Experiments with the Japanese oyster in the York River last year raised serious questions about the commercial value of that species in the Bay, but it bolstered hopes that a disease-resistant strain of oyster may be developed for the Chesapeake.
The test oysters grew slowly during the experiment and were heavily infested by worms native to the Bay, the researchers found, but the oysters proved to be resistant to the diseases that have devastated the native population, sending harvests to histori clows.
"Before, we just thought they might be resistant," said Stan Allen, of the Haskin Shellfish Research Laboratory at Rutgers University in New Jersey. "Now we're pretty sure of it."
Though laboratory work suggested the foreign species was resistant to disease, scientists supporting the research contended that those experiments did not adequately simulate real-world conditions. And although one of the two major diseases affecting Bay oysters - dermo - could be tested in a laboratory, the other - MSX - could not.
Last June, trays containing 200 Japanese oysters, Crassostrea gigas, and 400 native eastern oysters, Crassostrea virginica, were suspended about two feet off the bottom near the mouth of the York River.
Mortality among the native oysters, which became infected by both dermo and MSX, was about 99 percent, said Eugene Burreson, an oyster disease specialist at the Virginia Institute of Marine Science, which conducted the York River experiment.
By contrast, mortality was only about 25 percent among the gigas. "We can't attribute any of that mortality to disease," Burreson said. "They never did get MSX. They got a little bit of dermo, but they stayed very light infections."
Allen said that gigas also proved disease resistant in a experiment conducted in Delaware Bay last summer.
The exact cause of mortality among the York River gigas was not determined, but Burreson said the deaths occurred only during warm months. "As soon as the water cooled off in October they stopped dying," he said. "They only died in the summer."
While resistant to disease, the oyster could not fend off a native worm, polydora, also known as the whip mud worm. The worm does not affect the oyster meat, Burreson said, but forms a burrow on the inner shell surface that irritates the oyster, causing mud blisters to form.
The worm also affects the native oyster, but not to the degree of the foreign species. "Gigas, for reasons that we don't understand, just really got heavily infested by them," Burreson said. "On every one that we sampled, the whole inner surface of both shells was essentially covered with these mud blisters."
The worms may have contributed to another disappointing result: the lack of growth in the gigas. "They grew a little, but we really didn't see a growth spurt like we were expecting to see when the weather cooled off, and it's probably because they were s o heavily infested with this worm," Burreson said.
The poor growth, combined with the worm infestations, may make gigas an unattractive species for aquaculture, Burreson said.
With harvests of native oysters sharply down, some had hoped that the Japanese oyster - if it proved disease resistant - could be grown commercially in the Bay in controlled aquacultural facilities.
Nonetheless, Allen said disease resistance by the oysters offered hope that some genetic material from gigas may be transferred into the native oyster to bolster its ability to fend off disease.
"It has demonstrated that this source of raw material has something worth crowing about," Allen said.
Efforts to directly cross the eastern oyster and the Japanese oyster have not been successful. But scientists are trying some "end runs" to create a hybrid, Allen said.
For example, he said, biologists may attempt to cross the native species with another oyster and - if that proves successful - attempt to cross that hybrid with the Japanese oyster, a technique known as a "bridging cross." The idea, he said, is to get th e eastern oyster, during the first cross, the lose whatever mechanism is blocking the hybridization. Then, through the second cross with the Japanese oyster, incorporate the genes that help it resist disease.
If producing a hybrid does not work, it is still possible that the specific gene in the Japanese oyster that makes it disease resistant could be identified and implanted in the native species through genetic engineering.
But that could be years off, said Steve Jordan, a biologist with the Maryland Department of Natural Resources and director of its Cooperative Oxford Laboratory, which conducts oyster disease research. He said little work has been done to map the osyter's genetic code. "That's pretty expensive work," Jordan said.
"But," he added, "I think there will be some work in that area. The tools are becoming more efficient, cheaper, and more available for that kind of work. I'd like to encourage someone to get busy working on the molecular genetics of these beasts."