Jellyfish are ancient, beautiful and mysterious. But they’re becoming a global headache.

Overfishing, plastic pollution, warming temperatures and other impacts of human activities are changing the oceans — resulting in decreasing populations of everything from tuna to whales to dolphins. But humans are also causing one class of sea life to thrive: jellyfish. 

“What we are doing as a normal part of being human — our waste and our coastal construction and our fishing and our carbon dioxide, all of these things — we are creating a world for jellyfish that they're loving,” says Australian–based biologist Lisa-ann Gershwin. “We are giving them the biggest break of their entire history.”

Gershwin has a rare passion for this stinging sea creature. She has personally identified 200 new species and is one of the foremost experts on these ancient, intriguing beings. For better or for worse, she says, humans have created the ideal circumstances for jellyfish to flourish.

“If you're an animal and every day is a struggle to find food, a struggle not to be someone else's food, a struggle to grow fast enough to reproduce before you become food or die of some other way, imagine a world where somebody else does you the favor of taking out your predators and competitors,” Gershwin says.

And in this case, "somebody else warms up the water that you're living in so you grow faster, eat more … reproduce more and live longer to do more of it. You’d be pretty happy. Jellyfish don't have a brain, so I think they're not actually happy, but, man, they're loving it.”

Jellyfish have inhabited Earth’s waters for about half a billion years. Gershwin is drawn to how unique they are, and the fact that they're still so mysterious. 

“A lot of aspects of their biology and ecology are just fascinating, and yet they're so poorly studied and so poorly known that it's like every time you look at them, you can discover amazing new stuff, and I really like that aspect as a scientist — to be able to make so many discoveries of new species, new behaviors, new aspects of biology and ecology.”

Jellyfish function in ways that are hard for humans to relate to. “They have no brain, no blood, no heart, no bones. Pretty much they’re just a bag of goo with a stomach and a really primitive nerve net, and gonads. That's it.”

What’s even more fascinating, Gershwin adds, is that, while other creatures have evolved into a staggering array of life, jellyfish haven't had to: They're the same as they've always been. 

But now, in many places, they have become too much of a good thing. A jellyfish bloom (or group) can encompass millions of the creatures spread over many dozens of miles.

Not every species of jellyfish is rampant and out of control, Gershwin notes, but some are causing untold problems for all sorts of marine industries. For example, they’re hard on tourism (since they sting people), and they pose challenges for salmon farms, fishing vessels and some ecosystems — where, as the top predator, they’re taking over.  

“That just bends your mind backwards,” Gershwin says. “You think, ‘Hang on, jellyfish is the top predator? What about fish and sharks and whales?’ It's not that they're eating sharks, but they eat the food that the food of the food of sharks would eat, and so jellyfish are able to cripple an ecosystem at the ankles.”

Power plants are threatened if they draw cooling water from the ocean or an estuary, or anyplace where jellyfish could conceivably be, Gershwin explains. “When jellyfish bloom into ‘super-abundances,’ power plants suck in all these jellies and the engines that are cooled by the water shut down. It could be potentially catastrophic if the plant weren't shut down proactively.”

So, now that humans have managed to create for ourselves a serious problem with these creatures, what can we do about it?

“I think if we really want to change this dynamic, we have to actually change the reasons the jellyfish are blooming,” Gershwin says. “They're not blooming because they're evil. They bloom as a natural part of their lifecycle, and they respond to environmental conditions. They're blooming because the things that we are doing as humans are giving them the perfect conditions to bloom. So, as long as we keep giving them fewer fish, warmer water, more nutrients in coastal ecosystems, more coastal construction, etc., they're going to continue to bloom, because that's what they do.”

This article is based on an interview that aired on PRI’s Living on Earth with Steve Curwood.