They are called echizen kurage and they sound like monsters from the trashier reaches of Japanese science fiction. They are 6ft wide and weigh 450lb (200kg), with countless poisonous tentacles, they have drifted across the void to terrorise the people of Japan. Vast armadas of the slimy horrors have cut off the country's food supply. As soon as one is killed more appear to take its place.

Echizen kurage is not an extraterrestrial invader, but a giant jellyfish that is devastating the livelihoods of fishermen in the Sea of Japan. Nomura's jellyfish, as it is known in English, is the biggest creature of its kind off Japan and for reasons that remain mysterious its numbers have surged in the past few months.

The problem has become so serious that fishery officials from Japan, China and South Korea are to meet this month for a "jellyfish summit" to discuss strategies for dealing with the invasion. Japan's ruling Liberal Democratic Party has formed a jellyfish countermeasures committee and fishermen are at work on technology to keep the marauders out of their nets.

The problem first became obvious in the late summer when fishermen chasing anchovies, salmon and yellowtail began finding huge numbers of the jellyfish in their nets. Often the weight of the echizen kurage broke the nets or crushed the fish to death; those that survived were poisoned and beslimed by their tentacles. Fishermen on the northern tip of Honshu, Japan's main island, were forced to suspend work at the height of the lucrative salmon season. In Akita prefecture some communities saw their incomes fall by 80 per cent. The gizzard shad fishers of South Korea have also been plagued by the Nomura's.