For one, they prevent swimmers and beachgoers from entering the water. Some 150 million jellyfish stings occur annually worldwide.
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While not every jellyfish species has a sting that’s painful or even perceptible to people, some can be dangerous or even deadly.
Some types, like the Chironex fleckeri species of box jellyfish, can kill a human in 3 minutes.
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Box jelly venom targets the heart and nervous system, and packs such a punch that swimmers can drown or die of heart failure before reaching shore.
In January, nearly 4,000 people were stung in one weekend by blue bottle jellies that drifted ashore in Queensland, Australia.
Then, in 2018, more than 1,000 people were stung over the course of one week after jellyfish blooms popped up offshore near Volusia County, Florida.
In large numbers, jellies can clog power-plant pipes and force them to shut down.
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On December 10, 1999, 40 million people living on the Philippine island of Luzon lost power after thousands of jellyfish were sucked into the cooling pipes of a local coal-fired power plant.
In 2011, jellyfish overwhelmed the cooling system at a coal power plant near Hadera, Israel.
Two years later, jellies also got sucked into the cooling pipes at one of Sweden’s nuclear reactors, forcing a shutdown.
Jellyfish swarms can also be deadly for other marine creatures.
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In 2007, a mauve stinger jellyfish swarm 10 square miles in size killed 100,000 salmon in a fish farm off the coast of Ireland.
Overall, mounting evidence suggests, underwater ecosystem may be changing from one dominated by fish to one ruled by jellies. This is – in part – a situation of own making: A 2009 study noted that human-induced stresses, including overfishing, climate change, and habitat modification, “appear to be promoting jellyfish blooms to the detriment of other marine organisms.”