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Moon jellyfish's secret swimming powers earn it extra mileage

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In a fish-eat-fish ocean filled with sharks and other fierce swimmers, how has the delicate jellyfish survived – and thrived – through hundreds of millions of years of evolutionary culling? A team of biologists and engineers says it has discovered the "jelly’s" secret: They're incredibly efficient swimmers.

Jellyfish don't look like they're built for speed. Less than 1% of their body mass is devoted to muscle, compared with more than 50% in fish. But they’re competent predators — and perhaps even too successful. In many parts of the globe, they spread into ecosystems so thoroughly that they’re considered invasive species.

Researchers have recently found that the jellyfish is an extremely efficient swimmer, one that could put your Prius to shame.

“They get the efficiency of several hundred miles to the gallon,” said John Dabiri, a Caltech biophysicist and coauthor of a paper published in Proceedings of the National Academy of Sciences.

Of all the jellyfish species in the ocean, the moon jelly Aurelia aurita gets the most bang for its swimming buck. But thus far, researchers haven’t been able to figure out exactly how it manages this neat little trick.

They knew that the jellyfish first squeezes out a stream of water that pushes it forward and creates a vortex ring behind it. The jelly then relaxes before contracting again. But this rocket-like jet propulsion couldn’t entirely explain such remarkable efficiency.

“We saw this expected spike in velocity when the animal was contracting, but we would see this secondary peak during a period of time when the animal was actually pausing between contractions, after it had reopened and relaxed,” said lead author Brad Gemmell, a marine biologist at the Marine Biological Laboratory in Woods Hole, Mass. “So initially we dismissed it as noise, but it ended up showing up in every contraction and among all different sizes of animals and among different species.”

To figure out what they were missing, the researchers dropped little silver coated glass beads, each mere microns thick, into the aquarium with the swimming moon jellies. They then shined lasers into the tank. The  laser light bouncing off the sparkling glass beads revealed subtle flows previously invisible using a more traditional dye method.

They found that as the jellyfish relaxes, a second doughnut-shaped vortex of fluid pushes water against its underside while it fills its bell-shaped body to squeeze again. So the animal is getting extra mileage even in the moment when it’s doing nothing, traveling about 30% farther per swimming cycle.

“The animal can actually get a free ride on that second swirling current,” Dabiri said.

This free ride improves the cost of transport by 48%, according to the study. That’s some pretty hefty energy savings.

“Although we as engineers can be pretty clever, nature has out-clevered us in a lot of different ways,” Dabiri said.

The researchers are part of a group from the Office of Naval Research looking to tap into the jellyfish’s secret powers of propulsion for more efficient seafaring vehicles.

For example, a jellyfish-based system could allow scientific instruments deployed in the ocean for weeks or even months to hold their position in a water column, even on a limited battery, Gemmell said.

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