Escaping Flatland: three-dimensional kinematics and hydrodynamics of median fins in fishes

Abstract

SummaryFish swimming has often been simplified into the motions of a two-dimensional slice through the horizontal midline, as though fishes live in a flat world devoid of a third dimension. While fish bodies do undulate primarily horizontally, this motion has important three-dimensional components, and fish fins can move in a complex three-dimensional manner. Recent results suggest that an understanding of the three-dimensional body shape and fin motions is vital for explaining the mechanics of swimming, and that two-dimensional representations of fish locomotion are misleading. In this study, we first examine axial swimming from the twodimensional viewpoint, detailing the limitations of this view. Then we present data on the kinematics and hydrodynamics of the dorsal fin, the anal fin and the caudal fin during steady swimming and maneuvering in brook trout, Salvelinus fontinalis, bluegill sunfish, Lepomis macrochirus, and yellow perch, Perca flavescens. These fishes actively move the dorsal and anal fins during swimming, resulting in curvature along both anterio-posterior and dorso-ventral axes. The momentum imparted to the fluid by these fins comprises a substantial portion of total swimming force, adding to thrust and contributing to roll stability. While swimming, the caudal fin also actively curves dorso-ventrally, producing vortices separately from both its upper and lower lobes. This functional separation of the lobes may allow additional control of three-dimensional orientation, but probably reduces swimming efficiency. In contrast, fish may boost the caudal finʼs efficiency by taking advantage of the flow from the dorsal and anal fins as it interacts with the flow around the caudal fin itself. During maneuvering, fish readily use their fins outside of the normal planes of motion. For example, the dorsal fin can flick laterally, orienting its surface perpendicular to the body, to help in turning and braking. These data demonstrate that, while fish do move primarily in the horizontal plane, neither their bodies nor their motions can accurately be simplified in a two-dimensional representation. To begin to appreciate the functional consequences of the diversity of fish body shapes and locomotor strategies, one must escape Flatland to examine all three dimensions.

A fish perspective: detecting flow features while moving using an artificial lateral line in steady and unsteady flow

“…The importance of the third dimension to our understanding of fish propulsion has recently begun to be appreciated [24]; our work now brings a similar three-dimensionality to onboard flow sensing. Here, we used vertically distributed sensors on a three-dimensional craft to examine what is usually studied as a lateral two-dimensional wake, namely the KVS.…”

Volumetric imaging of fish locomotion

“…While these techniques have given us a great deal of insight into the fluid mechanics of fish locomotion, the current method of inferring three-dimensional wake structure from repeated two-dimensional PIV slices produces considerable room for error. A mechanism that instantaneously captures a three-dimensional wake structure is needed in order to understand fully the fluid interactions between fishes and their environment, and among the different fins used for propulsion [16,17].…”

New approaches for assessing squid fin motions: Coupling proper orthogonal decomposition with volumetric particle tracking velocimetry

“…Greater wave-like motions and a more downstream-positioned flap during arms-first swimming seemingly contribute to more lift generation, given higher observed lift forces for arms-first compared with tail-first swimming. This is not surprising considering that previous studies on fish and biorobotic systems showed that slight changes in fin behavior and mechanical properties can have a major influence on the timing, direction and magnitude of force production (Tytell et al, 2008;Tangorra et al, 2010;Neveln et al, 2014). Positioning the maximum fin amplitude farther downstream may also help counteract body pitching induced by rear-pointing jets.…”