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.
John O. Dabiri, Sean P. Colin and John H. Costello Journal of Experimental Biology 209, 2025-2033 (2006)
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Videos (click thumbnails):
Jet propulsion by the hydromedusae Aglantha digitale and Nemopsis bachei. In jet propulsion only a starting vortex is present in the wake and it is expelled from the bell of the medusae during contraction and travels rapidly away from the medusae. (Quicktime File 6,570 KB)
High magnification of bell kinematics of the hydromedusa N. bachei while swimming (shown at near normal speed and slow motion). Notice how the velum (the membrane along the bell margin) constricts the opening (or orifice) during pulsation. This serves to optimize the jet that is expelled. (Quicktime File 6,565 KB)