They called it "the blob that ate the Black Sea." And now the world's most dangerous American biological export -- a jellyfish about the size of a man's hand -- has moved east and is devouring the Caspian Sea.
This ecological monster, a voracious jellyfish called Mnemiopsis leidyi, was accidentally transported from the backwaters of the US eastern seaboard two decades ago in the hull of a ship. When the ship emptied its ballast water into the Black Sea, the jellyfish began its rampage.
Last month, a five-nation group of scientists decided that the best hope for saving the Caspian Sea's caviar and restoring the Black Sea's anchovies lies with yet another gelatinous stowaway. For three years beginning later this year, Iranian researchers will release small numbers of specially bred predator jellyfish named Beroe ovata along the coastline of the Caspian Sea, hoping they will consume their troublesome cousins and then die off.
The invasion of Mnemiopsis is one of the most startling cases of the global spread of alien species -- most of which hitchhike one way or another with traveling humans. Taken out of their natural environments, many aliens swiftly die off, consumed by predators against whom they have no defense. But some find no predators and swiftly become a plague.
The Pacific island of Guam is now overrun with the Australasian brown tree snake, which stole a ride on an American military transporter half a century ago. South Africa's water supplies are being sucked dry by non-native trees. And lakes and waterways across the planet are being clogged by the pretty but voracious water hyacinth, introduced from its native Brazil by admiring English colonial gardeners.
"The biological invasion by alien species constitutes the greatest threat to biodiversity," according to Jeffrey McNeely, chief scientist at the Geneva-based World Conservation Union.
Ballast water, carried in empty cargo vessels to maintain their stability as they return to their home ports, has become notorious for bringing alien marine species to foreign lands. Ships carry up to 10 billion tons of ballast water across the globe annually and much of it is infested with marine organisms. Biologists call it "ecological roulette."
Last month, the International Maritime Organization, a United Nations agency based in London, signed an agreement to clean up ballast water in ships "to prevent, reduce, or eliminate the transfer of harmful aquatic organisms." Beginning in 2009, ballast water will have to be checked for alien species by the ship's operators before it can be discharged into open water.
The American Association of Port Authorities welcomed the new rule, which will be policed in the United States by the Coast Guard. "We eagerly await the next big step -- the development of a mandatory national ballast water management program," the association's president, Kurt Nagle, said in a statement. The association estimates that some 80 million tons of ships' ballast is discharged into US waters each year.
America's most troublesome recent alien species, the zebra mussel, also arrived via ballast water, and -- coincidently -- hails from the same Caspian Sea that America's Mnemiopsis export is devastating. The clinging mollusc, which clogs pipes and covers boat hulls, arrived in the Great Lakes some 15 years ago and made it to Massachusetts five years ago, where it was first spotted in Salisbury's Twin Lakes.
The Mnemiopsis, a comb or non-stinging jellyfish, once lived modestly from Maine to Florida, grazing on plankton while being kept in check by countless other jellyfish-eating species.
But in the Black Sea it found abundant food and no predators. It munched its way through the eggs and larvae of a wide variety of fish, while consuming the plankton on which other fish fed.
A self-fertilizing hermaphrodite, Mnemiopsis bred as fast as it ate. It reaches maturity within two weeks and then produces 8,000 eggs a day. Its appetite is so great that it can double its size in a day. By 1990, its total biomass in the Black Sea had reached an estimated 900 million tons -- 10 times the annual fish catch from all the world's oceans.
A snorkeling marine biologist from the Ukraine, Yu Zaitsev, once calculated that there were 500 of the beasts in a single cubic meter of water in Odessa Bay. There were almost more jellyfish than water. Meanwhile, fish catches across the Black Sea declined by 90 percent. The valuable anchovy fishery was wiped out.
The jellyfish population stayed steady, until another American jellyfish showed up -- again most probably in ballast water.
Beroe ovata has a rather strict diet: It eats only its cousin Mnemiopsis. A sac-like creature, it opens itself up to its full extent and gobbles its relative in one go. Beroe ovata arrived in the Black Sea in 1997. Almost immediately, the Mnemiopsis population in the Black Sea began to decline.
But Mnemiopsis went on its travels again. In 1999, the gelatinous monster showed up farther east again, in the Caspian Sea. Just a few arrived at first, near where the Volga-Don canal, which links the two seas, enters the Caspian. They either swam up the canal or, more likely, hitchhiked in the bilges of ships.
Once again, the apparently inconsequential jellyfish found plenty of food and no predators. The waters of the Caspian Sea are shared between Iran, Russia, and three former Soviet states: Azerbaijan, Turkmenistan, and Kazakhstan. And in five years, each nation has seen its fishing stocks collapse by 50 percent as the alien has spread. Whole fishing communities along the shores of the world's largest inland sea have died.
Hardest hit has been the valuable kilka fish, which is also the preferred food of the sea's indigenous seal and its famed beluga sturgeon, source of most of the world's caviar. Both species are "under significant threat" from the invader, according to Hossein Negarestan, head of marine ecology at the Iranian Fisheries Research Organization in Tehran. The seals are already reeling from repeated epidemics of distemper and the sturgeon have been hit by an orgy of illegal fishing.
But during five-nation talks on the future of the Caspian region, held late last month in Azerbaijan, scientists unveiled plans to control the invader by breeding and releasing Beroe ovata. Iranian researchers had succeeded in acclimatizing the predator to the waters of the landlocked Caspian Sea, which are much less salty than the open ocean.
It seems the perfect solution. "Beroe ovata only preys on comb jellyfish, and the only comb jellyfish in the Black Sea are the invading Mnemiopsis," Negarestan told the Globe. Once Mnemiopsis has gone, he believes, Beroe will simply die out.