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ANDREW Sweetman has spent a lot of time throwing dead jellyfish into the sea. At the bottom of the Sognefjord, Norway’s largest fjord, a time-lapse camera recorded their fate. He’s trying to answer a simple question: does anything eat jellyfish?

That neatly sums up how little we really know about those alien creatures. Jellyfish carrion carpets the seabed, suggesting it is not a favourite food.”Why would you eat a jellyfish?” asks Sweetman, who now works at Heriot Watt University in Edinburgh, UK. “A jellyfish is 96 per cent water. You might as well just swim with your mouth open.”

For a long time, it didn’t matter that we knew so little about these animals. They were just a mucous mess that washed up on beaches, or a painful nuisance for swimmers. But then huge invading swarms – jellyfish blooms – started making the news. In their millions, jellyfish are capable of spectacular acts of sabotage. Yet nobody knows where these blooms come from or how to get rid of them. Time to ask more questions beyond whether anything eats them. Are numbers really going up? If so, why? More importantly, what can we do about it?

Jellyfish blooms can cause chaos. When the power went out on the island of Luzon in the Philippines in 1999, locals thought a long-feared military coup was under way. They were wrong. Sucked into water intakes, jellyfish had taken out vital services from power stations to data centres and water treatment plants. In 2006, a jellyfish bloom temporarily disabled the Ronald Reagan, one of the US navy’s flagship nuclear-powered supercarriers. Gelatinous bodies had clogged up the water intake for cooling the ship’s reactor. And in 2009 a Japanese fishing boat capsized when its crew tried to haul in a net filled with dozens of huge Nomura’s jellyfish (Nemopilema nomurai) – each 2 metres wide and weighing in at 200 kilograms.

Although it’s hard to pinpoint a single reason for the boom in jellyfish blooms, it seems clear that one or more human factors are in play (see “They get around“). What’s certain is that they can suck the life out of the sea. Jellyfish eat fish larvae as well as the plankton they live on, damaging already fragile fish stocks. Wild fish are hit hardest, but once a bloom gets its tentacles through the nets of an open-ocean fishery, it may as well shut up shop. South Korean researchers estimate that jellyfish are costing their country $70 million to $200 million a year in lost fish-related revenue.

That has led to some pretty far-out ideas to cull them. One comes from a team led by Hyun Myung at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea, who have created a fleet of jellyfish terminators called JEROS, for Jellyfish Elimination Robotic Swarm. Each robot looks like a small giraffe riding on a 2-metre-square catamaran, while under the surface, whirling blades churn through jellyfish like eggs in a blender.

Each craft has a camera to monitor the sea in front of it. When the software recognises a jelly in the water, sometimes with the help of drones overhead, a robot will summon the rest of the JEROS fleet. The robots sail forth, guiding the jellyfish into blade-filled traps. Each robot can shred 25 jellyfish a minute.

Not all jellyfish are created equal, however. JEROS can easily make mincemeat of moon jellyfish (Aurelia aurita), which are around 30 centimetres across, but Korean coasts also get blooms of Nomura’s jellyfish, and Myung’s robots are no match for them.

He wants to make the robots intelligent, so that they can teach themselves to aim for the right species. “A lot of international companies are interested in optimising JEROS for different environments and jellyfish species,” says Myung. Offshore industries in the Middle East are especially interested, he says.

Not everyone thinks robot terminators are a good idea. For a start, it’s a pretty gruesome solution. Rebecca Helm, a jellyfish researcher at Woods Hole Oceanographic Institution in Massachusetts, describes JEROS as her version of hell. Few think jellies feel pain, but the truth is we don’t really know.

Ferdinando Boero at the University of Salento in Lecce, Italy, is convinced they don’t. “They do not care,” he says. “They do not have a brain.” But of the University of British Columbia in Vancouver prefers to hedge his bets. “I don’t think humans have a good understanding of pain in other organisms,” he says.”When I dissect a large live jellyfish, it very visibly winces and contracts when I cut into it.” Such reactions may well be a reflex response, nociception, rather than a conscious experience of something painful, but no one can be sure.

