t was only four months after the conference closed, while Graham was still compiling its proceedings, that the spotted jellyfish launched its surprise invasion of the Gulf of Mexico.
"I've never seen anything like the spotted jellyfish," says Graham. He has boated and swum through countless jellyfish blooms, but this species was unfamiliar. "For about a month, I kept asking, 'Who is this?'" he recalls. Jellyfish can change color and size to suit different environments, and Graham finally recognized the spotted jellyfish (Phyllorhiza punctata), despite its uncharacteristically bloated bulk and light color.
The invaders wreaked havoc on the region's fishing industry, which accounts for 40 percent of all U.S. commercial landings, by intermittently shutting down its $60 million shrimp fishery. One retired Louisiana shrimper explains that it is virtually impossible to pull a shrimp net through a jellyfish swarm: "The weight stops even a 90-foot boat cold. It's like crashing into a brick wall. You can't go forward. You can't back up, because the nets get tangled in propellers. And the nets are too heavy with jellyfish to even pull them up."
In the fall, the spotted jellyfish vanished from the gulf. In all probability, says Graham, they had simply lived out their normal adult life span. But the next summer, they were baaaaaack. Several thousand spotted jellies reinhabited their Louisiana haunts -- perhaps a second generation related to the first, perhaps an entirely new invasion. Even more disturbingly, several hundred appeared for the first time in lagoons on Florida's east coast. These clusters were much thinner than the gulf swarms. But Graham fears that the Florida lagoons, which are similar to the spotted jellyfish's native Australian habitat, may be hospitable enough to support permanent populations.
Moreover, Graham warns, "You don't need large numbers of invaders to create problems down the road. Their life history is conducive to invasiveness." Jellyfish are hardy, reproduce rapidly, and can even shrink to survive when relatively little food is available.
When appearing in reasonable numbers, native jellyfish provide important ecological functions, such as serving as food sources for fish and turtles. But in addition to clogging fishing nets, the gulf's invading swarms could, under certain circumstances, damage the fishing industry by reducing stocks of some commercial fish. With each pulse of its bell, the spotted jellyfish removes the eggs and larvae of fish, shrimp, and crabs from the water, as well as the plankton that these species eat. Graham calculated that each spotted jellyfish in the gulf that summer ate about 2,400 fish eggs daily.
Where did the invaders come from? Most likely the Caribbean, where they have thrived for years. They may have been swept north by currents. Or, during their juvenile, wormlike stage, when they cling to solid surfaces, they may have attached themselves to the hulls of gulf-bound boats or other mobile structures. During the summer of 2000, the gulf was also visited by another Caribbean alien, a massive cannibal jellyfish that gulf locals nicknamed "Big Pinky." Distinguished by their Pepto-Bismol color, the bells of these 60-pounders stretched up to three feet across and were trailed by stinging tentacles more than 80 feet long.
But even without such aliens, the gulf is sometimes jammed with jellies. Two natives, the Atlantic sea nettle (Chrysaora quinquecirrha) and the moon jellyfish (Aurelia aurita), now sometimes swarm over thousands of square miles. One shrimper estimates that on about twenty nights out of a typical summer/fall season, the jellyfish are too dense for a boat to risk going out -- no small loss to most fishermen and fisherwomen.
What's causing the gulf's bounty of jelly? No one knows for sure. Analyzing jellyfish is particularly challenging. Their gelatinous bodies resist preservation for examination. Their stinging tentacles -- tantalizing though they may be to the scientist -- threaten the safety of handlers. Their life cycle often includes a particularly mysterious deep-dwelling stage.
But in one of the only comprehensive studies of jellyfish populations conducted in U.S. waters, Graham analyzed data on gulf marine life that have been collected regularly since 1985 by the National Marine Fisheries Service. His analysis shows that the gulf's Atlantic sea nettles and moon jellyfish have steadily increased their numbers and expanded their ranges since the mid 1980s. Graham suspects that these species' proliferation is encouraged by many factors. One may be the over-harvesting of menhaden fish, which compete with the jellyfish for food. Another factor may be the 4,000 oil and gas platforms planted in the same gulf coast areas that are commonly visited by jellyfish blooms. The forest of massive steel legs supporting these structures may serve as nurseries for wormlike juvenile jellyfish.
Also, says Graham, "Jellyfish play unique metabolic tricks" -- tricks that favor their survival in polluted waters like those of the gulf. The creatures are more than 95 percent water. (Humans are 65 percent water.) Dissolved within their fluid tissues is oxygen that fuels them when they enter the infamous Dead Zone -- 7,000 square miles of the Gulf of Mexico that becomes oxygen-starved during the summer. In the Dead Zone, oxygen is consumed by the decay of massive algae blooms, which are fertilized by the tons of fertilizer, sewage, and animal waste that continuously wash into the gulf, primarily from the Mississippi and Atchafalaya rivers. Most creatures, such as fish, shrimp, and crabs, either flee the Dead Zone or suffocate. But jellyfish, equipped with their built-in oxygen supplies, thrive in the Dead Zone and banquet on the ubiquitous plankton.