Kinematic comparison of bell contraction by four species of hydromedusae

Bell form strongly affects the hydrodynamic performance of swimming hydrome dusae. Although the relationship between bell shape and hydrodynamic parame ters has been documented for static models of different bell shapes, the dy namic differences in contraction characteristics of different bell shapes h ave not been described. This is an important issue in medusan motion becaus e 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 o blate (disc shaped) forms of hydromedusae. Our results indicate that propul sion by prolate medusae is fundamentally different than that of oblate form s. Prolate species were characterized by contraction over the entire length of the bell and produced a narrower, higher velocity jet. Oblate medusae c ontracted primarily near the bell margin, and produced a broader, lower vel ocity jet. Prolate medusae achieved higher velocities but lower Froude prop ulsion efficiencies than oblate medusae. The adaptive value of these patter ns are interpreted in terms of foraging and life history patterns.