Either way, chopping up jellyfish may do more harm than good. First, what happens to the shredded jellyfish? Some might get eaten – but researchers like Sweetman are only just working out where the animals fit in the food chain. Or the sticky mass could gunk up cooling systems with the pieces being even harder to get rid of than whole jellies. Plus tentacle fragments studded with stinger cells can wash over anti-jellyfish nets and on to beaches. The second problem is that when you cut jellyfish open, some release all their eggs and sperm at once. The cells fertilise each other and next season, your jellyfish bloom is worse. In some species, such as moon jellyfish, the shredded pieces can even turn into polyps themselves.

“Some species of jellyfish are actually biologically immortal“

So Myung’s team is tweaking their design. “We’ve rethought the shredder,” he says. A new version of JEROS scoops jellyfish out of the water and into a tank that holds more than 400 at a time. And there are a range of areas where they might find unexpected uses (see “Jelly versatility“).

Still, if shredding or hoovering them up seem crude, there may be a smarter way: manipulate their fertility. Konstantin Khalturin at the Okinawa Institute of Science and Technology in Japan has found a substance that makes moon jellyfish start producing clones of themselves within 48 hours. “It’s not birth control,” says Khalturin. “It’s uncontrolled birth.”

To understand how Khalturin’s jellyfish anti-pill works, and why it could be useful, you must first understand the jellyfish’s singular life cycle (see “Life of a jelly“). “That transparent, pulsating disc we all think of as a jellyfish – that’s just a phase,” Khalturin says. “The same DNA can make a creature that crosses oceans, wipes out fish, clogs up power plants, and lives six months at most, and then make a polyp that’s completely stationary but can live for a really long time,” says Khalturin. “Some polyps are actually biologically immortal.”

So polyps generally bide their time before reproducing to make active jellyfish. But dump 150 kilograms of hormone treatment in a cubic kilometre of water and the polyps living there will strobilate.

In theory, you could time blooms to coincide with the arrival of the JEROS swarm. “Or you could cause new jellyfish generations to die prematurely by making the polyps bloom in the winter, when there’s not enough food in the sea yet for the baby jellyfish,” says Khalturin.

One snag is that the polyps themselves can survive, so some want to use Khalturin’s work to produce a proper birth-control pill. But even if you could dump hormones in the sea by the kilo without harming other life forms, there’s another problem: finding the polyps.

Polyps are nearly invisible. They coat solid surfaces like rocks and concrete posts with a transparent carpet just 4 millimetres high. In South Korea they have started hosing down underwater surfaces where polyps are likely to form, hoping to wash them off into open water where they cannot survive. As a preventative measure, it’s also somewhat crude. Even if polyps are spotted, we do not know which species many polyps belong to. An adult jellyfish can drift far from the polyp that spawned it before it gets seen.

So it could be a lose-lose strategy. “Fight jellyfish?” says Boero. “Forget it.” Everything we’ve tried is useless. “Jellyfish shredders, hormones – you’re just treating the symptoms,” he says.

Marine zoologist Dror Angel at the University of Haifa in Israel agrees. “To portray this group in monochrome rather than presenting a more balanced picture where they are a natural part of the ecosystem is to do injustice to what really goes on – and to downplay our role in the outbreaks,” he says.

Simply removing jellyfish from the sea when we know so little about them and their role in the larger picture may be foolish. “Who knows what food shortage you’d be causing for sea creatures somewhere else?” says Boero. Which is where Sweetman’s work comes in.

When Sweetman sent his jellyfish carcasses to the sea floor they were accompanied by a selection of tasty mackerel. Yet he found that various types of fish and crustaceans devoured the jellyfish just as quickly as the mackerel. It is likely that many things do eat jellyfish, we just haven’t seen them at it. “And everything that normally eats jellyfish is for sale at the grocery store,” says Boero. For him at least the solution to the blooms is obvious. “Restore fish stocks and you’ve solved your jellyfish problem.”

Whatever strategy wins in the end, be it eradication, birth control, or better ecosystem management, it’s clear the picture is complicated. “Jellyfish are not a wave of pollution that simply needs to be removed from the sea,” says Angel. Can’t live with them, can’t live without them.

This article appeared in print under the headline “March of the jellies”