Stomolophus nomurai, also known as echizen kurage in Japan where they are often often found, are a truly gigantic species of jellyfish. Usually in the range of 6 feet wide and 450 pounds (2 meters and 200 kg for our metric friends), these jellyfish are also equipped with powerful nematocysts, from which at least eight people have died.
This species has recently become infamous for wreaking havoc on fisheries in China, South Korea and Japan. Their enormous girth breaks nets they become caught in, and they often crush or sting to death the fish within. While such incidents were no more than a nuisance in the past, the number of these jellyfish seem to have jumped prodigiously within the last year. In the Summer of 2005, thousands were caught in nets all across the South China Sea and the Sea of Japan, seriously hampering the salmon and yellowtail industry there.
Although there is no consensus yet about the reason for this sudden population surge, some scientists point to environmental changes. Heavy rains in mainland China have flushed additional nutrients into the South China Sea, and rising water temperatures may have contributed to a peak breeding year. Governments across the region are now developing jellyfish emergency plans, to cope with economic repercussions and devise ways of preventing these Lovecraftian horrors from further damaging local fisheries.
Parry, Richard Lloyd, "How do you tackle an invasion of giant jellyfish? Try making sushi," Times Online, Dec 7, 05, http://www.timesonline.co.uk/article/0,,3-1910322,00.html
Great picture of an echizen with this article. And by great I mean terrifying.