A silent, blobbing menace swarms the seas, thanks to overfishing, climate change and even "dead zones". Jellyfish seem set to regain their dominance of the oceans in future—and that could be bad news for humans. 

The two-meter long jellyfish known as Nomura have begun swarming year after year off the coast of Japan, 500 million or more of them fouling fishing nets thanks to agricultural runoff from China spurring plankton blooms. With fewer fish, the Nomura giants can dominate.

In fact, jellyfish are quite good at proliferating, even when circumstances are less than favorable. Just eight years after comb jellies first entered the Black Sea, they became the most numerous animal. And various jellyfish are among the only animals that can survive in the more than 400 ocean areas devoid of oxygen around the world—more commonly known as dead zones. 

Jellyfish mess with humans, too—from the deadly stings of box jellies to clogging water-intake pipes at nuclear power plants. This year the as much as 200-kilogram Nomuras actually capsized a 10-ton fishing trawler off the coast of Japan. 

Ultimately, our only hope may be to acquire a taste for the gelatinous creatures. Jellyfish burger, anyone?

—David Biello

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