Pectoral Fin Locomotion in Fishes: Testing Drag-based Models Using Three-dimensional Kinematics

Abstract

Paired fin propulsion in fishes has classically been divided into two categories which represent biomechanical extremes in the use of appendages for propulsion: lift-based and drag-based mechanisms of thrust production. Theoretical models predict that fishes using drag-based propulsion should have wedge-shaped fins with relatively blunt distal edges, a fin beat cycle that is oriented along the anteroposterior (x) axis, feathering of the fin to reduce drag during the protraction phase, and maximal fin area during the retraction phase as the fin sweeps posteriorly perpendicular to the body. In this paper we use a three-dimensional analysis of pectoral fin propulsion in the largemouth bass, Micropterus salmoides , to (1) evaluate the extent to which bass pectoral fin kinematics fit predictions of drag-based propulsion, and (2)demonstrate the complexity of fin movement when the traditional two-dimensional analysis is extended into three dimensions. We attached small markers to visualize the diaphanous distal fin edge, and we videotaped lateral and ventral views from which we could measure x, y, and z coordinates from the fin and body. We divided the fin into two triangular elements for which we calculated planar (three-dimensional) angles relative to each of three reference planes (XY, YZ and XZ) during the fin beat cycle. We show how angles of attack based only on two-dimensional data 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 much more complex than expected on a rowing model of drag-based propulsion, and that the pectoral fins may produce drag-based thrust even during protraction. Three dimensional kinematic data are critical to understanding the hydrodynamics of aquatic animal propulsion. Such data are a necessary foundation for reconstructing patterns of movement, modeling (both theoretical and empirical), and for assessing the extent to which motion is under active control or a passive consequence of fluid resistance.