A swimming adult medusa that regained symmetry after losing three arms as an ephyra. (Michael Abrams and Ty Basinger/CalTech)

Some animals that lose a limb simply regenerate a new one.

That's not what moon jellyfish do when they lose an arm or two. Nope. They work with what they've got. These translucent creatures actually reorganize their remaining arms to regain their lost symmetry.

CalTech researchers describe "symmetrization" as a previously unknown process and published their findings this week in the journal Proceedings of the National Academy of Sciences.

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Symmetry is super important to jellyfish, which need it so they can move and eat. When moon jellyfish lose an arm, they have a physical process that forces them back into shape.

"What we found was that they rely on the mechanical forces generated by their own propulsion machinery, [which is] their muscle and the viscoelasticity of their jelly material," said Michael Abrams, a CalTech graduate student and a lead author of the study.

Researchers studied a total of about 500 organisms from four different species. They anesthetized juvenile jellyfish, called ephyra, and amputated anywhere from one to seven of their arms.


One-day old Aurelia ephyra swimming with eight limbs. (Michael Abrams and Ty Basinger/CalTech)

While it took just a few hours for the wounds to heal, the jellyfish didn't create new cells. But two to four days later they were once again symmetrical.

What's more, the injured jellyfish that reorganized themselves matured into adults. The 15 percent of the young jellies that didn't reorganize successfully weren't able to mature normally.

"It broadens our understanding of the self-repair mechanism," Abrams said, adding that it's unclear what those implications could be. "So maybe we think about self-repair a little differently. It's not always about regeneration and regaining lost parts."


A single juvenile Aurelia jellyfish was split into a three-armed section (bottom) and a five-armed section (top). Each section mechanically reorganized itself to reform radial symmetry with its remaining arms. From left to right, the jellyfish are seen immediately after amputation, then six hours later, 18 hours later and 50 hours later. (Michael Abrams and Ty Basinger/CalTech)

To test this out, the researchers added muscle relaxers to the injured jellyfish's seawater. That slowed down muscle contractions, so it took them longer to reorganize. The opposite effect happened when researchers reduced the magnesium levels in the water.

"With each pulse of the arm, they contract their arms," Abrams said. "That pulls their material in and that means there's higher pressure of material that pushes against itself and repels against each other."

The jelly material facilitates the process, he added.

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One co-author, Chin-Lin Guo of Academia Sinica in Taiwan, helped develop a mathematical model to demonstrate how the jellyfish managed to force themselves back into symmetry by sheer force of their muscle contractions.

In this video, the dot represents the symmetrical center of the jellyfish:

One researcher compared the process to removing a front wheel from a four-wheel car, Abrams said. The car needs balance in order to move, so it would be like moving the remaining front wheel to the middle of the vehicle.

Self-repair is crucial to these creatures; an estimated 33 to 47 percent of seafloor invertebrates are injured at any given point in time, according to a 2010 study published in the journal Integrative and Comparative Biology.

Other researchers could take this principle of reorganizing and apply it to their work. "It's not out of the realm of possibility that we could try to build something that takes into account the mechanism of symmetrization," Abrams said.

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