Are aliens attacking the Sea of Japan? Not exactly. But these gigantic blobs are unwelcome visitors from another place. Called Nomura's jellyfish, the wiggly, pinkish giants can weigh up to 450 pounds (204 kilograms)—as heavy as a male lion—and they're swarming by the millions.

The supersize sea creatures—normally found off the coasts of China and North and South Korea—occasionally drift east into the Sea of Japan to feed on tiny organisms called plankton. But now one hundred times the usual number of jellyfish are invading Japanese waters. And local fishermen are feeling as if they are under siege.

The fishermen's nets are getting weighted down, or even broken, by hundreds of Nomura's. The jellies crush, slime, and poison valuable fish in the nets, such as the tuna and salmon that the fishermen rely on to make a living.

No one knows for sure what's causing this jellyfish traffic jam. It's possible that oceans heated by global warming are creating the perfect jellyfish breeding ground. Another theory is that overfishing has decreased the numbers of some fish, which may allow the jellies to chow down without competition for food. For now, all the fishermen can do is design special nets to try to keep the jellies out. Some of them hope to turn the catastrophe into cash by selling jellyfish snacks. Peanut butter and jellyfish, anyone?

Fast Facts

• Baby Nomura's jellyfish change from the size of a grain of rice to the size of a washing machine in six months or less.
• Jellyfish are 95% water.
  
• Jellyfish aren't actually fish, they're invertebrates—animals without backbones.

Text by Ruth A. Musgrave
National Geographic Kids magazine