One of the eeriest mysteries of the sea is the sudden surge in jellyfish over the last few decades. Consider the tale of the moon jellyfish. Since the first big moon jellyfish bloom was recorded in the Adriatic in 1910, moon jelly populations seemed to follow a predictable cycle. They’d turn up in normal numbers most years, and every couple of decades, clog the shores in huge gummy swarms.

Then in 1999, something alarming happened. The huge blooms that had stippled the Adriatic only every few decades now appeared year after year. The cycle had tightened—but what, exactly, had tightened it continues to mystify scientists.

Now a team of Slovenian marine biologists thinks they’ve found a key culprit: natural gas rigs.

Computer simulations by Martin Vodopivec and his team that recreate ocean dynamics and moon jellyfish life cycles suggest that gas platforms are helping to sustain moon jellyfish blooms in the Adriatic, according to a new peer-reviewed study in Environmental Research Letters.

How exactly do gas platforms help moon jellies survive? The answer involves the most fascinating—and disturbing—thing about jellyfish biology: the truly bizarre way in which they reproduce.

Growing a jellyfish garden

When two adult jellyfish—a.k.a. “medusas”—mate and produce a fertilized egg, that egg doesn’t just grow into a tiny version of themselves, like most creatures. Instead, that egg is actually a totally different creature—one that will never turn into a medusa like its parents.

Also called planulae or larvae, these free-swimming eggs are tiny and are “shaped a bit like a miniature flattened pear,” as the Smithsonian Museum of Natural History helpfully puts it. A planula drifts around until it bangs into a smooth, hard surface that it can latch on to. Once settled, it grows into a polyp, sprouting tentacle-like appendages so that it looks like a cross between a tiny tree and a sea anemone.

Then, when conditions are right, those arm-like things begin pulsating, and out pop a dozen or so baby jellies (which scientists call “ephyrae”). Those are the creatures that ultimately grow into that familiar umbrella-shaped beastie that we think of when we hear the word “jellyfish.” Polyps can repeat that process a few more times before they croak.

So far, so linear. But polyps have a nifty trick for whiling away the months (or years) until it’s showtime for baby-pulsating: They reproduce asexually. Meaning, they clone themselves.

And those clones? They clone themselves too, forming dense polyp families on whatever surface they’ve colonized. What look like delicate little gardens are actually high-octane jellyfish-making factories.

This seed-bank strategy is pure evolutionary gold—a hedge against the possibility that medusas might starve, get eaten by predators, or killed by bad weather before they can mate. It’s designed to create jellyfish en masse. Since medusas’ odds of producing fertilized eggs that make it to the polyp phase aren’t great, the more clones a polyp creates, the greater the chance of species survival.

But that all depends on little pear-shaped planulae finding a good home and hunkering down on a smooth, hard surface.

Richard A. Hoover Moon jelly polyps in Washington State getting ready to fire off zillions of baby medusas.

A few centuries ago, those homes were scarce. The best real estate going was typically a rock or a shell. Those lucky few that managed to find one didn’t tend to have much space for growing a clone colony.

An Adriatic gas rig, though—now there’s a nice place to settle down and raise a (very extended) jellyfish family.

The rise of the rigs

Drilling platforms first went up in the natural gas-rich Adriatic in the 1960s. Now there are around 150 of them, according to Vodopivec and his co-authors. That means there’s scads of space for polyps to expand their insane clone posses.

Mass polyp colonization certainly would help explain why blooms began taking off around the same time the Adriatic’s natural gas bonanza did. But finding evidence is tricky when the proof stands only a single millimeter tall.

In 2008, Italian researchers found moon jelly polyps clustered on a sunken iron motorboat (pdf) off the coast of Italy. The polyps—which were attached both to the oysters that had settled on the stern and directly to the ship itself—sat in clusters as many as 40 polyps per square centimeter. (At that density, more than 2,400 polyps would fit on a 3M Post-It Note.)

Across the Adriatic in Slovenia, Alenka Malej—a veteran jellyfish biologist and co-author of the latest paper—had been searching for moon jelly polyps since 2000, clocking more than 1,000 hours peering through at seafloor rocks through scuba masks. Malej herself never found any polyps. However, in 2009, an ecological survey team took a peek under the port of Koper. Encrusted with oysters, the dock pillars teemed with moon jelly polyps in maximum densities of around 27 per square centimeter. The scientists were restricted to surveying a single pier; they found polyps on all 574 of its pillars. According to their estimates, the Koper pier colony capably of releasing as many as 50 billion baby jellyfish (pdf, p.1) in the space of days.

Tihomir Makovec

Since then, similar moon jelly polyp colonies have been found in ports in Split and Ploče. Malej also identified a polyp colony on—you guessed it—on a gas platform.

Still, even though the simulations run by Vodopivec and Malej suggest a connection, we don’t know for sure that polyps are settling on rigs en masse—and leading to big jellyfish blooms—for the simple reason that they’re so hard to find.

Tihomir Makovec Aurelia sp. polyps from the Port of Koper in the Gulf of Trieste near Slovenia.

The idea that the blooms and the boom are connected isn’t far-fetched, though. There’s plenty more anecdotal and experimental evidence around the globe to support the hypothesis that the burgeoning of manmade marine surfaces drives coastal jellyfish blooms, as Malej and other jellyfish biologists argued in an exhaustive 2013 survey. For instance, 2014 study by leading jellyfish biologist Shin-ichi Uye found that after a new pier was installed in the Inland Sea of Japan, polyps quickly settled there; 25 million extra baby jellies appeared soon after. And a group of German scientists found a similar relationship between moon jelly abundance and wind farms in the Baltic Sea (paywall).

Still, the cryptic nature of these polyp colonies means a clear causal relationship remains elusive. Adding to the challenge is the fact that jellyfish blooms are also influenced by warming temperatures, overfishing, and eutrophication, to name just a few of many factors.

Jellyfish takeover—or human bias?

This mystery feeds into a much deeper jellyfish controversy. A slew of leading scientists are skeptical that a ”jellyfish takeover” is actually happening at all.

It’s clear that blooms are on the rise—both in magnitude and frequency—in some patches of the world, according to research done over the last couple decades. One of the only studies to quantify anecdotal information suggested that in more than three-fifths of large marine areas, jellyfish abundance was on the rise. Only 7% of large marine areas reported a decline.

However, the reigning counterargument to the “global rise in jellyfish” was put forth by many of the all-stars of jellyfish biology in 2013. Jellyfish populations, these scientists argued (pdf), go through 20-year ”oscillations.” The “oscillation” camp notes that while there has been a small linear rise in jellyfish blooms since the 1970s, more data are required to determine whether this trend marks a true shift in the baseline of their abundance—or just another oscillation.

To their chagrin, academic journals seemingly love the idea of a global jellyfish takeover. A recent analysis found that a whopping half of published papers suffered from jellyfish invasion bias—a narrative with horror-movie appeal that the media merrily runs with.

It may be a while before biologists know for sure whether the jellyfish invasion is temporary or here to say. But let’s hope the oscillation hypothesis is correct—not just because of the havoc the creatures wreak on fishing and coastal plants, but for the sake of tourists trying to enjoy a sting-free seaside vacation.