Design Features and Mechanics of Axial Locomotion in Fish

SYNOPSIS. Locomotion is the result of transfer of momentum fromthe fish musculature to the surrounding water. The present paperdiscusses some basic principles of this momentum transfer andshows the effects of various adaptations of body shape and finshape, size and positioning. Muscles take up a large part of the fish body volume in manycases. The effects of distribution of muscle mass on externalshape, and drag (with its reciprocal influence on the muscularsystem) are analysed. Fins provide an effective means of momentumtransfer, by allowing large amounts of water to be moved bysmall body masses. Fin shape, variable flexibility and positioningall interact to influence thrust producing performance. A frameworkfor understanding the various combinations of fins, their shapesand motion is presented. Reasons for shifting the center ofpropulsion to the rear part of the fish, in anguilliform, andmuch more so in carangiform swimmers are discussed. This shiftis shown to result from considerations of propulsive efficiency.Double-tailed fin configurations, defined as dorsal and ventralfins placed at the same longitudinal positions so as to producea "continuous" are analysed. Examples of both fast starters(such as esocids) and cruising species (scombrids, etc.) areused to point out the advantages of such fin placement.