Self-repairing symmetry in jellyfish through mechanically driven reorganization

Significance Animals are endowed with the capacity to repair injuries. In this study, we found that, upon amputation, the moon jellyfish Aurelia aurita rearranges existing body parts and recovers radial symmetry within a few days. This unique strategy of self-repair, which we call symmetrization, requires mechanical forces generated by the muscle-based propulsion machinery. We observed a similar strategy in a number of other jellyfish species. This finding may contribute to understanding the evolutionary pressures governing biological self-repair strategies. Beyond biology, this finding may inspire a mechanically driven, self-organizing machinery that recovers essential geometry without regenerating precise forms. What happens when an animal is injured and loses important structures? Some animals simply heal the wound, whereas others are able to regenerate lost parts. In this study, we report a previously unidentified strategy of self-repair, where moon jellyfish respond to injuries by reorganizing existing parts, and rebuilding essential body symmetry, without regenerating what is lost. Specifically, in response to arm amputation, the young jellyfish of Aurelia aurita rearrange their remaining arms, recenter their manubria, and rebuild their muscular networks, all completed within 12 hours to 4 days. We call this process symmetrization. We find that symmetrization is not driven by external cues, cell proliferation, cell death, and proceeded even when foreign arms were grafted on. Instead, we find that forces generated by the muscular network are essential. Inhibiting pulsation using muscle relaxants completely, and reversibly, blocked symmetrization. Furthermore, we observed that decreasing pulse frequency using muscle relaxants slowed symmetrization, whereas increasing pulse frequency by lowering the magnesium concentration in seawater accelerated symmetrization. A mathematical model that describes the compressive forces from the muscle contraction, within the context of the elastic response from the mesoglea and the ephyra geometry, can recapitulate the recovery of global symmetry. Thus, self-repair in Aurelia proceeds through the reorganization of existing parts, and is driven by forces generated by its own propulsion machinery. We find evidence for symmetrization across species of jellyfish (Chrysaora pacifica, Mastigias sp., and Cotylorhiza tuberculata).