Locomotion in sturgeon: Function of the pectoral fins

Pectoral fins are one of the major features of locomotor design in ray-finn ed fishes and exhibit a well-documented phylogenetic transition from basal to derived clades. In percomorph fishes, the pectoral fins are often used t o generate propulsive force via oscillatory movements, and pectoral fin pro pulsion in this relatively derived clade has been analyzed extensively. How ever, in the plesiomorphic pectoral fin condition, exemplified by sturgeon, pectoral fins extend laterally from the body in a generally horizontal ori entation, have been assumed to generate lift to balance lift forces and mom ents produced by the heterocercal tail, and are not oscillated to generate propulsive force. The proposal that pectoral fins in fishes such as sturgeo n generate lift during horizontal locomotion has never been tested experime ntally in freely swimming fishes. In this paper, we examine the function of pectoral fins in sturgeon swimming at speeds from 0.5-3.0 L s(-1), where L is total body length. Sturgeon were studied during steady horizontal locom otion as well as while sinking and rising in the water column. Pectoral fin function was quantified using three-dimensional kinematics to measure the orientation of the fin surface, digital particle image velocimetry (DPIV) w as used to describe flow in the wake of the fin and to estimate force exert ed on the water, and electromyography was used to assess pectoral fin muscl e function. Sturgeon (size range 25-32 cm total length) swam horizontally u sing continuous undulations of the body with a positive body angle that dec reased from a mean of 20 degrees at 0.5 L s(-1) to 0 degrees at 3.0 L s(-1) . Both the angle of the body and the pectoral fin surface angle changed sig nificantly when sturgeon moved vertically in the water column. Three-dimens ional kinematic analysis showed that during steady horizontal swimming the pectoral fins are oriented with a negative angle of attack predicted to gen erate no significant lift, This result was confirmed by DPIV analysis of th e pectoral fin wake, which only revealed fin vortices, and hence force gene ration, during maneuvering, The orientation of the pectoral fins estimated by a two-dimensional analysis alone is greatly in error and may have contri buted to previous suggestions that the pectoral fins are oriented to genera te lift. Combined electromyographic and kinematic data showed that the post erior half of the pectoral fin is actively moved as a flap to reorient the head and body to initiate rising and sinking movements. A new force balance for swimming sturgeon is proposed for steady swimming and vertical maneuve ring. During steady locomotion, the pectoral fins generate no lift and the positive body angle to the flow is used both to generate lift and to balanc e moments around the center of mass. To initiate rising or sinking, the pos terior portion of the pectoral fins is actively moved ventrally or dorsally , respectively, initiating a starting vortex that, in turn, induces a pitch ing moment reorienting the body in the flow. Adjustments to body angle init iated by the pectoral fins serve as the primary means by which moments are balanced.