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Attack of the killer jellyfish
March 10, 2004

Written by Fred Pearce
Posted by HW on July 23, 2021

A ship-born alien that played havoc with the ecosystem of the Black Sea is now threatening the neighbouring Caspian. The toxic invader is a gelatinous monster that can double its weight in a day.

Ecological terrorist
They call it an ecological terrorist and "the blob that ate the Black Sea". Now the world's most dangerous alien species, a jellyfish about the size of your hand, is invading the Caspian Sea, where the region's scientists last week proposed setting another equally voracious jellyfish to gobble it up. Welcome to the biological equivalent of the war on terror.

Mnemiopsis leidyi is an opaque comb jellyfish. The comb jellyfish is like an ordinary jellyfish, but without the sting. It comes from the backwaters of the US eastern seaboard, all the way from Massachusetts to Florida, where it once lived modestly enough, grazing on plankton, while being kept in check by countless jellyfish-eating species.

But two decades ago it entered the Black Sea, probably hitch-hiking in a ship's ballast water. There 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.

Breeding, eating fast
A self-fertilising hermaphrodite, Mnemiopsis breeds as fast as it eats. It reaches maturity within two weeks and then produces 8,000 eggs daily. 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 estimate 900 million tonnes, 10 times the annual fish catch from all the world's oceans.

500 in a cubic metre
One snorkelling marine biologist from the Ukraine, Yu Zaitsev, calculated that there were 500 of the beasts in a single cubic metre of water in Odessa Bay. There was almost more jellyfish than water. Meanwhile, fish catches across the Black Sea had declined by 90 per cent. The valuable anchovy virtually disappeared.

Jelly eats jelly
It was like a plague of locusts on the land. But unlike locusts, the jellyfish did not seem to eat itself out. Its population stayed steady, until another comb jellyfish showed up - again, probably in ballast water from the US. Beroe ovata has a rather more strict diet than its cousin Mnemiopsis. It likes eating its cousin. A sac-like creature, it simply opens itself up to its full extent and gobbles its close relative in one go. Beroe ovata arrived in 1997. Almost immediately, the Mnemiopsis population in the Black Sea began to decline.

Caspian Sea
Crisis over? Not exactly. For Mnemiopsis went on its travels again. In 1999, the gelatinous monster showed up further east again, in the Caspian Sea. Just a few arrived at first, around where the Volga-Don canal, which links the two seas, enters the Caspian. They either swam up the canal or, more likely, hitch-hiked again in the bilges of ships on the canal.

Fishing stocks collapse
And once again, the jellyfish found it had plenty of food and no predators. The waters of the Caspian Sea are shared between Iran, on its southern shore, Russia and three former Soviet states: Azerbaijan, Turkmenistan and Kazakhstan. And in the past five years, each nation has seen its fishing stocks collapse by 50 per cent or more as the alien has spread. Whole fishing communities along the shores of the world's largest inland sea have been devastated.

Hardest hit are stocks of the anchovy-like kilka fish. But that's not all. For the kilka, besides being the staple of local fishermen, is also the preferred food of the sea's indigenous seal, as well as of the famed beluga sturgeon, source of most of the world's caviar. Both species are "under significant threat" from the invader, says Hossein Negarestan, the head of marine ecology at the Iranian Fisheries Research Organisation in Teheran. The seals are already reeling from repeated epidemics of distemper in the past decade, and the sturgeon have been hit by a mafia-sponsored orgy of illegal fishing. According to Negarestan, "the impact of Mnemiopsis on the Caspian Sea ecosystem may be much worse than in the Black Sea." And any threat to the local caviar trade is a threat to the entire regional economy.

Breeding predators
But during five-nation talks on the future of the Caspian region, held at the end of last month in Baku, the capital of Azerbaijan, ministers heard the results of a meeting of their fisheries scientists in Teheran a few days earlier, which had proposed breeding and releasing Beroe ovata in an attempt to control the invader. According to Negarestan, his researchers have been experimenting for more than two years with breeding the predator and acclimatising it to the less salty waters of the landlocked Caspian Sea. The trick, they say, is to breed it in salty water and then move the offspring into tanks containing ever less salty water over a period of a few weeks, until they are used to their new environment. Then the offspring can be released into the Caspian Sea, ready to be deployed as a "bioweapon".

