on the outside of the turn) experiences higher curvatures and changes in area than either the weak side or steady swimming fins. Middle rays of the weak side fin curve into oncoming flow, demonstrating active control of fin conformation. EMG experiments confirm differential function of discrete muscle bundles within the abductor superficialis, corresponding with observed ray kinematics. Sonomicrometry during steady swimming in the abductor superficialis reveals strains of 1-2% increasing with speed while during maneuvering strains are as high as 6%. Particle image velocimetry results and dye visualization experiments show the effect of different fin motions on the water and suggest mechanisms of fin force generation. We conclude by summarizing progress towards understanding the generation of forces by pectoral fin muscles and the transmission of forces via the fin membrane and rays to the fluid.