Giant Jellyfish Invasion May Be Imminent in Japan

Julian Ryall in Tokyo
for National Geographic News
July 29, 2009

Giant jellyfish seem poised to invade Japan, and experts are warning local fishers to brace themselves for an inundation that could wreak havoc on their industry.

Nomura's jellyfish is one of the largest jellyfish species in the world, growing up to 6.5 feet (2 meters) wide and weighing as much as 440 pounds (200 kilograms). (See giant jellyfish pictures.)

The giant jellyfish last swarmed western Japan in vast numbers in 2005. Their huge bodies damaged fishing nets, and their toxic stings poisoned the catch and even injured some fishers.

Now the jellyfish could be gearing up for a similar assault, say experts who recently conducted some of the first surveys of the giants' spawning grounds.

"We have reports of massive bloomings of young jellyfish near the Chinese coast, where the ecosystems of the Yellow Sea and the East China Sea are favorable for breeding," said Shin-ichi Uye, a biological oceanographer at Hiroshima University. (See a map of the region.)

Efficient Jellyfish

Relatively little is known about Nomura's jellyfish, so Uye and colleagues across Japan have been studying the jellyfish in the lab to learn more about its habits and reproductive strategies.

Based on captive breeding, Uye's team has found that the jellyfish are extremely efficient at filtering tiny creatures called zooplankton out of the water.

As long as a jellyfish is healthy, it devotes all its energy to eating. Even during the spawning season, its reproductive system remains immature.

But if the jellyfish is injured or weakened, it quickly switches to producing offspring, Uye said.

Giant Jellyfish "Typhoons"

The scientists are still not sure why thousands of the creatures float across the Sea of Japan in some years but not others.

"It is possible that they have a 'rest stage' or hibernation period in their development over several years, but then their numbers shoot up given certain environmental stimuli," Uye said.

Researchers also don't know why the giant jellyfish are becoming more regular visitors to Japan's shores.

In the early 1900s large numbers of the giants were reported only every 40 years or so.

But in recent years the jellyfish have been appearing with alarming frequency: Japan experienced unusually large outbreaks almost every year between 2002 and 2007.

One contributing factor may be a decline in the number of jellyfish predators, including sea turtles and certain species of fish known to eat young jellyfish.

According to Uye, right now giant jellyfish outbreaks are like typhoons—they can't be controlled, but they can be predicted.

He and his colleagues are currently working on a system for creating accurate jellyfish forecasts, so fishers will hopefully be able to better prepare themselves.

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