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.

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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.

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.