Jellyfish have a nifty trick: Out of nowhere one day, they’ll invade a patch of sea, hundreds of millions of them, clogging nets, beaches, even power-plant cooling systems that happen to be nearby. And then, just like that, they’ll disappear again for another few decades.

These cryptic cycles raise a lot of questions. After all, a surge in jellyfish should mean a surge in jellyfish reproduction. So why, as is often the case, do only a few jellyfish turn up the year following a huge bloom? And how do these enormous population explosions happen after decades of dormancy? And, above all, why do jellyfish blooms appear to be getting bigger, more frequent, and more destructive?

Though scientists have only begun to unravel the secrets to jellyfish reproduction, what they’re finding is frightening. Research emerging in the last year or so increasingly confirms that apocalyptic blooms aren’t thanks to biology alone. Jellyfish are getting a lot of help—from humans.

The “known unknown” of jellyfish blooms

AP Photo/Junji Kurokawa The giant jellyfish that’s attacking China’s coast.

It turns out that baby jellyfish don’t come from adult jellyfish—or at least, not directly. The creatures with the blobby bell and tentacles that you see washing up on beaches are in the “medusa” phase, in which jellyfish float around the ocean. But there’s an earlier stage of the jellyfish lifecycle: when they exist as polyps stuck to hard surfaces. This stage is critical to understanding blooms, says Lucas Brotz, a zoologist at the University of British Columbia.

“Jellyfish polyps have been one of the major ‘known unknowns’ for a long time,” he tells Quartz. “That is to say, we’ve suspected that they’re an extremely important factor controlling medusae populations, but we just don’t know that much about them.”

Once fertilized, jellyfish eggs float around the ocean until they find a hard, flat surface—oyster shells, for example—to stick to. They then sprout a dozen or so tentacles, making them look sort of like microscopic sea anemones. In this form, they’re called polyps.

"Regulation of Polyp-to-Jellyfish Transition in Aurelia aurita," Fuchs et al., 2014 The strobilation process of moon jellyfish polyps. (Ephyra is the scientific name for “baby jellyfish medusa.”)

All of a sudden one day, these polyps begin flowering, separating into frilly, pulsating ridges that, after a few days, detach and swim free—a process scientists call “strobilating.” These turn into baby jellyfish “medusas.” Those that survive eat like crazy until they grow into the gelatinous blobs that sting swimmers and gum up power plants.

Jellyfish reproduction: science or sc-fi/horror film?

Sun Song A moon jellyfish strobilates.

Polyps can go through the baby-jellyfish-creating process a couple times before they die. In fact, polyps of lion’s mane jellyfish (Cyanea nozakii) are known to strobilate up to seven times.

That right there is some freaky math. But it isn’t even the most terrifying aspect of jellyfish reproduction.

While they wait to strobilate, polyps have something else besides feeding to keep busy with: they clone themselves. In one of the most common methods, an appendage pops out of the polyp’s stem, reaching outward until it sticks firm to a patch of surface nearby. The rest of the polyp then follows that appendage to the new location, leaving behind in its place a little mound called a “podocyst.” Since that’s sort of hard to imagine, Quartz made a gif (but be forewarned—it might look a little NSFW).

Created from a presentation by Shin-ichi Uye A cartoon version of how a Nomura polyp clones itself.

The polyp does this over and over again while it waits to release medusae. One study found that, in less than three months, a single polyp of a species of sea nettle on the US’s east coast produced 52 podocysts and six polyps.

So what about the left-behind pods? They can lay dormant for many years until, seemingly out of the blue, fingers poke through their casing and grow, until what was once a pod is now a polyp. And like the polyp that created it, this new clone starts its own little pod-creating march around the surface of the shell or rock it’s settled on.

And that’s just one of the more common ways jellyfish polyps copy themselves. The moon jellyfish (Aurelia sp.), which now blooms in huge number all over the planet, is a cloning master, with at least five other means of duplicating itself.

Dong et al. in "Jellyfish Blooms," Pitt et al., 2014 Different types of asexual reproduction in moon jellyfish. a) Typical lateral budding; b) lateral budding by means of stolons; c) reproduction from parts of stolons; d) podocysts; e) motile bud-like tissue particles; f) internally produced propagules; g) longitudinal fission; h) transverse fission. All scale bars = 1mm.

Clone survivors

It’s a genius evolutionary hack. Polyps tend to be much more likely to strobilate when environmental factors are ideal for baby jellyfish survival. As long as nothing eats them or they don’t run out of space, generations of polyps can in theory keep building their clone colonies infinitely. And even if polyps are eaten, or are threatened by changes in their environment, pods act as a backup population, ensuring jellyfish survival. These polyp colonies mean that jellyfish medusas don’t need to bloom every year for their species to endure.

Warmer seas are helping polyps pick up their pace

Flickr user tamra hays (image has been cropped) Moon jellyfish in the Bosphorous. The species now blooms all over the world.

The fact that environmental factors often aren’t ideal for jellyfish probably explains why blooms are intermittent. But when conditions are right, look out.

