Einstein Science Reporting for Kids
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Are you ready for this jelly?

Moon jellyfish, Gijón Aquarium. Photograph by Julio Arrontes.
[Courtesy of José Luis Acuña and Julio Arrontes, University of Ontario]

There are places in the ocean where giant jellyfish rule. They look like dinner plate-sized water balloons, hundreds and sometimes thousands of them floating together. When people catch too many regular fish like sardines or anchovies, they leave behind an empty ocean neighborhood that the jellyfish move into in a big way.

But jellyfish are sort of the couch potatoes of the ocean world. They swim, but not very fast. They find their meals by bumping into food, rather than darting around and searching for stuff to eat like a sardine does. So scientists are wondering: will jellyfish rule for long in their new homes, or will the quick and clever regular fish soon take back their old neighborhoods?

José Luis Acuña of the Universidad de Oviedo in Spain and his fellow scientists decided to compare the snacking strategies of jellyfish and regular fish, to see which one wins the contest for top predator in their section of the sea. It seems like no contest, right? Shouldn't a fast and furious swimmer like a sardine gobble up more food than a lazily drifting water balloon? But it turns out that jellyfish eat just as well as the fish. The researchers show that jellyfish use their big size to increase their chances of bumping into food. They also don't need to eat as much as fast swimmers because they burn less energy by mostly floating around.

Acuña and the others say that jellyfish may do just fine defending their new neighborhoods against regular fish. And if people keep pulling too many regular fish out of the seas, we might just see a "jelly ocean" in our future.

The jellyfish study appears in the 16 September issue of the journal Science.