This understanding of jellyfish has come under scrutiny in recent years as marine biologists have used new tools to figure out what eats what in the sea.

Prey leave a chemical signature in the predators that consume them. Elements like oxygen and nitrogen in the muscles of animals can reveal the kinds of prey they consume.

As it turns out, a number of fish species carry a jellyfish signature in their muscles.

Scientists have also invented new ways to peer inside the guts of ocean predators. Rather than searching for pieces of half-digested jellyfish, researchers began rummaging for their DNA. And they found a lot of it came from jellyfish.

In eel larvae, for example, researchers found that 76 percent of prey DNA belonged to jellyfish. From the feces of albatrosses, scientists determined that jellyfish made up 20 percent of their diet.

By mounting miniature cameras on marine animals, biologists have been capturing days’ of video. Footage of penguins has revealed that they also eat jellyfish. In fact, the birds actively seek them out even when other options are available. Jellyfish may make up over 40 percent of a penguin’s diet.

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These new techniques “have allowed us to scratch the surface and get a glimpse of another world,” said Julie McInnes, a biologist at the University of Tasmania.

It’s a world with a tremendous appetite for jellyfish. So why are many animals are eager to eat a seemingly useless food?