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The Relentless Symmetry of a Jellyfish

When a moon jellyfish is injured, it grows new tissue to become more symmetrical—but not necessarily to replace lost limbs.

For many sea creatures, regrowing body parts is a routine matter of survival. The ocean is full of predators to whom the jellyfish, for example, is the perfect hor d'oeuvre: a tasty, gelatinous morsel.

So when a jellyfish finds itself with a tangle of tentacles ripped off, or a bite-sized chunk of its jelly suddenly missing, the ability to heal quickly is critical. This trait is so marked, in fact, that scientists often look to hydras—a tiny jellyfish cousin—to better understand how cell regeneration works.

Among moon jellyfish, though, the healing appears to be focused on restoring anatomical symmetry—not replacing limbs. That's according to scientists at the California Institute of Technology who discovered a self-repair mechanism that shows jellyfish don't always regenerate lost appendages but instead reorganize existing limbs to become symmetrical again after an injury.

“This is a different strategy of self-repair,” said Lea Goentoro, an assistant professor of biology at Caltech, in a statement published by the university. “Some animals just heal their wounds, other animals regenerate what is lost, but the moon jelly ephyrae don't regenerate their lost limbs. They heal the wound, but then they reorganize to regain symmetry.”

The researchers, whose work was published this week in the Proceedings of the National Academy of Sciences, found that wounded jellyfish shifted remaining limbs to become symmetrical again even when they lost as many as six legs—leaving only two remaining.

This happens because, while humans have distinct right and left sides of their bodies, jellyfish have what's called radial symmetry. Like a daisy or a sea anemone, their bodies are pie-shaped, with distinct characteristics on the top and bottom rather than on any given side. Radial symmetry is key to how moon jellies propel themselves and eat. (The two functions are related: The same flapping motion that moves a jellyfish through the water also sends food-rich water flowing into its mouth.) So you can see how being lopsided—having too many legs on one side, for example—would be more problematic to a jellyfish than losing some of the propulsion power that comes with having all eight legs in working order.

Humans may lack the regenerative power that enables coelenterates to rearrange their own body parts—but our species still gravitates toward symmetry in its own way. Several studies have explored the extent to which humans value mirror-image features. Researchers have found that men who were perceived to have more symmetrical bodies were also considered better dancers, and much scholarship has been devoted to explaining why people with symmetrical facial features are often deemed more attractive than their asymmetrical pals.

One longstanding theory is that people’s preference for symmetrical faces evolved as a way to assess a potential mate’s overall health. A face pocked by scars from a childhood illness wouldn’t be symmetrical, for example. So perhaps humans, over time, learned to avoid those features. But a 2014 study, drawing on a large cohort of British children, calls that hypothesis into question. Researchers looked for correlations between childhood illness and facial asymmetry among nearly 5,000 British children. They found none.

“If preferences for facial symmetry do represent an evolved adaptation,” the authors of that study wrote, “then they probably function not to provide marginal fitness benefits by choosing between relatively healthy individuals on the basis of small differences … but rather evolved to motivate avoidance of markers of substantial developmental disturbance and significant pathology.”

Asymmetry can still be disorienting for humans, particularly when faced with our own reflections. Unlike moon jellies, lopsided human features are not a matter of life and death, but more likely a confluence of vanity and perception. Symmetry above sea level, at least, is about form rather than function.