PECTORAL FIN LOCOMOTION IN FISHES - TESTING DRAG-BASED MODELS USING 3-DIMENSIONAL KINEMATICS

Paired fin propulsion in fishes has classically been divided into two categories which represent biomechanical extremes in the use of append ages for propulsion: lift-based and drag-based mechanisms of thrust pr oduction. Theoretical models predict that fishes using drag-based prop ulsion should have wedge-shaped fins with relatively blunt distal edge s, a fm beat cycle that is oriented along the anteroposterior (x) axis , feathering of the fin to reduce drag during the protraction phase, a nd maximal fin area during the retraction phase as the fin sweeps post eriorly perpendicular to the body. In this paper we use a three-dimens ional analysis of pectoral fin propulsion in the largemouth bass, Micr opterus salmoides, to (1) evaluate the extent to which bass pectoral f in kinematics fit predictions of drag-based propulsion, and (2) demons trate the complexity of fin movement when the traditional two-dimensio nal analysis is extended into three dimensions. We attached small mark ers to visualize the diaphanous distal fin edge, and we videotaped lat eral and ventral views from which we could measure x, y, and z coordin ates from the fin and body. We divided the fm into two triangular elem ents for which we calculated planar (three-dimensional) angles relativ e to each of three reference planes (XY, YZ, and XZ) during the fin be at cycle. We show how angles of attack based only on two-dimensional d ata may result in gross errors that severely compromise understanding of the mechanics and hydrodynamics of pectoral propulsion. Furthermore , three-dimensional analysis revealed that bass fin kinematics are muc h more complex than expected on a rowing model of drag-based propulsio n, and that the pectoral fins may produce drag-based thrust even durin g protraction. Three-dimensional kinematic data are critical to unders tanding the hydrodynamics of aquatic animal propulsion. Such data are a necessary foundation for reconstructing patterns of movement, modeli ng (both theoretical and empirical), and for assessing the extent to w hich motion is under active control or a passive consequence of fluid resistance.