They poison fish, sting humans and even attack nuclear power stations. They are 6ft wide, up to 440lb in weight, and they are pink, slimy and repellent. They sound like rubber monsters from a Godzilla film, but they are Echizen kurage, or Nomura’s jellyfish, an authentic horror of the deep about to launch its latest assault on Japan.

Four years after they last caused havoc, and for reasons that remain mysterious, an armada of the gelatinous giants is gathering in the Yellow Sea off China and the Korean peninsula. It is expected to drift into the Sea of Japan in the next few months.

“The arrival is inevitable,” Professor Shinichi Ue, from Hiroshima University, told the Yomiuri newspaper. “A huge jellyfish typhoon will hit the country.”

In 2005, fishermen looking for anchovies, salmon and yellowtail began finding huge numbers of the jellyfish in their nets. When the Nomuras grow larger than a metre in diameter, half a dozen of them can destroy a fishing net. The fish caught alongside them are poisoned and covered in slime and rendered unsaleable.

So serious was the situation that salmon boats in northern Japan stopped going out, and in some places fishermen lost 80 per cent of their income. Even staff at some of the nuclear power plants along the Japan Sea coast found that the jellyfish got sucked into the pumps which take in sea water to cool the reactors.

No one is sure about the reasons for the slimy plague. One theory is that global warming is heating up the sea water and encouraging them to breed. Another blames effluent from rivers in China, which carries nutrients on which the jellyfish feed. Another blames over-fishing of other species, leaving a surfeit of plankton for the Echizen kurage to feed on.

Meanwhile, enterprising Japanese are making the most of the maritime catastrophe. Fishermen have devised ways of keeping the jellyfish out of their nets with sharp wires. Scientists have discovered a method of extracting collagen form the jellyfish, an ingredient of cosmetics and pharmaceuticals, increasingly valued as a health food, and usually extracted from pig or fish carcasses.

Most unlikely of all, a company called Tango Jersey Dairy sells vanilla and jellyfish ice cream, created by soaking diced cubes of Echizen kurage in milk. The resulting desert is described as “slightly chewy”.

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