As rogue jellyfish wreak havoc in the Caspian Sea, ecologist Henri Dumont warns that time is running out for indigenous species and local fishermen alike.

What happens when an organism that has evolved in one environment accidentally, or intentionally, invades a new one? While ecologists are unable to provide a comprehensive answer, it appears that in the majority of cases the immigrant will find itself in a hostile environment and will die.

But occasionally, a new habitat will prove to be a nirvana, offering plentiful food and few, if any, predators and competitors. In some cases, the organism will multiply so quickly that sections of its new ecosystem will collapse, causing what might be termed an ecological catastrophe - particularly if economic interests are at stake.

But how do organisms get carried around the globe? For species that live in river estuaries, one of the main routes is in the ballast water of ships. This has become increasingly common since the 1950s. Ballast water is used to stabilise ships. Water is pumped into tanks or cargo holds at one port and released at another. Estuaries are rich in organisms, and ships will often carry surprisingly diverse collections of living organisms in their tanks. What happens when these are released will vary, but the more frequently ballast water is released, the higher the probability of a successful invasion by the immigrant.

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The quantities of ballast water transported have grown hugely in recent years: North America's Chesapeake Bay received more than 20 million tonnes of ballast water in 1994. This is the official quote, but the figure could be higher. Jim Carlton, of Williams College, Connecticut, calls this "ecological roulette". Carlton and his colleagues sampled ballast water from 60 vessels in Baltimore harbour and collected 221 species of crustaceans and fish, including comb jellies.

While many comb jellyfish live only in sea water, others can adjust to a broad range of sal****er environments. These typically live in river estuaries, where salt levels fluctuate with the tide. If they also possess the ability to withstand broad temperature fluctuations, they are perfectly suited to hitchhiking around the world. The comb jelly Mnemiopsis has all these characteristics.

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Mnemiopsis's native home is the brackish estuaries along the east coast of America. It prefers water that is about one-third as salty as sea water, but it can survive levels below or above that. It is a hermaphrodite, so a single organism can colonise a new environment. This is what happened in the Ponto-Caspian region, where a few organisms were probably released in about 1980.

The Ponto-Caspian basin covers the Black and Caspian seas, both of which are enormous brackish water lakes. They may be the ideal habitat for specific foreign estuary organisms. When Mnemiopsis arrived in the Black Sea, it found itself in a physically attractive environment with no natural predators. Fish that sometimes feed on comb jellies, such as mackerel, had been fished to extinction in the Black Sea in the 1970s. Other potential predators, such as the comb jelly Beroe ovata , had never made it to the Black Sea, presumably because there was nothing for them to eat there.

Mnemiopsis found a rich diet: plenty of zooplankton, and the eggs and larvae of various sardine-like fish, which in the 1980s provided the main income for local fishermen. This two-pronged attack had a devastating effect on the sardines. Mnemiopsis can consume up to ten times its weight in food every day, andby 1989 its biomass had increased to more than a billion tonnes, more than the entire weight of fish caught in the world that year. By contrast, the sardine catch fell from about 700,000 to 100,000 a year, putting thousands of fishermen out of business.

In the mild Black Sea winter, Mnemiopsis retreated to subsurface levels where the temperature never sinks below 8C, well above the 4C that is fatal to the jelly. By the summer it was reproducing again. The pattern continued for a decade.

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East of the Black Sea lies the Caspian. Both seas share their geological origins, but the Caspian is a closed basin extending from north to south for more than 1,000km. It is fed at its north end by the River Volga, which brings fresh water to the lake. As this evaporates and becomes more salty, it creates a north-south gradient of different levels of saltiness, with the Iranian shores in the south being one-third sea water. The climate in the north and south is also different: the shallow water in the north freezes in the winter, but the surface temperature in the deep south never falls below 10C - making it ideal for Mnemiopsis . But how did the jelly get there?

In the 1930s, the Soviets linked the Don to the Volga, via the Lenin canal, opening the Caspian to international shipping. Ships first had to move upriver through a stretch of fresh water, which Mnemiopsis cannot cross. But ballast tanks gave it a hand. In an article published in 1995, I warned of this danger, and only a year later, the minister of the environment in Turkmenistan told me fishermen in the Gulf of Krasnovodsk had reported finding some strange jellies in their nets. I spent a week in the gulf, but found nothing because the density of the jellies was still low. The first certified sightings came in November 1999 from scuba divers along the east of Kazakhstan. From that time, the situation deteriorated fast: by July 2000 the Caspian was infested with Mnemiopsis . Between early August and early October this year, the jelly biomass doubled. This is an astonishing speed of reproduction and, although its biomass is only 5 per cent of the maximum reached in the Black Sea, it could take less than two years for the Caspian to catch up.

What are the likely consequences? The Caspian, once the producer of about 90 per cent of the world's caviar, has lost most of its sturgeon and has only one commercial fish left: a number of sardine-like fish known collectively as kilka. Like the sardines in the Black Sea, these feed on zooplankton and produce free-floating eggs. They are the first domino that will topple. Thousands of fishermen will lose their livelihoods. Kilka is also the food of the lake's main carnivore, the Caspian seal, of which between 100,000 and 300,000 remain after being subjected to water pollution and viral infections. Deprived of food, the seals will quickly become extinct.

How can we prevent this? At the height of the Black Sea crisis, the United Nations called together experts to look for a solution. Among the proposals was one to introduce milkfish, an estuary fish that is Mnemiopsis's natural predator. Objections were raised because similar plans had gone wrong in the past when the predator had itself become a pest. But in the Caspian the milkfish might offer new commercial possibilities for fishermen. Moreover, the Caspian is a closed lake and the danger that the milkfish might escape into the Black or Mediterranean seas is limited. An alternative would be to introduce Beroe ovata, a comb jelly that feeds on Mnemiopsis . It spontaneously invaded the Black Sea in 1997 and by 2000 had started to have an impact on the Mnemiopsis. The zooplankton has also begun to recover.

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We therefore have two potential tools for combating Mnemiopsis in the Caspian. All we need now is the will of the five countries in the region to act. We only have two to three years to save the kilka, the seals and numerous small invertebrates that live in the lake. If we allow history to repeat itself, and wait until Beroe ovata finds its own way to the Caspian, it will surely be too late.

Henri Dumont is an ecologist at the University of Ghent. A full account of this research will appear in The Journal of Biological Invasions in December.

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