GOLDMAN, E.B.*; DANIEL, T.L.: Tuning for swimming performance in hydrozoan jellyfish depends on non-linear springs and temporal patterns of force production

Radial symmetry and primitive neuromuscular organization make jellyfish ideally-suited for exploring the dynamic consequences of the interplay between muscle forcing, material properties, geometry, and fluid motion. Jellyfish, composed predominantly of mesoglea, have a characteristic nonlinear response to an applied strain. Given that non-linearities are pervasive in biological systems, it is important that we understand their functional consequences. We approach this issue with a combination of experimental and theoretical analyses. We combine measured swimming kinematics and measured non-linear material properties with a mathematical model, in which we balance the forces of muscle contraction against hydrodynamic and structural forces. We find that 1) a fluid-solid coupled non-linear model of jellyfish locomotion effectively captures details of jellyfish swimming kinematics; 2) the extent of the non-linearity in bell structural properties has a profound effect on swimming performance, often giving rise to increasing swimming speeds with increasing non-linearity; and 3) subtle details of the timing of muscle force production (its Fourier components) are increasingly important with ever greater non-linear structural forces, revealing patterns of muscle forcing that maximize swimming speed and efficiency. Taken together, these results suggest that, in addition to classic mass and spring resonance behavior, tuning for swimming performance resides in a wide range of other possible parameters, including the non-linearity of the spring, mean bell stiffness, and the subtle details of the temporal pattern of force production.