Jellyfish shape as a mechanical balance.

Why are jellyfish round? Animals get their shapes as they develop. After development, however, how animals keep their shapes is less understood. Moon jellies respond to perturbations to body shape, such as being halved or quartered, by reorganizing existing body parts and regaining radial symmetry, i.e., their round shape. The robust recovery of radial symmetry led us to investigate, in this study, how being round is encoded. We tested perturbing shape by grafting body sections in varying configurations. Testing these perturbations confirms the moon jellies' ability to recover their round shape from many perturbations. However, in response to some perturbations, the jellies can also adopt other stable body shapes, such as oval, quadrilateral, and triangular. Thus, although the jellies are characterized by a radially symmetrical body plan, perturbations can lead to them recovering to bilateral shapes. Employing mathematical modeling, we find that interactions between forces from muscle contractions and viscoelastic tissues can explain the recovery to different shapes. A stable body shape is achieved when the mechanical forces are locally balanced, regardless of symmetry. Consistent with the model prediction that stable shape is the outcome of balancing mechanical forces, modulating the mechanical parameter in the system, i.e., the muscle contraction rate, can produce shape-shifting. Maintaining shapes dynamically as the balance of mechanical forces may enable the animals to readily adapt to changing physical environments.