An undulatory swimming gait is characterized by short lateral displacement waves that propagate backwards along the body of the swimmer faster than it swims. Hydrodynamic theory of elongated bodies predicts that if the amplitude of the displacement waves does not increase toward the caudal end, the part of the swimmer posteriad of the dorso-ventrally widest point takes no part in propulsion. It also predicts that if the amplitude does increase, then the hydrodynamic propulsion efficiency suffers. Cusk eels have their widest point located in the anterior half of the body with the bulk of their locomotive muscles located posteriad of it; indeed, they swim so that the amplitude of the propulsion wave increases toward the caudal end. Anguillid eels have their widest point posteriad of the mid-body, and their locomotive muscles are distributed along their entire length-but they swim as cusk eels, using the posterior half only. Apparently, both use hydrodynamically inefficient gaits. The paper questions the definition of propulsion efficiency and shows that biomechanical considerations are more important than hydrodynamic, and that most probably fish adjust their gait to maximize the ratio between the energy made good (the product of thrust and distance) and the chemical energy consumed by the muscles. The role of body shape is discussed.
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