Contents

A Question of Survival

Formidable Foes

Legisltating to Fight Disease

Review: Yesterday's Oysters Today

Free-swimming oysters

By Merrill Leffler

For more than a half century Maryland and Virginia have regulated the harvest of oysters in an attempt to slow a long decline. Now, with the continuing devastation of two oyster diseases - MSX and Dermo - some question remains whether even tight restrictions can save the Chesapeake's oyster populations.

Oysters under Siege

"Diseases," says Victor Kennedy of the University of Maryland Center for Environmental Science (UMCES), "are a natural component of biological systems." Those such as Dermo - and MSX, which some researchers believe was accidentally imported into the Chesapeake - might have been present for many years. That they are so virulent now may be the result of the Bay's changing ecology. These changes could be related to pollution, to runoff, and to the overharvesting of oysters themselves - and consequent changes in habitat and bottom-dwelling communities, most notably the destruction of large reefs.

Some scientists argue that in addition to the destruction of reefs and other habitat changes, the years of intense harvesting in the Chesapeake may have also removed substantial numbers of oysters, and their potential progeny, with a natural resistance to parasitic disease. If left undisturbed - though for how long is uncertain - oysters with a natural resistance could begin, through natural selection, to survive and repopulate themselves.

Whatever the reasons, these two diseases are now firmly entrenched, though their virulence depends on environmental factors, particularly temperature and salinity.

In the face of social and economic pressures resulting from the demise of the oyster fishery in the Chesapeake and with the recognition that oysters play a critical role in the ecology of estuaries, the U.S. Congress appropriated funds in 1989 for the Oyster Disease Research Program, which is supporting scientists and resource managers to develop new ways of combating disease (see the sidebar, "Legislating to Fight Disease").

Understanding the Enemy

According to Chris Dungan, of the Maryland Department of Natural Resources Cooperative Shellfish Laboratory, the devastation of MSX and Dermo will "wax and wane" year by year, in a game of climatic roulette. The amount of rainfall affects salinity and both diseases are worse at higher salinities. In 1996, for example, Maryland's oysters got a break, according to Dungan, when record precipitation led to lower salinities in many parts of the Chesapeake. In 1996, he reports, the state's annual survey showed MSX restricted primarily to Tangier Sound, the more saline part of Maryland's Bay.

Dermo also slacked off slightly, according to Dungan. "We found a few places well north of the Bay Bridge that had Dermo last year that didn't have it this year." Otherwise, says Dungan, "Dermo is still everywhere."

"Dermo was actually discovered in the Chesapeake Bay first," Dungan says, but he points out that once MSX arrived its staggering virulence tended to attract all the attention. Now even when MSX lets up during periods of lower salinity, Dermo tends to keep on killing.

According to Eugene Burreson at the Virginia Institute of Marine Science (VIMS), the fresh water flows of 1996 helped hold down disease in Virginia as well, at least in the rivers. "We saw no MSX in the James River or the Rappahannock," he said, "and only a little in the York." MSX could still be found in the mainstem Bay and in Tangier and Pocomoke Sounds, though mortalities were down. And while still present in their samples, the severity of Dermo disease was also lessened by the rains. "We haven't seen levels this low in eight years," says Burreson, but he points out that Dermo is still widely distributed and found "just about every place we look for it."

In fact, Burreson says, "if I had to pick one of the diseases as the worst, I guess I would have to pick Dermo." This may seem surprising, since MSX has decimated the oyster industry in Virginia, but, as Dungan also notes, even though Dermo does not kill as quickly or as completely as MSX, it is more persistent. "When salinities drop below 10 parts per thousand, MSX will disappear for a while," says Burreson - at least until salinities rise again. But Dermo, he says, will "hang on" even in low salinities, and as soon as conditions become more favorable the disease will spread.

A major front in the search to understand the diseases is in trying to discover the fundamentals of the parasites' behavior, says Chris Dungan, the conditions that favor growth, survival, and virulence. "We need to know our enemy," he says.

