Undulatory median fin propulsion of two teleosts with different modes of life

Abstract

A simple fluid flow model, based on momentum considerations, is employed to calculate the hydromechnanical efficiency of the undulatory dorsal fin propeller of the electric fish (Gymnarchus niloticus) and the seahorse (Hippocampus hudsonius). The undulatory fins of G. niloticus and H. hudsonius are representative of two extreme kinematic styles. The dorsal fin of G. niloticus is characterized by waveforms which are propagated at low frequency and a leading edge which "sweeps out" a large area. In contrast, the leading edge of the dorsal fin of H. hudsonius sweeps out a comparatively small area and waveforms pass down the fin at a high frequency. It is shown that the propulsive efficiency of the dorsal fin of G. niloticus can be up to twice that of H. hudsonius at similar swimming speeds. Possible explanations for the evolution of the two kinematic modes are discussed in relation to the mode of life of the animals.