I saw this on Discovery a few nights ago.
Since last summer, Japanese waters have been inundated with the massive sea creatures, which can grow 6.5 feet (2 meters) wide and weigh up to 450 pounds (220 kilograms).
Apparently when the Japanese fishermen cut these guys up into pieces, they release millions of eggs and sperm resulting in what is now millions of giant jellies turning the Nomura's otherwise modest numbers into an armada.

[attachment=446:G
antJelly3.jpg]
Of course, Global Warming has been blamed.