Jellyfish recently clogged the cooling water intake pipes at a nuclear power plant in Sweden, forcing the world’s largest boiling water reactor to shut down. And this isn’t the first time jellyfish have tangled with nuclear reactors.

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Nuclear engineers at the Oskarshamn nuclear power plant on the Baltic Sea in southeastern Sweden discovered a giant swarm of moon jellyfish had blocked the cooling water intake pipes, the New York Times reported. The jellyfish caused the plant to shut down but were prevented from entering the reactor itself by filters.

Moon jellyfish, called Aurelia aurita, are a common type found worldwide and have an umbrella-shaped body. While moon jellyfish aren’t considered dangerous to humans, it’s debatable whether they’re actually harmless. A spokesman for the power plant told the Times last night they hoped they had solved the problem by clearing the jellyfish but weren’t sure because they could come back.

Jellyfish and power plants have a tense history. In December 1999, jellyfish clogged the cooling pipes for a coal-fired plant in the Philippines, sending millions of people into temporary darkness during the holidays.

The Oskarshamn plant reported a similar incident in 2005. Then in 2006, a mass of jellyfish stopped up the seawater cooling system at a nuclear reactor in Hamaoka, Japan. Fortunately the water intake system automatically shut itself down. Plants in Israel and in Florida were also threatened.

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Jellyfish aren’t just messing with power generation, either. They’ve thwarted mining operations, asphyxiated salmon and pushed sturgeon closer to extinction, Smithsonian Magazine’s Abigail Tucker reported. It’s tempting to attribute the rise of the jellyfish to global warming, and indeed many have.

But there are still questions about whether this is a new population explosion or part of a boom-and-bust cycle for the creatures. Either way, looks like we’re in a boom time now.

Photo: A purple moon jellyfish. Credit: Philippa Warr.