GLOUCESTER, VA. -- Scientists are inducing a genetic change in some Chesapeake Bay oysters in the hope that the new strain will resist parasites that have driven the shellfish industry to virtual ruin.

Roger Mann, a researcher at the College of William and Mary's Virginia Institute of Marine Science, said scientists still are years away from understanding the parasites Dermo and MSX.

The two parasites have cut the harvest in the bay to 1 percent of what it was three decades ago.

Mann and colleague Michael Castagna propose giving the oyster an extra set of chromosomes. What this would do is sterilize the shellfish. Energy used for spawning would instead be used for growth.

Since spawning and the major infectious months for the two parasites coincide during the summer, Mann and Castagna believe the sterilized oysters would stand a better chance against the parasites.

"We'd be providing a different animal for a different market," Mann said. "Instead of the flat, flaccid oyster common in the summer, these would be fat, plump animals."

Oysters, like humans, are diploid -- having two sets of chromosomes. When the egg is fertilized by the sperm and begins cell division, it actually has three sets of chromosomes from the mother and two from the father. The father's extra set and the mother's extra sets are shed during the division, Mann said.

"What we can do is treat the eggs with a vanishingly small amount of chemical that will keep it from throwing away one of the mother's extra sets of chromosomes. This apparently will make the organism sterile but robust," he said.

An organism with the three sets of chromosomes is called a triploid.

Because only a minuscule amount of the chemical is needed and the process involves the microscopic egg stage of the oyster, Mann said millions of animals could be altered in a container the size of a mixing bowl.

Mann and Castagna are growing two batches of oysters over the next several months.

One batch will be diploid, the other triploid. The oyster larvae will be moved to the bay where their progress over the next two years will be monitored.

The process is not new. Oyster growers in the Pacific Northwest regularly market triploid Japanese oysters during the summer months.

VIMS researchers will be looking to see if the triploid oysters grow at a better rate than the diploid and if they are resistant to disease.

Mann said the new oyster, if successful, would mean a change in the waterman's way of life.

"It will tend to be a cultured animal rather than a throw-it-out-there-and-forget-it one," he said. "It will not be self-perpetuating."

Bill Goldsborough, a fisheries specialist for the Chesapeake Bay Foundation in Annapolis, said all possibilities ought to be looked at, though he doubts the triploid oyster would replace the natural strain. "I don't see it as a risk to the bay," he said.

Many of the watermen and the foundation opposed an earlier proposal by Mann to introduce triploid Japanese oysters into the bay because of fears that not all the shellfish would be sterile. Environmentalists were concerned that fertile Japanese oysters would reproduce and drive out the native oysters.

Mann wanted to see if the Japanese oyster is resistant to the two diseases.