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This article is based upon reports in New Scientist Issue No. 2255, National Geographic Online & The Russian Academy of Sciences, Shirshov Institute of Oceanology.

Battle of the Black Sea Jellies

 

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Mnemiopsis sp.

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Scientists are hoping that two alien ctenophore species will battle it out in the Black Sea and eventually restore its fragile ecological balance. The latest comb jelly is acting as a form of biological control, attacking the earlier invader that ran riot when it reached the sea in the 1980s.

The seeds of disaster were sown in the 1970s as increasing salinity, pollution and overfishing began to bite. "The Black Sea ecosystem was about as screwed up as something could get," says Monty Graham, a biologist at the Dauphin Island Sea Lab in Alabama. Then the comb jelly Mnemiopsis leidyi, a relative of jellyfish, arrived on the scene, probably in the ballast water of a ship traveling from the East Coast of the United States.This invador, with a plentiful food supply and a lack of predators, underwent a population explosion.

 

Mnemiopsis has a great appetite for the zooplankton that many fish also eat, as well as for actual fish eggs and larvae. Its dominance was so rapid that by 1989 there was about a billion tonnes of it, while the numbers of fish, including those of the commercially valuable anchovy, had plummeted. The situation became critical, causing the virtual collapse of the anchovy, scad and sprat fisheries and an increase in toxic red algae blooms.

 

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Zooplankton: Fish eggs & larvae

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Beroe ovata
- note flashing comb -plates

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Beroe ovata
- mouth agape

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Biologists considered introducing one of Mnemiopsis's predators such as Beroe ovata, another comb jelly to rebalance the food web. However the idea seemed too risky because attempts to use biocontrols, such as the introduction of the cane toad to Australia, can go horribly wrong. For example Beroe may also begin to eat native comb jellies or other species that are important to the ecosystem and therefore cause the original problems to escalate.

Then, without any intervention by humans, in 1997, B. ovata established itself in the Black Sea of its own accord, either by migrating naturally from the Mediterranean or possibly in ship's ballast water again. Initial occupation of coastal areas spread rapidly and by 1999 Beroe populations in the entire northeast region of the Black Sea.

"I could not believe it," says Tamara Shiganova of the Shirshov Institute of Oceanology in Moscow. Since Beroe arrived there has been a massive decline in the Mnemiopsis population while some species of plankton have begun to increase, Shiganova and her colleagues report in a paper that will appear in Hydrobiologia. Because B. ovata feeds upon animals similar to itself, such as Mnemiopsis, as opposed to the smaller zooplankton and fish populations, the initial reports look positve. However, the long term effects are much more difficult to predict. It will take many years of monitoring population cycles, ecosystem dynamics and seasonal variations in order to determine the true impact that this invasion will have.

 

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Beroe ovata (left) devouring Mnemiopsis

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For more information contact:

Monty Graham, Senior Scientist, Mississippi-Alabama Sea Grant Research Scientist Dauphin Island Sea Lab, (O) 334-861-7555, Email: mgraham@disl.org

Dr. Tamara. Shiganova , Scientist, Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, +7 (095) 129-23-27, Email: shoganov@thip.fio.rssi.ru

 

 

    Recent dramatic changes in the Black Sea ecosystem: the reason for the sharp decline in Turkish anchovy fisheries.
 Seasonal and Long-term Modifications on the Temporally-Evolving Black Sea Epipelagic Ecosystem: A Synthesis of Observations and Model Simulations

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