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Australian Jellyfish Invade U.S. Waters

Scott Norris
for National Geographic News
August 27, 2007
 
Invasive Australian jellyfish are appearing in large numbers this summer in waters off the southeastern United States.

Swarms of the dinner-plate-size creatures have been reported from east Texas to Florida in the Gulf of Mexico and along the Atlantic coastlines of Florida and South Carolina.

The jellyfish are only mildly venomous and do not pose a threat to humans who may come in contact with them, experts say. (Related photo: "Giant Jellyfish Invade Japan" [January 19, 2006].)

But some commercial fishers and shrimp trawlers in the Gulf are finding their nets fouled with the gelatinous blobs, which may weigh up to 25 pounds (11 kilograms).

Interference with fishing operations can result in a reduced catch, said Monty Graham of the Dauphin Island Sea Lab and the University of South Alabama.

But an even greater concern for biologists and fisheries managers is that the invaders may harm native fish and other marine species.

The jellyfish feed on tiny fish larvae and may also compete with adult fish for other sources of food.

"We have good evidence that [jellyfish] can, at times, clear water of eggs and larvae of fishery species," Graham said. "The scale at which this occurs may or may not be large enough to force a reduction in [fish reproduction] over the entire range of the stock."

The same jellyfish species previously invaded Gulf waters in 2000, but had been seen only sporadically since then. Compared to 2000, Graham said, this year the jellyfish seem to be less numerous in any one place and are instead spread out across a far larger area.

"This is the first year we've heard about [jellyfish sightings] north of Florida," he said.

Reasons to Swarm

The Australian spotted jellyfish has been on the move for decades, appearing in numerous locations far from its native range in the South Pacific and Indian oceans.

But scientists are still not certain what factors may explain the species' explosive spread into the Gulf of Mexico in 2000 and again this year.

The dynamics of jellyfish invasions are often hard to track, experts say, because of the creatures' complex life cycle.

Most people think of jellyfish only as the gelatinous drifters that biologists call medusae.

But medusae start off as polyps, which are tiny sedentary creatures that attach themselves to hard surfaces such as rocks or ship hulls.

Graham believes that ship traffic is what first brought the Australian spotted jellyfish—in polyp form—to the Gulf of Mexico.

Changing ocean currents and nutrient availability may help trigger the jellyfish blooms, he said. But the Gulf invaders are probably coming from source populations of polyps that have become established nearby.

"It's hard to explain year-to-year variations ... by looking only at the free-floating phase of the life cycle," Graham said. "The numbers and spread of polyps will ultimately control the medusae."

But oceanographer Donald Johnson, of the University of South Mississippi in Ocean Springs, is not so sure nearby polyps are the source of the invasion.

He said that unusual current patterns during the 2000 invasion may have carried drifting jellyfish in from the Caribbean, where a permanent population exists.

"The most recent invasion looks more likely to have also come from the Caribbean, through the Yucatán Straits," Johnson said.

Changing Ecosystems

Jellyfish invasions have become an increasingly common phenomenon worldwide. (Related: "Jellyfish Invasion Puts Sting on Europe Beaches" [August 18, 2006].)

Johnson said that while there has been speculation that changing sea temperatures may be involved, a more likely overriding factor is depletion of ocean communities through destructive fishing practices.

Studies have shown that jellyfish sometimes take advantage of resources made available as fish populations decline.

"When the door is left open, you never know who will enter," Johnson said.

Dominance of marine communities by jellyfish changes the flow of energy through food webs, making it harder for depleted fish populations to recover, Graham said.

Normally energy flows upward, as smaller fish are eaten by larger ones. But while jellyfish consume fish eggs and larvae, they are seldom eaten by larger predators.

"We're concerned about the shifts in energy flows in ecosystems towards this so-called trophic dead-end of jellyfish," Graham said.

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