Instead of feeling dislocated, the newcomer, Mnemiopsis leidyi, began reproducing in a major way. Pronounced NEH-me-op-sis, the fist-sized hermaphrodite ate fish eggs and larvae, as well as crustaceans and other food for fish. In other words, the creature simultaneously preyed on fish and competed with them.

The results could fairly be called a population explosion: By 1990, the weight of Mnemiopsis in the Black Sea was estimated at 1 billion tons -- about equal that of all the fish caught in all the oceans that year.

All this growth came at the cost of fish that traditionally fed Russians, Turks and others living around the Black Sea, says Richard Harbison, a biologist at the Woods Hole Oceanographic Institution. The anchovy catch plummeted from 500,000 tons in the early 1980s to 100,000 tons in 1989. Although that has since rebounded to about 300,000 tons, catches of a second food fish, the Azov Sea kilka, have fallen to zero.

Wanted: Alien enemy!

When Harbison became interested in finding a way to control the Mnemiopsis, he immediately wondered about fighting fire with fire: If the problem was exotic species, then the solution might be a second exotic species -- one that made a living eating Mnemiopsis.

This is a standard practice in the fight against exotic species. Also called "alien species," these invaders originate not in outer space but rather in another ecosystem. Exotics are wrecking habitats as diverse as Africa's Lake Victoria where the introduced Nile perch is killing off many of the 500 closely related species of cichlid fish, and Hawaii where exotic insects, rats, pigs and plants have eliminated native species from most of the lowlands.

Biologists explain that introduced species can run wild because their new habitats lack the predators and diseases that controlled them in their native habitats. That was particularly true of Mnemiopsis in the Black Sea, Harbison says. The salinity is so low that "there just aren't any predators that can deal with the Mnemiopsis."

Harbison knew the place to find an organism that deems a relative of jellyfish high cuisine was back in the organism's home, the North Atlantic Ocean between Florida and the Grand Banks. "When you look for a control for an exotic pest, you go to the home range and look for a predator that's evolved with the pest species," Harbison says.

Buttered butterfish, anybody?

That search focused on the butterfish -- a small, edible fish that eats Mnemiopsis and jellyfish. Harbison's preliminary tests indicates that the fish can survive in Black Sea conditions.

If it indeed can live in the Black Sea, would the introduction become just another disturbance there? Conservation biologists warn that biological controls like this can be risky. The mongoose, after all, was brought to Hawaii to control rats, but instead ended up preying on endangered birds.

To Harbison, such cavils are highly irrelevant to the Black Sea. He says the danger is not so much extinctions as the hole left in the diet by the decline of the anchovy and kilka: "A lot of this attitude has to do with the way we Americans look at the sea. We don't regard it as a grocery store, but as a place for recreation."

Finding a useful fish "solves a number of problems at once," he adds. "If the butterfish population gets out of hand, you just eat them."

People have been eating frogs' legs for years. But what about frogs that don't have legs?