Lagrangian coherent structure analysis of swimming jellyfish reveals trade-offs between efficient swimming and effective foraging*

Many jellyfish species benefit from passive energy recapture (PER), a secondary swimming acceleration that occurs if the animal pauses between contraction cycles. Utilization of PER can dramatically reduce the cost of transport (COT)—e.g. by ~50% in Aurelia aurita. However, many species do not take full advantage of the phenomenon. Oblate jellyfish, which rely on the fluid flow generated by their bell contractions for both propulsion and feeding, keep their pauses short, truncating the PER benefit. Here, we investigate the hypothesis that longer pauses increase PER but reduce prey entrainment rates. CFD simulations of free-swimming Aurelia aurita with various pause durations show varied amounts of PER benefit and propulsive energy savings. We compare these savings to the changes in prey clearance rate, found from Lagrangian coherent structures via the finite-time Lyapunov exponent (FTLE) field, and estimate the associated energy gained from feeding. The trade-offs between efficient swimming and effective predation are key to understanding the cosmopolitan success of jellyfish despite their simple biomechanics.