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. 
 
 Abstract 
 
 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). ||||| View Images Moon jellyfish (pictured at the Vancouver Aquarium) can restore their symmetry within four days. 
 
 Photograph by Joel Sartore, National Geographic 
 
 In the spring of 2013, biologist Michael Abrams had cut two arms off a young jellyfish when he witnessed something he'd never seen before. 
 
 "He started yelling... 'You won't believe this, you've got to come here and see what's happening,'" recalls his Ph.D. advisor Lea Goentoro , a biologist at Caltech in Pasadena. 
 
 The team expected that Abrams' moon jellyfish (Aurelia aurita) had regrown its limbs, since many other marine invertebrates —including the polyp stage of moon jellyfish—regenerate themselves this way. (See " Pictures: 5 Animals That Regrow Body Parts .") 
 
 Instead, the moon jelly had rearranged its six remaining arms until they were evenly placed around the body. Muscles in the jellyfish's body had pushed and pulled on the remaining arms until they were once again evenly spaced. Being symmetrical is crucial for moon jellyfish movement. 
 
 The scientists had stumbled upon a phenomenon completely new to science, which they call "symmetrization." Because jellyfish often suffer from injuries—sometimes inflicted by unsuccessful predators—symmetrization is an important method to heal themselves. 
 
 "This is an amazing study and a fantastic piece of detective work," says Sara Lindsay , a marine biologist at the University of Maine, who was not involved with the study. 
 
 Flexing Muscles 
 
 Goentoro and colleagues could have easily missed their discovery: They had originally set out to study limb repair in Turritopsis dohrnii, also known as the immortal jellyfish . While they waited for specimens to arrive, the scientists practiced their experimental techniques on the much more common moon jelly. 
 
 After observing the jellyfish putting themselves back together, "I repeated the experiments several times because I thought I might have made a mistake," Abrams says. (See jellyfish pictures from the National Geographic archives .) 
 
 Watch a video of jellyfish in their natural habitat. 
 
 Dance of the Jellyfish De-stress and drift into space with these hypnotic and exotic creatures. Pure escapism for the soul. 
 
 
 
 
 
 Once he had reassured himself that the jellyfish really was rearranging its arms to restore symmetry—a process which takes between 12 hours to 4 days—Abrams and his team had to figure out how the animal manages such a feat. 
 
 After several false starts, the researchers focused on the jellyfish's musculature. Giving the moon jellies a muscle relaxant made them unable to complete the symmetrization process. On the other hand, when the team increased the muscular pulses of the juvenile jellyfish, the process occurred faster than usual. 
 
 This suggests that the muscles push against each other to get the arms as widely spaced as possible. 
 
 "It's like squeezing one side of a stress ball and you get a protrusion on the other side," says Goentoro, whose study appeared June 15 in the journal Proceedings of the National Academy of Sciences. 
 
 He started yelling... 'You won't believe this, you've got to come here and see what's happening.' Lea Goentoro, Caltech biologist 
 
 Back on Track 
 
 Restoring symmetry is crucial to the marine animals: Asymmetrical jellyfish have difficulties propelling themselves through the water and feeding properly. A jellyfish can lose a limb due to predators. 
 
 View Images Moon jellyfish (pictured, animals from the Toledo Zoo) are common in the world's oceans. 
 
 Photograph by Joel Sartore, National Geographic 
 
 It's like a car: Remove one wheel and you won't be able to drive it. But if you shift the remaining wheel from the side of the vehicle into the center, the car will once again be able to move about, the researchers say. 
 
 "This isn't replacing lost parts, it's replacing their function. That's pretty cool," Lindsay says. 
 
 Goentoro adds that understanding the newly discovered phenomenon could provide new insight for scientists studying regenerative medicine. (Related: "Will We Ever Regenerate Limbs?") 
 
 "We hope it will inspire new technology for biomaterials—not by regenerating what was lost, but by restoring function," she says.
– When Caltech biologist Michael Abrams cut two arms off a young jellyfish in 2013, he figured it would do what many marine invertebrates do—grow new ones. But no. "[Abrams] started yelling... 'You won't believe this, you've got to come here and see what's happening,'" his PhD adviser Lea Goentoro tells National Geographic. Reporting this week in the journal Proceedings of the National Academy of Sciences, Abrams says he watched the jellyfish, which relies on being symmetrical to move about, not regenerate the missing arms but rather rearrange its remaining six limbs so that they were symmetrical again. The phenomenon, dubbed symmetrization, has never before been observed in nature, and Abrams was floored. The jellyfish was using its own muscles to push and pull on its remaining six arms to space them out evenly again. (They confirmed this by observing that muscle relaxants made the jellies unable to rearrange their arms, while increasing muscular pulses allowed them to rearrange their arms faster.) And the discovery was accidental; Abrams and his team had only been cutting into the common moon jellyfish to practice for their future study on what are called immortal jellyfish, which had yet to arrive in the lab. They've since observed symmetrization in moon jellies many times, and it takes anywhere from 12 hours to four days to complete. (Scientists recently made another staggering observation, this one in Norway.)