( CNN ) -- Giant jellyfish descend on the Sea of Japan , causing untold devastation to coastal villages and leaving a trail of destruction and human misery behind . A diver attaches a sensor to a Nomura 's jellyfish off the coast of northern Japan in October 2005 . Sounds like a great sci-fi flick . But it 's not . It 's real and a nightmare for Japanese fishermen . The massive sea creatures , called Nomura 's jellyfish , can grow 6 feet ( 1.83 meters ) in diameter and weigh more than 450 pounds ( 204 kilos ) . Scientists think they originate in the Yellow Sea and in Chinese waters . For the third year since 2005 , ocean currents are transporting them into the Sea of Japan . Monty Graham , a marine biologist at Alabama 's Dauphin Island Sea Lab , said the jellyfish grow to an enormous size as they are transported by ocean currents . He said they stay together in packs and as they drift northward , they get caught in fishermen 's nets . The giant jellyfish are one of about 200 species of coastal jellyfish or large jellyfish that exist around the world . But Nomura 's stands out because of its enormous size . `` The sheer size of them , individually , makes them fairly spectacular , '' Graham said . Spectacular , perhaps , to scientists , but perilous to villagers along the Japanese coast who have seen the destructive habits of these colossal creatures in the past . They had giant-jellyfish invasions in 2005 and 2007 , and because they 've recently been spotted in the Sea of Japan , they 're bracing for another , potentially harmful wave this summer . The jellyfish destroy fishermen 's nets , getting trapped in them , tearing holes and ruining catches . Fishermen often use expensive mazelike nets that stretch for hundreds of kilometers . When swarms of giant jellyfish tear them , the result is devastating . `` Communities of fishermen and these fishing villages own these nets , '' Graham said . `` When these nets get wiped out , it actually has this economic devastation for an entire community . '' The good news is that previous attacks have prompted Japan to put in place a warning system for fishermen . While they still risk losing a big catch , they can , at least , save their pricey nets from the invasion of the giant jellyfish . It 's not clear why waves of Nomura 's jellyfish have made it to the Sea of Japan in recent years . Some have speculated that overfishing , pollution or rising ocean temperatures may have depleted the kinds of fish that prey on Nomura 's jellyfish in the polyp stage . However , no one is certain , Graham said .
Nomura 's jellyfish seen in Sea of Japan for third year since 2005 . Species can grow 6 feet ( 1.83 meters ) in diameter , 450 pounds ( 204 kilos ) and live up to 100 years.