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.
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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