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First published online May 18, 2006
Journal of Experimental Biology 209, 2025-2033 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02242

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Fast-swimming hydromedusae exploit velar kinematics to form an optimal vortex wake

John O. Dabiri^1,*, Sean P. Colin² and John H. Costello³

¹ Graduate Aeronautical Laboratories and Bioengineering, California Institute of Technology, Mail Code 138-78, Pasadena, CA 91125, USA
² Biology and Marine Biology, Roger Williams University, MNS 241, Bristol, RI 02809, USA
³ Biology, Providence College, Providence, RI 02918, USA

^* Author for correspondence (e-mail: jodabiri{at}caltech.edu)

Accepted 27 March 2006

Fast-swimming hydromedusan jellyfish possess a characteristic funnel-shaped velum at the exit of their oral cavity that interacts with the pulsed jets of water ejected during swimming motions. It has been previously assumed that the velum primarily serves to augment swimming thrust by constricting the ejected flow in order to produce higher jet velocities. This paper presents high-speed video and dye-flow visualizations of free-swimming Nemopsis bachei hydromedusae, which instead indicate that the time-dependent velar kinematics observed during the swimming cycle primarily serve to optimize vortices formed by the ejected water rather than to affect the speed of the ejected flow. Optimal vortex formation is favorable in fast-swimming jellyfish because, unlike the jet funnelling mechanism, it allows for the minimization of energy costs while maximizing thrust forces. However, the vortex `formation number' corresponding to optimality in N. bachei is substantially greater than the value of 4 found in previous engineering studies of pulsed jets from rigid tubes. The increased optimal vortex formation number is attributable to the transient velar kinematics exhibited by the animals. A recently developed model for instantaneous forces generated during swimming motions is implemented to demonstrate that transient velar kinematics are required in order to achieve the measured swimming trajectories. The presence of velar structures in fast-swimming jellyfish and the occurrence of similar jet-regulating mechanisms in other jet-propelled swimmers (e.g. the funnel of squid) appear to be a primary factor contributing to success of fast-swimming jetters, despite their primitive body plans.

Key words: locomotion, wake, vortices, jellyfish, Nemopsis bachei

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