Passive energy recapture in jellyfish contributes to propulsive advantage over other metazoans

Abstract

Gelatinous zooplankton populations are well known for their ability to take over perturbed ecosystems. The ability of these animals to outcompete and functionally replace fish that exhibit an effective visual predatory mode is counterintuitive because jellyfish are described as inefficient swimmers that must rely on direct contact with prey in order to feed. We show that jellyfish exhibit a novel mechanism of passive energy recapture, which is exploited to allow jellyfish to travel 30 percent further each swimming cycle, thereby reducing metabolic energy demand by swimming muscles. By accounting for large interspecific differences in net metabolic rates, we demonstrate, contrary to prevailing views, the jellyfish (Aurelia aurita) is one of the most energetically efficient propulsors on the planet, exhibiting a cost-of-transport (J kg-1 m-1) lower than other metazoans. We estimate that reduced metabolic demand by passive energy recapture improves cost-of-transport by 48%, allowing jellyfish to achieve the large sizes required for sufficient prey encounters. Pressure calculations, using both computational fluid dynamics (CFD) and a new method from empirical velocity field measurements demonstrate that this extra thrust results from positive pressure created by a vortex ring underneath the bell during the refilling phase of swimming. These results demonstrate a physical basis for the ecological success of medusan swimmers despite their simple body plan. Results from this study also have implications for bio-inspired design where low-energy propulsion is required.