Swimming in the electric eels and knifefishes

Abstract

The kinematics of steady forward swimming in six species of Gymnotidae and three species of Notopteridae are described. All the gymnotids and one notopterid (Xenomystis nigri) are propelled by the action of an undulatory anal fin (gymnotiform mode). Notopterus notopterus and Notopterus chilata employ the body and anal fin as a single propulsive unit and generate a body wave. Rapid bouts of burst swimming activity (e.g., escape responses) are generated by large amplitude motions of the entire body in all species studied. Experimentally determined drag coefficients exceed the theoretical rigid body predicted minimum values for the case of a laminar boundary layer and reasons for this are suggested. Values of the drag coefficient inferred from hydromechanical theory are within 20% of the experimental values. It is concluded that fish swimming in the gymnotiform mode may be subject to significantly less viscous drag than fish of equivalent size swimming at the same speed in the subcarangiform mode. Hydromechanical theory indicates a significant enhancement of fin added mass (due to the presence of a rigid body) over that of an isolated fin. The hydromechanical efficiency of the undulating fin system ranged from about 0.6 to 0.9 over a speed range of 0.2–5.0 lengths∙s−1 (corresponding to a Reynolds number range of about 103–105). It is suggested that undulatory median fin propulsion in the electric eels and knifefishes is an adaptation to swimming with high hydromechanical efficiency at low absolute forward speeds. Similarities and differences in body form between the species and the evolution of gymnotiform swimming are discussed.