MADDEN, P.G. *; LAUDER, G.V.; Harvard University; Harvard University: Pectoral fin locomotion in fishes: fin ray deformation and material properties

Fish pectoral aspect ratio and planform have been studied but little attention has been paid to fin ray flexibility and planform changes during a fin beat. Such changes in fin shape may dramatically affect the propulsive forces produced. We used high-resolution high-speed cameras (1280�1024 pixels at 250 fps) to measure detailed pectoral fin ray kinematics during propulsion and maneuvering in fish with very flexible fin rays (sunfish, Lepomis macrochirus) and with stiffer fin rays (perch, Perca flavescens). The flexible rays of sunfish undergo large deformations during adduction and abduction, with ray tips sometimes pointing almost perpendicular to the fin ray base. During the transitions between adduction and abduction the whole ray can assume an S-shape. In contrast, the stiffer rays of perch show significantly less curvature. During abduction, the ventral three fin rays in sunfish appear to actively form a fin surface that is concave in a direction pointing both anterior and lateral. Fin planform is modulated by bringing the rays closer or further apart. The high tip curvature of sunfish rays during abduction makes the distal ends almost parallel to the fish body and may therefore increase the lateral force produced. During adduction, the angle of the distal ends of the rays lags behind the angle of the ray bases to form a trailing edge fin surface almost perpendicular to the fish body, probably increasing the drag-based propulsive force. Each fish fin ray is composed of two segmented bony elements (hemitrichia). Using an axial relative sliding motion, the hemitrichia may actively adjust fluid flow by changing fin ray curvature. Concurrent studies of the mechanical properties of individual rays will improve the understanding of the significance of propulsor flexibility.