Jellyfish have been around for at least 500 million years — they're older than dinosaurs and even trees.

Science writer Juli Berwald calls them "ghosts from the true garden of Eden".

"An intelligence of a sort has allowed them to make it through the millennia," she says.

And they're not going anywhere.

In fact, the brainless, spineless, eyeless, bloodless creatures are booming in numbers — and causing mayhem around the world.

Some scientists think jellyfish numbers are increasing as the climate changes. ( Getty: Nicolas Montealegre/EyeEm )

Their propensity to breed fast and prolifically means jellyfish can disrupt ocean ecosystems in a flash.

And their effects aren't contained to the sea.

In places like Sweden, Israel, the US and the Philippines, power plants have been affected by blooms of jellyfish.

"So many jellyfish were swept into the power system … that it shut down the power system through much of this one island in the Philippines," Ms Berwald says.

"People thought that perhaps there was a coup going on, but there wasn't, it was just the jellyfish."

Jellyfish being removed from the cooling system of a power plant in Hadera, Israel. ( Getty: Jack Guez )

Jellyfish have also caused plants to shut down in Japan.

"One jellyfish scientist from Japan told me that the first threat to the electric system in Japan is earthquakes, but the second is jellyfish," Berwald says.

"We are dealing with a ubiquitous creature."

A human cause

Some scientists think jellyfish numbers are increasing as the climate changes — the creatures reproduce well in warmer waters.

Jellyfish also fare better than many other sea creatures in polluted waters, as they don't need much oxygen.

Berwald says that can give them the upper hand over predators.

"They can sort of slip into polluted waters, into low oxygen waters, and hide from predation there better than a fish that has a higher oxygen demand," she says.

The World Wildlife Fund says land-based activity accounts for 80 per cent of ocean pollution. ( Getty: Sergio Hanquet )

Overfishing is also part of the story.

"There's a situation in Namibia ... where there was uncontrolled illegal fishing for many, many years, so it depleted the ecosystem," Berwald explains.

"Then there was a sort of warming that happened … and it allowed these two jellyfish to come into the ecosystem there.

"[Jellyfish] numbers have proliferated so much that when fisherman fish there now they will get two to three times the [mass weight] of jellyfish to fish."

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Many scientists don't think the ocean ecosystem can revert back to what it was, and it's unclear as to whether fish can come back to rebalance the system.

"It's a really terrible situation because the region used to be one of the richest fishing grounds in the world," Berwald says.

Depleted fish stocks have also affected other animals in the area that rely on fish for their diet.

"There's stories that seals and birds are actually starving there because there aren't enough fish left to eat," Berwald says.

200kg Nomura

The proliferation of jellyfish is also affecting the largest of the creatures: the Nomura, which live in waters around Japan.

It can grow up to 2 metres in size and can weigh as much as 200 kilograms.

These jellyfish were once seen only every 30 years, but now they're being sighted every year.

Giant Nomura jellyfish are being sighted more frequently in Japan. ( Getty: The Asahi Shimbun )

"A fisherman would tell their son 'this year there were these giant jellyfish everywhere' and then 30 years later the son would remember the father telling them," Berwald says.

"But in the 21st century those jellyfish have occurred every single year".

She says the numbers can be "just enormous" — and sometimes "catastrophic".

In 2009, a 10-tonne fishing boat capsized after Japanese fisherman attempted to haul in masses of Nomura.

Extra measures are now being taken for precaution.

"They are trying to come up with systems to predict what years will be big jellyfish years and which years won't be," Berwald says.

What can jellyfish tell us?

According to the World Wildlife Fund, most of the waste we produce on land eventually reaches the oceans — everything from plastic bags to pesticides.

Berwald says the lesson from jellyfish is that we need to care for our oceans, and work to tackle pollution.

"Because we are terrestrial creatures we haven't really thought about the health of the oceans," she says.

"We've considered it as this place that is so big we can never really damage it that badly. The message from jellyfish is that we are.

"It's quite important that we start doing things more responsibly when it comes to the oceans."