KINEMATICS OF PECTORAL FIN LOCOMOTION IN THE BLUEGILL SUNFISH LEPOMIS MACROCHIRUS

Abstract

The pectoral fins of ray-finned fishes are flexible and capable of complex movements, and yet little is known about the pattern of fin deformation during locomotion. For the most part, pectoral fins have been modeled as rigid plates. In order to examine the movements of different portions of pectoral fins, we quantified the kinematics of pectoral fin locomotion in the bluegill sunfish Lepomis macrochirus using several points on the distal fin edge and examined the effects of swimming speed on fin movements. We simultaneously videotaped the ventral and lateral views of pectoral fins of four fish swimming in a flow tank at five speeds ranging from 0.3 to 1.1 total lengths s-1. Four markers, placed on the distal edge of the fin, facilitated field-by-field analysis of kinematics. We used analyses of variance to test for significant variation with speed and among the different marker positions. Fin beat frequency increased significantly from 1.2 to 2.1 Hz as swimming speed increased from 0.3 to 1.0 total lengths s-1. Maximal velocities of movement for the tip of the fin during abduction and adduction generally increased significantly with increased swimming speed. The ratio of maximal speed of fin retraction to swimming speed declined steadily from 2.75 to 1.00 as swimming speed increased. Rather than the entire distal edge of the fin always moving synchronously, markers had phase lags as large as 32 with respect to the dorsal edge of the fin. The more ventral and proximal portions of the fin edge usually had smaller amplitudes of movement than did the more dorsal and distal locations. With increased swimming speed, the amplitudes of the lateral and longitudinal fin movements generally decreased. We used two distal markers and one basal reference point to determine the orientation of various planar fin elements. During early adduction and most of abduction, these planar fin elements usually had positive angles of attack. Because of fin rotation, angles of attack calculated from three-dimensional data differed considerably from those estimated from a simple lateral projection. As swimming speed increased, the angles of attack of the planar fin elements with respect to the overall direction of swimming approached zero. The oscillatory movements of the pectoral fins of bluegill suggest that both lift- and drag-based propulsive mechanisms are used to generate forward thrust. In addition, the reduced frequency parameter calculated for the pectoral fin of Lepomis (sigma=0.85) and the Reynolds number of 5x10(3) indicate that acceleration reaction forces may contribute significantly to thrust production and to the total force balance on the fin.