As predators, jellyfish appear to be slow and passive. Unable to swim to and chase their prey, most drift along, creating tiny eddies to guide food toward their tendrils. Yet in waters from the Sea of Japan to the Black Sea, jellyfish, like those pictured here, are thriving as many of their competitors are eliminated by overfishing and other human impacts. How have these drifters reversed millions of years of fish dominance, seemingly overnight? Writing in the journal Science, biologist José Luis Acuña of the University of Oviedo in Spain and his colleagues suggest that jellyfish are just as effective at catching prey and turning it into energy as fishes. In fact, they have set the stage for a takeover—dubbed the “gelatinous ocean” by some scientists. “We need research to be sure of what new ecological scenarios are arising,” Acuña says. “It is time to take [jellyfish] seriously.”