If I had to pick which disease was worse, I would pick Dermo.

Scientists in the mid-Atlantic were first able to culture Dermo several years ago (see Marine Notes, Volume 11, Number 4), an advance which has allowed scientists to undertake studies on the oyster's immune system that would otherwise not have been possible. In the last year, researchers at VIMS have developed new molecular tools to discover the life cycle of MSX. According to Dungan, Eugene Burreson not only has a new diagnostic tool and an "excellent team," he is also driven to solve the disease problem.

"He has a thing for MSX," says Dungan. "I am really excited about their work."

Such excitement is understandable. Though MSX has ravaged Bay oyster bars for four decades, neither scientists, resource managers nor watermen can say how the parasite moves around or how it infects the oyster. Burreson hopes to change that, using tools never before available. "We have already found positive samples" says Burreson, meaning that genetic material from MSX is present in the sediment and water column. "Whether these are free spores or developmental stages [of MSX] we don't yet know," he says. Burreson's team is just now proceeding with its analysis.

Developing Oysters that Can Resist Disease

In addition to the development of new techniques for culturing Dermo, and new probes for detecting both Dermo and MSX, the disease research program has focused on developing a hardier oyster. If successful, the program could speed up what natural selection might otherwise do.

These studies, says Rutgers University scientist Standish Allen, include:

• Breeding disease resistant oysters through traditional genetic techniques.
  
• Evaluating the Chesapeake's native species, the eastern oyster (Crassostrea virginica) in other regions where Dermo is prevalent (such as Gulf Coast waters) and where oysters may have already developed improved resistance to disease.
  
• Transferring genes for disease resistance from other Crassostrea species, such as the Pacific oyster (Crassostrea gigas) into the Bay's eastern oyster.
  
• Hybridizing the eastern oyster with strains of the same species that are more disease resistant, and also with the Pacific oyster.

"Perhaps closest to making a difference in the relatively near term," says Allen, "is a program where we have taken MSX-resistant oysters as a foundation stock to breed oysters for resistance to Dermo as well."

Hal Haskin began those foundation stocks shortly after MSX invaded Delaware Bay in 1956, virtually eliminating the oyster industry there. Scientists at what is now Rutgers' Haskin Shellfish Research Laboratory continued that breeding program and have reared oysters in the Rutgers hatchery that would tolerate MSX. Those stocks have been used for research, and for aquaculture. When Dermo invaded Delaware Bay in 1990, however, these specially bred stocks were vulnerable - those that have survived are now being used as the basis for creating a second generation, says Allen.

Allen is working with Kennedy Paynter of the UMCP Department of Zoology and Don Meritt of UMCES and Mark Luckenbach and Eugene Burreson at VIMS in a project they call Cooperative Regional Oyster Selective Breeding or "CROSBreed." The team is growing the specially bred Rutgers oysters at three sites - one in the Delaware Bay and two in the Chesapeake (in Maryland's Choptank River and Virginia's Mobjack Bay). In addition to these specially bred oysters, all three sites have been given control stocks from Delaware Bay, and from local oyster beds.

Researchers Standis Allen and Ximing Guo examine oysters at Rutgers' Haskin Shellfish Research Laboratory

After one year, says Allen, the oysters are doing well. Does that mean that they are more resistant to disease? It is too early to say - this past summer was unique, says Allen. Like Dungan and Burreson, Allen notes that in 1996, "We had a lot of rain." The lower salinities may have held disease in check on the experimental bars, just as on the natural bars.

We need another growing season, says Allen, to see where we go next.

If all the CROSBreed oysters outperform the controls, that could mean these are the ones to use as spawning stock for restoration and aquaculture. On the other hand, if CROSBreed oysters do better in one place and the local strain in another, researchers may need to breed different strains for different locations.

It could take countless growing seasons if the Chesapeake is to once again have sizable oyster populations - but large achievements are the result of many small ones and there is, at least, some optimism that the small ones have begun.

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