It’s especially worth being vigilant in Japan; the seas there see frequent blooms of the giant Nomura jellyfish—which spans up to 2 meters (6.6 feet) across and can weigh up to 440 pounds (200 kilograms). Historically, blooms of these jellyfish would hit Japan’s coast every 40 or so years. Since 2002, however, they’ve swarmed the Sea of Japan three years out of every four, costing fisheries there hundreds of millions of dollars. Recent research by Shin-ichi Uye, a Hiroshima University marine biologist and the foremost expert on Nomura jellyfish, offers some hints to why—and they’re alarming.

Sun Song Nomura polyps begin strobilating.

Uye and his colleagues found that the warmer the water gets, the faster Nomura polyps clone themselves, something he’s also documented in other species. It’s probably no coincidence that over the last few decades, the East China Sea—where the Nomura polyp colonies are assumed to be—has been warming rapidly (pdf).

Uye and his colleagues found that the warmer the water gets, the faster jellyfish polyps clone themselves

Nomuras aren’t the only jellyfish plaguing the East China Sea. Invasions of this and two other species—the lion’s mane and moon jellyfish—have dramatically altered (pdf) the catch at fisheries in the Yangtze Estuary, clogged the intake screens at a Shandong province power plant, closed beaches in the resort town of Qingdao, and filled the waters at density levels bordering on epic.

Recently, a $4.5-million research initiative by the Chinese Academy of Science’s Institute for Oceanology helped shed new light on the behavior of these jellyfish polyps. Lead researcher Sun Song and his team raised dense polyp gardens of the three species on glass, plastic, and stone slabs in their lab. Then they deposited the slabs on the seafloor at six sites on China’s eastern coast, and waited for the jellyfish to bloom.

Tihomir Makovec Moon jellyfish polyps in the Gulf of Trieste near Slovenia. The one in the middle is starting to strobilate.

And bloom they did. The preliminary findings suggest polyps released medusas when the water ranged from 50-59°F (10-15 °C), reports Nature. They were also more likely in areas where pollution and overfishing have wiped out shrimp, fish, starfish, and other creatures that graze on polyps. However, the blooms were especially big when temperatures swung to extremes.

Big swings in temperatures = big jellyfish blooms?

Reuters/Awashimaura Fisheries Association Coop. Nomura jellyfish from the Sea of Japan.

Though this research hasn’t yet been published, let alone peer-reviewed, the evidence gathered so far suggests that changes in temperature and salinity, along with fishing activity and environmental modification, are all “substantially conducive to explaining the causes of jellyfish blooms in China’s coastal sea,” says Sun.

UBC’s Brotz says the finding that extreme temperature swings led to larger blooms “is concerning given what we know about global warming”—namely, that it seems to be promoting more frequent and dramatic swings in temperature.

Recent research by Hiroshima University’s Uye sheds light on why that might be happening. Lab experiments on podocysts of moon jellyfish—which have infested waters everywhere from Denmark to the Gulf of Mexico (pdf)—and the giant Nomura jellyfish suggest that it takes an environmental shock—something like a big temperature spike, or a sudden change in the seawater’s salt or oxygen levels—to prompt their metamorphosis into polyps.

"Bloom or non-bloom in the giant jellyfish Nemopilema nomurai," Kawahara et al., 2012 The lifecycle of the Nomura jellyfish.

Jellyfish clones’ manifest destiny

Perhaps the most disquieting observation about the rise of jellyfish, however, has to do with new polyp habitats. A few centuries ago, when a jellyfish larva—i.e. a fertilized egg—looked for a surface to start cloning on, it had to make do mostly with the odd seabed rock or oyster shell. If a larva couldn’t find such a surface, it would be eaten or die out.

The odds of finding a place to settle used to be pretty long. But humans are bettering those larvae’s chances of survival. Bridges, ports, drilling platforms, ship hulls—these are just a few examples of miles upon miles of smooth surfaces that polyps are colonizing. Research published last year reported polyps of numerous species taking over everything from buoys to floating plastic cigarette packaging (paywall).

"Ocean sprawl and jellyfish blooms," CM Duarte et al. The many surfaces where jellyfish polyps thrive.

A study on moon jellyfish published in October offers a more direct link between booming coastal development and jellyfish blooms. The research team, which included the prolific Shin-ichi Uye and three other marine biologists, counted the number of baby moon jellyfish in a bay before and after a new floating pier was installed. The jellyfish polyps rapidly colonized the new pier’s underside, resulting in a four-fold surge in their numbers after the dock’s arrival.

Humans are doing jellyfish colonies a big favor

Flickr users Steve and Sara Emry (image has been cropped) Teaming up to take over the world.

Though this latest research is building a strong case that man-made disturbances to the ocean are amplifying blooms, the lack of historical data on jellyfish means these links still aren’t certain. In fact, other leading scientists argue the recent spate of jellyfish explosions worldwide merely follow a 20-year cycle. Confusing things further, says UBC’s Brotz, is that various factors—notably overfishing and pollution, as well as coastal development and climate change—play out differently in different places around the globe. Still, whether they’re infesting Chesapeake Bay or Tasmania, jellyfish enjoy two advantages: their talent for cloning, and the way humans are helping these clone colonies thrive.