The oysters in the cove behind Robert Jensen's home looked like dinner plates in the water. There was a thicket of these monsters, maybe 50 in total, some with shells eight inches across.

"Oh, God, they felt heavy," Jensen said. "They felt like over two pounds."

These oysters, with a meat bigger than a McDonald's patty, were the biggest anybody had seen here in Virginia's Northern Neck in years.

There was a reason for that: They weren't Virginia oysters, but rather a new Asian species that many hope will be able to replace the Chesapeake Bay's devastated native population.

These Asian oysters -- apparently sterile and apparently left behind by scientists after an experiment in 2001 -- became the first of their kind to grow wild in the Chesapeake region.

Their discovery last month has sent waves of excitement and alarm across the bay. Some scientists worry that such massive shellfish might become destructive invaders, the snakeheads of the bay bottom.

To others, including Maryland and Virginia state officials, these shellfish are a hopeful sign that the oyster -- once the bay's signature species -- could be on its way back.

"There was great concern and consternation" that the oysters had been left behind accidentally, said Peter Kube, an environmental scientist with the U.S. Army Corps of Engineers. "And on the reverse side, there were people that were just amazed."

The oysters were discovered in late April by girls canoeing in an inlet of the Rappahannock River. Jensen, an amateur oyster expert and a retired Navy lieutenant whom everybody calls "Captain Bob," made the call to Virginia authorities: "They're loose!"

Reporter Larry S. Chowning wrote a story about the oysters for the Rappahannock Record newspaper, which ran with a photo of one of them dwarfing Jensen's palm.

Soon, everybody who knows oysters was talking about them.

"It's a big deal," said Mike Fritz, a scientist with the U.S. Environmental Protection Agency's Chesapeake Bay Program. He said the oysters are "a demonstration of phenomenal growth capacity."

Native Chesapeake oysters grew this big at one time, back when oysters were so plentiful that their reefs blocked the passage of ships.

Until late in the last century, oysters seemed as plentiful as water in the bay: Watermen routinely harvested 2 million to 3 million bushels a year in Maryland, and the Eastern Shore town of Crisfield had so many oysters that its downtown was built on a massive foundation of shell.

But in the past 50 years, scientists say, the oyster has been hurt by overharvesting and then ruined by a pair of diseases that kill most of them before they reach 3 or 4 years old.

Last year's oyster harvest was a paltry 23,165 bushels -- less than 1 percent of the haul 30 years ago. The oysters found in Washington area supermarkets now generally come from North Carolina and the Gulf Coast, experts say.

Some scientists are still trying to breed disease-resistant versions of the native oyster. But many in the bay area have given up hope.

"The native oyster isn't going to come back," said Larry Simns, president of the Maryland Watermen's Association.

So the hopes of watermen and state officials have turned increasingly to the Asian oyster, or Crassostrea ariakensis. Many believe that the native of China is immune to local oyster diseases.

Scientists say that this oyster's meat, which can grow as large as a portobello mushroom cap, tastes very similar to the native oyster but has a slight metallic aftertaste. One potential drawback: Some of the Asian oysters' meat is orange.

In addition to helping watermen, the Asian oyster could also help clean the bay, by filtering water as it feeds.

"It will literally be a rebirth of the bay, in ecological terms," said W. Pete Jensen, deputy secretary of the Maryland Department of Natural Resources.

But some scientists wonder what kind of a birth it might be.

They say there are numerous examples of outside animals being introduced into the bay ecosystem with disastrous results. Besides the most recent example -- the voracious, air-breathing northern snakehead recently found in the Potomac River -- there have been mute swans and the nutria, a rodent that turned out to eat huge amounts of the grasses crucial to the bay's ecosystem.

To these scientists, the Asian oyster is scary, even though it can't bite anybody or even move an inch on its own. It could introduce a new disease, they say, or eat so much that native species starve.

"It's a different animal, it's a different species," said Bill Goldsborough, a scientist with the Chesapeake Bay Foundation, a nonprofit advocacy group. "We have no idea [if] that animal would fit into the Chesapeake Bay ecosystem."

The matter has been put on hold while the Army Corps of Engineers studies the potential effect of the Asian oyster. The study is to be released next year; in the meantime, the oysters are supposed to be tested only when they are sterile and kept in mesh bags. None is supposed to be growing wild.

To scientists who are cautious about the Asian oyster, the discovery behind Robert Jensen's house was particularly troubling. What if the scientific studies had been short-circuited and the oysters were already here?

"Once something like that would be established in the bay, it would be irreversible," said Julie Thompson, an oyster expert with the U.S. Fish and Wildlife Service. She is one of several scientists from federal agencies who have urged caution about the Asian oysters; Maryland and Virginia officials have been more gung-ho.

At the Virginia Marine Resources Commission, the state agency that put the oysters in Jensen's inlet in 2001, officials have said they don't know how these oysters were left behind. Their guess is that somebody accidentally stepped on them, forcing them down into the mud and out of sight. The oysters then worked their way back to the surface.

These officials say that a few of the oysters were tested and confirmed to be sterile.

But Robert Jensen said it is not that simple. There were 40 or 45 more oysters out there that scientists didn't study -- in fact, the oysters just vanished. He's heard that boys from the neighborhood picked them off the bottom and sold them on the street.

Now, looking at the muddy, shallow water, he said he believes there might be even more of the oysters out there.

"In my view, the Asian oyster is a reality, and it's here," Jensen said. "The question is, how do we deal with it?"

Here's how he plans to deal with it: He has filed for a permit to farm the Asian oysters in the inlet. The money from selling them, he said, would go to his nonprofit organization, the Rappahannock Preservation Society.

Its goal: to restore the Chesapeake Bay's vanishing native oysters, which have already been written off by so many.

"Maybe this would be a great way to fund my organization without having to beg anybody for money," Jensen said.

The Asian oysters found in the Rappahannock River taste similar to the native oyster but have a slight metallic aftertaste. Some of the meat is orange.