Parasite kills Asian oysters in N.C. test

Infestation raises doubts about use in Chesapeake

December 17, 2003|By Howard Libit | Howard Libit,SUN STAFF

The Asian oyster might not be a miracle cure for the Chesapeake Bay after all.

Researchers at the Virginia Institute of Marine Science have discovered a potentially fatal parasite - never before seen in the Mid-Atlantic region - in a batch of experimental Asian oysters raised in waters off North Carolina.

The parasite was previously found only in oysters off New Zealand, France and Maine. Its appearance here raises serious concerns about whether the Asian oyster, Crassostrea ariakensis, can fulfill the aspirations of scientists, watermen and politicians as a substitute for the dying native Chesapeake Bay species.

The one-celled organism, a new species of the genus known as Bonamia, attacks the oysters' blood and is usually fatal.

"It's certainly something we weren't expecting, beyond my wildest dreams," said Eugene Burreson, the VIMS researcher who made the discovery. "The question is, if the parasite spreads, is it going to limit the use of ariakensis in the Chesapeake Bay?"

Industry officials and environmental advocates agree that the parasite is of great concern. But neither group wants the states to back away from their ambitious studies of Asian oysters in the bay.

"It's distressing news, except that we are assured and reassured that it has never been found anywhere in the Mid-Atlantic and never in the Chesapeake Bay," said Frances W. Porter, executive director of the Virginia Seafood Council, which is helping to pay for a 1 million- oyster experiment.

"We just have to hope that when they do the research, there's no sign that it will spread beyond the one site where it's been found so far."

Scientists do not believe the new parasite threatens the native species of oyster.

Robert Brumbaugh, a scientist at the Chesapeake Bay Foundation, said the VIMS discovery underscores why in-depth study is needed before Maryland and Virginia commit to distributing Asian oysters widely in the bay.

"This is the kind of thing that illustrates why there is a need for a concerted research effort," Brumbaugh said.

Scientists in Virginia and Maryland began testing the Asian oyster in the waters of the bay after the collapse of the native Crassostrea virginica.

Maryland officials expect a record-low oyster catch this season, less than half of the 53,000 bushels caught last year and a tiny fraction of the 2.5 million bushels harvested as recently as the 1980s.

The Asian oyster has been seen as a particularly attractive alternative because it is resistant to the two diseases that have destroyed the native oyster species - dermo and MSX.

Eventually, Maryland officials and watermen hope to establish reproducing populations of the Asian oyster in the bay, providing a new source of harvest and the environmental benefits of creatures that filter harmful pollutants from the water.

So far, scientists at VIMS are testing Asian oysters that have been genetically altered to prevent reproduction, raising them only within secure cages to ensure that none are let loose.

About 1 million of the sterile variety are growing in Virginia's Chesapeake Bay waters and almost 500,000 off North Carolina. A Maryland researcher intends to raise 3,900 Asian oysters at three sites in the northern part of the bay once federal permits are secured, probably next month.

So far, none of the oysters in the Virginia experiment - nor any of those intended for Maryland - has been found with Bonamia, Burreson said.

And while the Asian oysters have been placed in several locations along North Carolina, the parasite has been detected in only one area - Bogue Sound, near Morehead City.

Two batches of oysters at that site suffered large numbers of deaths, first in August and again in the fall. Scientists initially attributed the mortality to high salinity in the hatchery, combined with unexpected infestations of marine worms.

But in-depth tests of tissue samples - including DNA molecular analysis - showed that as many as 60 percent of the oysters are infected with Bonamia.

"We were just stunned," Burreson said. "There's no question that's what it is. It seems to be a new species that's never been reported, and right now, it's restricted just to Bogue Sound."

The ariakensis oyster is known to be highly susceptible to the Bonamia parasite, Burreson said. But until now, scientists believed that it thrived only in areas of cold water and high salinity - such as the waters off Maine, southern New Zealand and the English Channel adjacent to France.

The European flat oyster, raised in all of those areas, has been a frequent target of the parasite.

"We have no idea where it's coming from, but it's clearly something that is being picked up in Bogue Sound," Burreson said. "Perhaps there is some other mollusk there has it, and it's spreading to ariakensis. We're not sure."

The possibility of a Bonamia infection was raised earlier this year in a National Research Council report on Asian oysters in the Chesapeake Bay. But the authors speculated that it might spread here from Maine, not appear on its own.

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