"Beroe ovata only preys on comb jellyfish, and the only comb jellyfish in the Black Sea are the invading Mnemiopsis," Negarestan says. "It would be an ideal biological control agent. And we have established that there is no risk of transfer of bacteria or parasites to the Caspian with Beroe ovata." Once Mnemiopsis has gone, be believes, Beroe will simply die out.

The spread of Mnemiopsis is one of the most startling cases of the global spread of alien species - most of which hitch-hike one way or another with travelling humans. Taken out of their natural environment, many aliens swiftly die off, consumed by predators against which they have no defence. But some find greener pastures. And without predators, there is nothing except a limit on their food supply to prevent their proliferation.

There are famous cases of alien species being deliberately released by humans. Fish such as the Nile perch and trees such as the Australian eucalyptus are ubiquitous. Australia is also famous for being populated by other peoples' species, not least the humble rabbit. Many of the most voracious aliens were introduced to combat other aliens. The poisonous cane toad, currently proliferating in the wilderness of northern Australia, was introduced 70 years ago to eat the grubs of the cane beetle, then infesting sugar-cane crops.

Botanists and gardeners took the water hyacinth, a pretty but voracious water-weed native of Brazil, across the world for display. But it escaped, and today is choking canals, lakes and harbours in 50 countries. East Africa's giant Lake Victoria has been all but covered by the weed in some years. In South Africa, they are currently clearing huge areas of non-native trees that are soaking up the country's scarce underground water supplies. Hundreds of tropical islands have been stripped of their ground-nesting birds, thanks to rats that jumped ship during colonial times. The brown tree snake has wrecked ecosystems, terrorised human beings and eaten its way through countless power lines in Guam since the reptile hopped to the islands aboard US military aircraft in the 1950s. And 18 months ago, just before the Gulf War, US troops halted military preparations on their Indian Ocean base at Diego Garcia to erect a snake-proof fence round the airfield, after fears that a brown tree snake had hitched a ride there from Guam.

And Britain has not escaped the alien invasion, either. We have suffered from an influx of Japanese knotweed, the Colorado beetle, the ruddy duck and the grey squirrel. The spread of the hedgehog through the Outer Hebrides in the past 30 years has triggered a succession of crises for its island bird populations.

Some scientists call the invasion of alien species a bigger threat to the planet's biodiversity than any human activity. But not everyone is quite so opposed to the aliens in our midst. Where would Britain be without them? asks the naturalist Richard Mabey. He says that we should welcome the diversity that they bring to our landscape. According to the Wildflower Society, two thirds of our wild flowering plants are aliens, many of them semi-tropical imports that hopped over the fence from suburban gardens and are now establishing themselves in the wild.

And some people don't even like the term "aliens". The US social scientist Betsy Hartmann of the Hampshire College in Amherst, Massachusetts, says: "The ideas of ecologists about aliens sound so similar to anti-immigration rhetoric. Green themes such as scarcity and purity and invasion all have right-wing echoes. Hitler's ideas about environmentalism came out of the idea of purity, after all."

It may be little consolation to the fishermen of the Caspian Sea. But perhaps we should spare a thought for the aliens, after all.

* Mnemiopsis leidyi was first recorded in the Black Sea in the 1980s, where it has no native predators. It radically affected the pelagic ecosystem, achieving enormous biomass levels (up to 1.5 to 2kg m-2 in the summer of 1989).
* The jellyfish has had a huge impact on populations of anchovy (Engraulis encrasicolus) and kilka (Clupeonella spp.), which provide food for the beluga sturgeon. Beluga caviar is crucial to the health of the local economy.
* Mnemiopsis was first reported in the Caspian Sea in 1999, having migrated through the Don-Volga canal in ships' ballast water.
* To control the problem, scientists propose introducing another comb jellyfish, Beroe ovata, which has a very specific diet: Mnemiopsis.
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