Huge amounts of jellyfish have forced the shutdown of nuclear power plants in Japan, already hit by the earthquake and tsunami, Scotland and a coal-powered plant in Israel in the past few weeks.

And a sustained explosion in the population of jellyfish throughout the world's oceans has the potential to be "quite catastrophic" if it is not checked, said jellyfish expert Dr Jamie Seymour from James Cook University in Queensland.

Last week at the Orot Rabin Electric Power Station in Hadera on Israel's west coast - which uses seawater to cool its reactors - tonnes of jellyfish clogged up the filters.

At the Torness power station on the south-east coast of Scotland both reactors were shut down after jellyfish were found in their seawater filters at the end of last month.

Around the same time, the cooling system at one nuclear reactor of the Shimane plant in western Japan was blocked after a jellyfish invasion.

The seasonal warmer waters in the northern hemisphere - conducive to the growth of the cold-blooded creatures that have existed for about half a billion years - could be one explanation as to why the power plant incidents occurred in quick succession, experts said.

But global warming, the nitrification of oceans through fertiliser run-off and overfishing have also created the  environment for a huge expansion of the animals nicknamed the cockroaches of the sea, studies showed.

"All these things individually can potentially lead to more jellyfish, and then we add them all together," Monty Graham, co-author of a jellyfish blooms study published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) in June, told the Sarasota Herald-Tribune.

"Global warming increases the water temperature. These animals are cold-blooded so the warmer you make it the quicker they grow," Dr Seymour said.

"An increase of nutrients in the ocean [from fertiliser run-off] increases the amount of algae, so that increases the amount of zoo plankton or little critters, and that's what the jellyfish are eating."

"The third one is overfishing. So you remove all the fish and all the major predators in the ocean and there's nothing left to eat the jellyfish."

Dr Seymour said it was difficult to predict how much the worldwide jellyfish populations would grow in the next few years, but warned that if their numbers continue to increase, humans could be in "a world of hurt" within a few decades.

"It has the potential to be quite catastrophic. We really don't know [what will happen in the long term]. The general consensus is it's not a matter of if it happens but when it happens.

Jellyfish, which appear according to seasons, might allso become present for longer periods of time.

"You'll see the way people use beachfronts change completely. So instead of being able to get onto the beach and seeing one or two jellyfish, you are going to [see] beaches closed through the world."

He said fishing industries in the Gulf of Mexico, the Mediterranean, Japan and Korea were already destroyed due to the dominance of jellyfish over other species. Australia had so far not experienced such massive jellyfish blooms as it hasn't depleted its fisheries in the same manner yet.

So can the trend be reversed?

Dr Seymour said stopping fertiliser run-off into our oceans and a halt in overfishing would go a long way in limiting the ocean environment that encourages jellyfish blooms.

Another co-author of the PNAS study, Professor Deborah Steinberg, told Wired magazine: "We're a long way from jellyfish taking over the world, but humans are changing food webs in the ocean by our activities.

"It's an experiment, a big experiment, and we don't know yet what the outcome is going to be. We need to be careful."

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