A federal experiment in restoring oysters to the Chesapeake Bay this summer unexpectedly turned into an underwater buffet for shellfish-loving predators, with about $45,000 worth of oysters quickly eaten, scientists said yesterday.

In June, the U.S. Army Corps of Engineers dumped about 1 million oysters into Virginia's Great Wicomico River. But within weeks, about 750,000 of the oysters were consumed by a creature called the cownose ray.

Corps of Engineers leaders said they had no idea that the rays would cause so much damage. But that was cause for chortling in the seafood business, where oyster growers say they have dealt with the predator for years.

"The rays show up every year at the same time, in the same place. They go through the shellfish beds, and they love oysters," said Rich Pelz, president of the Circle C Oyster Ranch in Ridge, St. Mary's County.

"It's not something they should have missed," Pelz said.

The demise of the oysters, first reported yesterday by the Virginian-Pilot in Norfolk, is not the first time that nature has crossed scientists' attempts to revive wildlife in the bay.

In 1985, for instance, a moratorium was imposed to protect the declining population of rockfish. That was good news for rockfish, but it turned out to be bad news for blue crabs, one of the fish's favorite meals.

The bay's native oysters are in worse shape than rockfish or crabs: A combination of disease, pollution and heavy harvesting has driven their numbers to record low levels. The situation is so bad that many people want to write off the native oyster and bring an Asian species to the bay.

The Corps of Engineers, however, is testing to see whether the bay could be reseeded with native oysters that have been genetically engineered to prevent disease.

One of the testing grounds is in the Great Wicomico River, which flows into the bay on Virginia's Northern Neck, said Doug Martin, the program manager for oyster work in Virginia.

Before releasing oysters there, Martin said, scientists prepared for the oyster's top predator -- the blue crab -- by bringing in oysters more than 40 millimeters in size, thought to be too hefty a meal for a crab.

But he said no one had prepared them for the cownose ray, a relative of the stingray that migrates into the bay during the summer.

The ray, which can grow to a wingspan of several feet, sucks clams, oysters and other shellfish off the bottom, smashes them in its mouth and spits out the shells -- something like a baseball player eating a sunflower seed.

"The cownose ray was never identified to be a major, major problem," Martin said.

That perception changed in June, when the Corps of Engineers began dumping a test batch of 1.3 million oysters into the Great Wicomico. A swarm of rays showed up even before the dumping was done, Martin said, and the release was stopped with 300,000 oysters still to go.

When the rays finished a three-week feeding spree, just one-quarter of the $78,000 batch of oysters was left.

But Martin said Corps of Engineers scientists learned lessons for future oyster releases and still considered the release a success, since some oysters survived.

"This is just a minor hiccup," he said.

The appetite of cownose rays for oysters has been known to watermen for decades, said Ronald Bevans, who owns the Bevans Oyster Co. in Kinsale, Va.

In one oyster ground near his business, Bevans said, rays decimated a population of about 7 million oysters in a few weeks. Now, he said, scraping the bed turns up not live oysters, but fragments of shells smashed by the rays.

"This problem goes back years and years," he said.

To solve it, some oyster farmers put their bivalves in mesh bags, and others protect them with cages or fencing.

The Corps of Engineers will probably follow their lead this fall, when it plans to dump 15 million more oysters into the Great Wicomico, Martin said.

"We have built fences to keep out soldiers and special forces" in other spots around the world, Martin said. "We feel pretty confident that we can build a fence that can keep out the cownose ray."

The cownose ray is partial to oysters, as the U.S. Army Corps of Engineers learned in an expensive lesson as part of its bay restoration effort.