They have no backbone and their slimey bodies are made up of more than 90 per cent water but they threaten to turn beaches into no-go zones within two decades.

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.

A digger clears away jellyfish after they blocked the water supply to a power plant in Hadera, Israel. Credit:AFP

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.