Last week, Sweden’s Oskarshamn nuclear power plant, which supplies 10% of the country’s energy, had to shut down one of its three reactors after a jellyfish invasion clogged the piping of its cooling system. The invader, a creature called a moon jellyfish, is 95% water and has no brain. Not what you might call menacing if you only had to deal with one or two.

En masse, jellyfish are a bigger problem. “The [moon jellyfish swarm] phenomenon…occurs at regular intervals on Sweden’s three nuclear power plants,” says Torbjörn Larsson, a spokesperson for E.ON, which owns Oskarshamn. Larsson wouldn’t say how much revenue the shutdown cost his company, but noted that jellyfish also caused a shutdown in 2005.

Coastal areas around the world have struggled with similar jellyfish blooms, as these population explosions are known. These blooms are increasing in intensity, frequency, or duration, says Lucas Brotz, a jellyfish expert at the University of British Columbia.

Brotz’s research of 45 major marine ecosystems shows that 62% saw an uptick in blooms (pdf) since 1950. In those areas, surging jellyfish numbers have caused power plant outages, destroyed fisheries and cluttered the beaches of holiday destinations. (Scientists can’t be certain that blooms are rising because historical data are too few.)

The proliferation of jellyfish appears in large part to be related to humans’ impact on the oceans. The toll we take on the seas may augur a new world order of jellyfish disasters, which, in turn, could devastate the global economy.

AP Photo/Junji Kurokawa The Nomura jellyfish invasion has plagued Japanese fishermen—and probably stems from pollution in China’s Yangtze River.

The blight of the jellyfish

Oskarshamn-like disturbances are happening all over the world. Throngs of jellyfish have disrupted power generation everywhere from Muscat to Maryland, from South Korea to Scotland. Things are worse in the fishing business, where blooms have wiped out billions of dollars in earnings over the last few decades. They’re also a nightmare for fishermen, who must contend with bursted nets and clogged trawl lines. Japan’s now-annual bloom of Nomura jellyfish, which each grow to be the size of large refrigerator, capsized and sank a 10-ton trawler when the fishermen tried to haul up a net full of them.

Reuters/Ronen Zvulun One of the invaders that shut down Israel’s Orot Rabin coal-fired power plant in 2011.

Tourism has taken a hit, too. This summer, a pileup of a million jellyfish along a 300 kilometer (186 miles) swath of Mediterranean coastline shortened swimming season for hundreds of thousands of tourists on beach holidays, reports The Guardian. Some 150,000 people are now treated for jellyfish stings in the Mediterranean each summer.

Flickr user rezendi How Australia warns people about box jellyfish.

The box jellyfish: the deadliest creature on the planet

Those swimmers are getting off easy, though. Residents of Australia and Southeast Asia share shores with the dread box jellyfish, whose sting “is the most explosive envenomation process presently known to humans,” wrote a team of scientists. Venom injected from its 10-foot-long tentacles ”turns the tissue into soup,” as one marine biologist put it, and causes the heart to seize. Death usually occurs within four minutes. In the Philippines each year, between 20 and 40 people die from box jellyfish stings.

Then there’s the Irukandji. The box jellyfish’s diminutive cousin, the Irukandji has mastered the closest thing to the perfect murder in the animal kingdom. Usually the size of a sugar cube, the Irukandji is hard to see, and its stinger leaves no trace. Around 10 minutes after contact, victims suffer everything from excruciating lower back pain to incessant vomiting to constricted airways and the “creeping” skin frequently associated with methamphetamine usage. Unlucky victims sometimes succumb to brain hemorrhaging, extreme high blood pressure or, in 30% of cases, experience some form of heart failure, according to Scientific American. And one out of five victims ends up on life support. “It’s difficult to know how many victims the Irukandji have claimed,” writes biologist Tim Flannery in a must-read piece, since “many deaths have doubtless been put down to stroke, heart attack or drowning.”

AP Photo/Brian Cassey The deadly Irukandji: Nearly one in three people stung experience heart failure.

Australia is known for its menagerie of lethal beasts. But now both types of jellyfish are found in Florida and elsewhere. Six box jellyfish nearly killed endurance swimmer Chloe McCardel last June (she miraculously survived despite having sucked a “spaghetti”-like tentacle into her mouth). Reports of stingings now come from India, Cape Town and even Wales.

An eating, reproducing machine that’s almost impossible to kill

No one’s sure how box jellyfish and the Irukandji are spreading. Jellyfish species are turning up in new habitats every year—and thriving. That’s probably because, from an evolutionary standpoint, jellyfish are biologically primed to swarm the seas. Here’s why:

They have few predators. The ones they do have include sea turtles, salmon, mackerel and albatross—animals that are increasingly scarce. And of course, when they’re transported to new ecosystems, jellyfish often have no natural predators.

