Kinematic comparison of bell contraction by four species of hydromedusae

Abstract

Bell form strongly affects the hydrodynamic performance of swimming hydromedusae. Although the relationship between bell shape and hydrodynamic parameters has been documented for static models of different bell shapes, the dynamic differences in contraction characteristics of different bell shapes have not been described. This is an important issue in medusan motion because the way in which medusan bells contract may influence the effect of bell shape on swimming performance. We measured differences in bell morphologies and wake velocities during swimming by two prolate (streamlined) and two oblate (disc shaped) forms of hydromedusae. Our results indicate that propulsion by prolate medusae is fundamentally different than that of oblate forms. Prolate species were characterized by contraction over the entire length of the bell and produced a narrower, higher velocity jet. Oblate medusae contracted primarily near the bell margin, and produced a broader, lower velocity jet. Prolate medusae achieved higher velocities but lower Froude propulsion efficiencies than oblate medusae. The adaptive value of these patterns are interpreted in terms of foraging and life history patterns.