Japanese oysters appear to be resistant to the two oyster diseases that have devastated native oyster populations in the Chesapeake Bay — a finding that may boost efforts to find a disease-resistant gene that could aid the native species.

The finding came from a controversial experiment that began June 29 when trays containing 200 Japanese oysters and 400 Bay oysters were placed in the York River.

Since then, researchers from the Virginia Institute of Marine Sciences found that 95 percent of the native eastern oyster, Crassostrea virginica, died from the diseases MSX and Dermo. None of the Japanese oysters, Crassostrea gigas, died as the result of disease.

Eugene Burreson, an oyster disease specialist at VIMS, said the field study confirmed earlier laboratory work indicating that although the Japanese oyster could be infected by Dermo, the number of parasites remained low and did not prove fatal as in native oysters. Dermo infected about 25 percent of the Japanese oysters, he said.

“The more important news is that the gigas did not get MSX at all,” Burreson said. “And we had a very heavy challenge — there was lots of MSX in the water.”

Burreson said the finding was somewhat of a surprise to researchers. “We thought there would be a low level of infection, but there wasn’t,” he said. “We didn’t see any MSX in the gigas at all.”

The latter finding was particularly significant because, unlike Dermo, scientists have been unable to conduct studies with MSX in the laboratory.

About 20 percent of the Japanese oysters died during the summer and fall, but not as the result of disease, Burreson said. The reason for their death is uncertain, but Burreson speculated it may have stemmed from the use of a needle to withdraw a blood sample from each Japanese oyster before being used in the experiment. “They had a wound, essentially, and it may be partially related to that,” Burreson said.

Whatever the cause, Burreson said the 20 percent mortality rate was not alarmingly high.

Although the Japanese oyster proved disease resistant, it did not grow well in the York River during the summer, Burreson said, though growth appeared to increase in the fall as temperatures cooled.

“It may be just as simple as it’s very hot here in the summer and they’re not used to those sorts of conditions,” Burreson said. “Salinity can get quite low here as well, and they’re mainly a cool-water, high-salinity organism. We just don’t know what they’re going to do in these conditions.”

The study is scheduled to end after a year, and scientists hope to learn more about the Japanese oyster’s growth as the experiment continues through the winter and spring.

“I think we’ve probably learned what we wanted to learn about their disease susceptibility,” Burreson added, “but we haven’t learned all that we want to learn about their growth and survival.”

The York River study was prompted by scientists who were curious whether the Japanese oyster, which has proved to be more disease resistant elsewhere, would fend off the parasites that have caused Bay oyster harvests to fall more than 90 percent in the past two decades.

The proposal was strongly opposed by the state of Maryland, environmental groups, and others who feared the experiment would lead to an introduction of the foreign oyster. Such an introduction, they argued, could eventually eliminate the native species from the Bay and pose unforeseen ecological consequences to the Chesapeake.

Most of the opposition subsided after researchers promised that all oysters used in the experiment would be certified as being triploid. Triploid oysters have been chemically treated to produce three sets of genes instead of two. Triploid oysters typically cannot reproduce and, if they do, the young cannot survive beyond larval stages. Each oyster used in the experiment had a blood sample taken to make sure the treatment worked.

Proponents of the experiment have said that if the Japanese oyster could survive in the Bay, it could lead to efforts to identify a gene or combination of genes in the foreign species that might be used to make the native species resistant as well.

“That’s a long ways off, of course,” Burreson said. “It involves a lot of genetic research.”

VIMS, the state of Maryland, and environmental groups are strongly opposed to any outright introduction of reproducing Japanese oyster stocks in the Bay.

About Oyster Diseases

Since the early 1980s, diseases have overwhelmed oyster populations in the Chesapeake Bay, causing losses of up to 90 percent of stocks in some areas. These heavy mortalities have been linked to the spread and intensification of two parasites: Dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni). Though deadly to the oysters, neither affects humans, whether the oysters are eaten raw or cooked.

Dermo is a disease caused by a single-celled organism (protozoan) that infects oysters, eventually reaching such high numbers within the oyster host that the oyster can no longer maintain its physiological functions, and dies. The exact mechanisms by which the parasite kills the host are not understood. Dermo tends to have its largest impact on oysters about the time they reach market size.

MSX disease is also caused by a protozoan. It is more virulent than Dermo, although as far as is known, it also kills by reproducing to vast numbers within oyster tissues and overcoming the oyster’s ability to maintain its functions. MSX does more damage to young, or small, oysters than Dermo.

Source: Maryland Department of Natural Resources