The ones they do have include sea turtles, salmon, mackerel and albatross—animals that are increasingly scarce. And of course, when they’re transported to new ecosystems, jellyfish often have no natural predators. They’re eating machines. The comb jellyfish, which wiped out the Black Sea’s $350 million fishing industry, can put away 10 times its body weight in food in a single day. This is even though it needs to eat only 16% of its body weight to keep growing. The rest of that food goes toward making it bigger and bigger.

The comb jellyfish, which wiped out the Black Sea’s $350 million fishing industry, can put away 10 times its body weight in food in a single day. This is even though it needs to eat only 16% of its body weight to keep growing. The rest of that food goes toward making it bigger and bigger. They play dirty against competitors. Not only do jellyfish compete with smaller fish for the same food, but they also eat those fishes’ eggs. That collapses fish populations.

Not only do jellyfish compete with smaller fish for the same food, but they also eat those fishes’ eggs. That collapses fish populations. They’re world-class proliferators. Jellyfish don’t have baby versions of themselves the way most animals do. They create polyps—little bundles of clones—that attach to hard surfaces and wait for their opportunity to release small jellyfish. However, while they’re waiting, polyps clone themselves, creating more bundles of future baby jellyfish.

Jellyfish don’t have baby versions of themselves the way most animals do. They create polyps—little bundles of clones—that attach to hard surfaces and wait for their opportunity to release small jellyfish. However, while they’re waiting, polyps clone themselves, creating more bundles of future baby jellyfish. They’re (almost) invincible. One reason jellyfish blooms are so disastrous is that they’re almost impossible to get rid of. In fact, cutting some species open actually creates exponentially more of them. When the cells of one species, named the Benjamin Button jellyfish, are released through post-mortem decomposition, they somehow find each other again and from a whole new polyp.

Reuters/Monty Graham, Dauphin Island Sea Lab A bloom hits the Gulf of Mexico.

Why is this happening now?

They may have gamed evolution. But in written human history, jellyfish blooms have never before infested the seas. So why now?

Throughout history, the intricate lattice of ocean life has kept jellyfish in check. Thanks to overfishing, pollution and other factors, though, “jellyfish populations are exploding into superabundances and exploiting these changes in ways that we could never have imagined… and in some cases driving them,” explains biologist Lisa-Ann Gershwin in her brilliant book “Stung! On Jellyfish Blooms and the Future of the Ocean,” both a fascinating read and a crucial reference for this story. Here’s an illustration of some of those effects, explained in more detail below:

"The jellyﬁsh joyride: causes, consequences and management responses to a more gelatinous future," Richardson, et al. ​

Dismantling the food chain

Overfishing creates more opportunity for jellyfish to feed and breed. The plundering of, say, salmon removes one of the jellyfish’s few predators. Without a curb on their population, growing hordes of jellyfish start eating the eggs of smaller fish, as well as their food supply. Jellyfish also wreak havoc on the food chain when they’re introduced to new ecosystems, usually via ballast water that shipping tankers take on and release as a counterbalance to cargo.

Outlasting everything else

Other contributors to the jellyfish boom are the “dead zones” created by what scientists call “eutrophication.” That’s when farming pesticides and sewage pumped into rivers meet the ocean. This affects phytoplankton, the teeny aquatic plants that are the dinner buffet for vast numbers of sea creatures. Normally phytoplankton live on nutrients from chemicals the seabed releases. But their populations explode when doped up on nitrogen and phosphorous, forming algal blooms like the one in Qingdao, China, each summer.

The whole food chain starts chowing down, creating more excrement and more dead creatures. Those float to the bottom, stripping the water of oxygen. Since most creatures can’t survive in areas with little oxygen, their numbers fall. Not jellyfish; they need very little oxygen to survive. So as other animals dwindle, jellyfish colonies expand. The best example comes from China, where pollution from the Yangtze River in western China has formed huge dead zones in the East China and Yellow Seas (paywall). Scientists think dead zones are behind the surge in Nomura jellyfish in Japan:

Reuters/Awashimaura Fisheries Association Coop Catch of the day.

How humans have exacerbated jellyfish cloning

Humans are also helping jellyfish reproduce. Polyps—those clone sacks that churn out baby jellyfish—are “key to their ability to bloom in such incredibly rapid fashion and shocking numbers,” writes Gershwin.

A few centuries ago, the hard surfaces available for polyps to cling to included mainly seabed rocks and oyster shells; those polyps that couldn’t find such surfaces couldn’t clone. Thanks to the proliferation of human structures, the world is now their oyster shell. Piers, drilling platforms, plastic cigarette packets, offshore wind turbines, boats—those are just a few of the new surfaces polyps can cling to:

"Is global ocean sprawl a cause of jellyfish blooms?" Duarte, et al. ​

Big blooms may be the new normal

Humans may eventually act to reverse the boom in jellyfish blooms, given the material damage they cause. But as the research of Gershwin, Brotz and other scientists suggests, those efforts may not work. The peculiar biology of jellyfish means that once their numbers surge, the tide may be impossible to turn.