Design Features and Mechanics of Axial Locomotion in Fish

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Abstract
SYNOPSIS. Locomotion is the result of transfer of momentum from the fish musculature to the surrounding water. The present paper discusses some basic principles of this momentum transfer and shows the effects of various adaptations of body shape and fin shape, size and positioning. Muscles take up a large part of the fish body volume in many cases. The effects of distribution of muscle mass on external shape, and drag (with its reciprocal influence on the muscular system) are analysed. Fins provide an effective means of momentum transfer, by allowing large amounts of water to be moved by small body masses. Fin shape, variable flexibility and positioning all interact to influence thrust producing performance. A framework for understanding the various combinations of fins, their shapes and motion is presented. Reasons for shifting the center of propulsion to the rear part of the fish, in anguilliform, and much more so in carangiform swimmers are discussed. This shift is shown to result from considerations of propulsive efficiency. Double-tailed fin configurations, defined as dorsal and ventral fins placed at the same longitudinal positions so as to produce a “continuous” are analysed. Examples of both fast starters (such as esocids) and cruising species (scombrids, etc.) are used to point out the advantages of such fin placement.

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