Jellyfish never stop. Twenty-four hours a day, seven days a week, they move through the water in search of food such as shrimp and fish larvae, on journeys that can cover several kilometers a day. They are more efficient than any other swimmer, using less energy for their size than do graceful dolphins or cruising sharks. “Their cost of transport—the oxygen they use to move—is 48 percent lower than any other swimming animal,” says Bradford J. Gemmell, a marine biologist at the University of South Florida. By studying moon jellies, the species Aurelia aurita, Gemmell and other researchers have recently found that jellyfish pull off this feat by creating zones of high and low pressure around their body that alternately suck and push them forward.