A common type of jellyfish has a neat trick up its sleeve -- or arms -- to compensate for a missing limb. Instead of regrowing a lost limb, moon jellies rearrange the bodies and remaining limbs to remain symmetrical, researchers found.

Never before observed, the method of natural self-repair the scientists have dubbed "symmetrization" likely helps the jellies survive in the wild, where they are a preferred meal for sea turtles and where at least a third of marine invertebrates such as jellyfish may have injuries at any given time.

While many invertebrates can regrow lost limbs, the moon jellies' strategy of rearrangement rather that regrowth has never been observed before, notes study researcher Michael Abrams of the California Institute of Technology (Caltech).

"We've now observed another self-repair mechanism," says Abrams, a graduate student in biology and biological engineering at Caltech. "It kind of broadens our definition, a little bit, of self-repair."

To study the moon jellies, the researchers focused on their juvenile stage, also known as the ephyra stage, when their simple body plan -- eight symmetrical arms attached to a disc-shaped central body -- would make any tissue regeneration easy to spot.

The researchers performed amputations on anesthetized ephyra, creating examples with two, three, four, five, six, or seven arms, instead of the usual eight, then returned them to a seawater habitat to monitor their response to the injuries similar to what a sea turtle might inflict.

Although the wounds created by the amputations quickly healed, the researchers were surprised to discover that the jellyfish were not regenerating tissue to replace the lost arms but instead reorganized their existing arms, moving them around their disklike bodies to be symmetrical and evenly spaced.

The resymmetrization occurred with whatever number of legs the jellyfish were left with, the researchers reported in Proceedings of the National Academy of Sciences.

"Pretty quickly, we realized that they were doing something very different than what anyone had ever talked about before," Abrams says.

The jellyfish have likely evolved the ability because their symmetry is vital for their survival, the researchers suggest.

"Jellyfish move by 'flapping' their arms; this allows for propulsion through the water, which also moves water -- and food --past the mouth," Abrams explains.

As they swim a boundary layer of thick fluid forms between their arms, creating a continuous surface that aids their paddling motion, he says.

"And you can imagine how this paddling surface would be disturbed if you have a big gap between the arms."

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