[Maryland Marine Notes masthead]
Volume 13, Number 3 • Summer 1995
Table of Contents
Download pdf



New Oyster Wars

Can America Save Its Fisheries

On Another Front: Juvenile Oyster Disease

Bay Commission Asks: Are Blue Crab Stocks Stressed

Also visit:



The New Oyster Wars:
Battling Disease in the Lab and Bay

By Merrill Leffler

In the final decades of the last century, oyster wars in the Chesapeake pitted watermen against the oyster police and each other as they battled over the riches of the Bay's "winter gold."

No more.

With those riches gone, oyster wars in the final decade of this century are being fought below water, not by watermen but by poorly defended oysters and marauding protozoans. Known as Dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni), these microscopic parasites have been battering oyster populations throughout the Chesapeake. One measure of this onslaught can be seen in commercial harvests - over these last five years, harvests have fallen so low their landed value in Virginia, says Roger Mann of the Virginia Institute of Marine Science (VIMS), "is less than the sale of one median house in Hampton Roads."

So entrenched is Dermo on bottom grounds in the Bay that even in summers with good sets of new oyster larvae, the chances of oysters surviving to harvest size by the second or third year are at best slim, at worst, nonexistent. According to Eugene Burreson, a scientist at VIMS, Dermo commands all of Virginia's oyster bars except the upper James River. With all the emphasis on Dermo, people have tended to forget MSX, says Burreson - that's a mistake. "This year," he says, "we have had the highest MSX infection since 1959."

[Oyster injection]

Probing the immune system of the oyster, researchers witness a raging molecular battle between relentless parasites and the mollusc's faltering defense mechanisms.

Why are oysters so defenseless? Why aren't they able to mount an effective counterattack against Dermo and MSX, as they have against other pathogens? Or conversely, why are these protozoans so successful in eluding defenses the oyster immune system throws at them? And can anything be done to reverse the devastation that these diseases have been wreaking?

Until five years ago, there were few answers that evoked optimism, and no long-range plan for help. That is no longer the case. In the last several years, says Roger Mann, "we have made quantum leaps in some areas of understanding." One reason for these rapid advances has been a Congressionally funded program of research on oyster disease that has made consistent support possible on numbers of fronts, from molecular studies on the interaction between protozoans and the oyster immune system, to development of sophisticated techniques for monitoring the presence of Dermo and attempts at breeding strains of oysters that may eventually be able to resist the attacks of Dermo and MSX.

[oyster harvest]

Once the Chesapeake's most lucrative fishery, the oyster has fallen on hard times. Maryland's 1994-95 harvest of some 162,000 bushels represents a fraction - about 14% - of the harvests of only a decade ago.

The Cellular Front

Scientists have long known that hemocytes, cells in the oyster's circu-lating fluid, play a major role in fending off invaders. Analogous to the human body's white blood cells, though far less sophisticated, hemo-cytes are the oyster's first line of defense: in general, when a microbe invades, the hemocyte binds, then surrounds the attacker, and engulfs it in a process called phagocytosis. The cells release bursts of toxic com-pounds, specifically reactive oxygen intermediates (ROIs) such as hydrogen peroxide, says Robert Anderson of the University of Maryland Center for Environmental Science (UMCES). When Anderson exposes these hemocytes to Dermo, however, the hemocytes engulf the parasite but he doesn't see the ROIs. The question, of course, is why not? Dermo may survive for a number of reasons, says Anderson. The parasite may prevent oyster hemocytes from triggering the ROIs, it may for some reason be able to withstand them, or it may produce substances that are toxic to hemocyte cells.

New molecular tools have been making it possible for Anderson and other scientists to better examine the chemical weaponry that both oysters and protozoans deploy. For example, using molecular probes and chemiluminescent analysis to detect and quantify ROI production, Anderson no longer depends on counting cells through a microscope. "You could go blind," he says. "These new methodologies give you a chance to count three million cells, not 300."

Many of these studies depend on large amounts of Dermo. Thanks to recent breakthroughs, scientists now have that advantage. The ability to grow Dermo in continuous culture in the lab resulted from a near-simultaneous discovery two years ago by Mohamed Faisal and Jerome F. La Peyre at VIMS, Sharon Shrunk and Stephen Kleinschuster at Rutgers University, and Gerardo R. Vasta and Julie D. Gauthier at the University System of Maryland's Center of Marine Biotechnology (COMB).

Before having that capability, it was difficult to obtain Dermo in pure form. Moreover, says Anderson, you could not get enough of it. "Now you can make it by the bucketful - it's duck soup." Growing Dermo in petri dishes makes it possible to study its life cycle and how different environmental conditions - for instance, salinity, temperature, heavy metals, chemicals - affect its growth and behavior.

"Culturing the Dermo cell," says Chris Dungen, a research scientist at the Oxford Cooperative Lab, "was a breakthrough whose major benefits we have yet to realize." Two paths of investigation, one in Gerardo Vasta's lab at COMB and another in Mohamed Faisal's lab at VIMS, have been revealing molecular armaments by Dermo that are especially provocative, though the work is in early stages of investigation.

Mohamed Faisal and Jerome La Peyre are tracking enzymes that Dermo releases when it attacks an oyster cell. Called proteases, these enzymes break down oyster tissue and likely contribute to the oyster's demise. The researchers found that Dermo can grow and divide in hemocytes of infected oysters, suggesting, Faisal says, that "factors other than hemocytes may be important in resistance."

While they have observed the presence of Dermo in the Pacific oyster (Crassostrea gigas), that oyster appears more resistant than the Eastern oyster (Crassostrea virginica). Faisal is focusing on the possible presence of special inhibitors in the Pacific oyster. If he can identify these, he could potentially develop "protease blockers" that would act something like antibiotics in fighting the parasite.

Still another molecular battle may be taking place over iron.

Iron is critical for growth, both to parasites like Dermo and host organisms like oysters. Because iron is generally much less available than other metabolic needs, competition for iron between the parasite and the host cell is intense, says Gerardo Vasta. Vertebrates have developed strategies against malaria, Vasta points out, by producing iron-binding proteins to reduce the levels of iron available to malarial parasites - "this slows the parasite's growth rate and reduces the pathogenicity of the infection." Recent studies in his lab, he says, "indicate that Dermo has a strong requirement for soluble iron and its growth rates are correlated with iron availability."

Environmental factors in the Chesapeake may increase the availability of iron. For example, low concentrations of oxygen - or its complete absence (anoxia) - occur in the Chesapeake Bay during summer months and trigger chemical reactions in the sediments that release iron into the water. "This may help explain," says Vasta, "why Dermo is more prevalent during the summer months in oysters that are located in low dissolved oxygen estuaries like the Bay."

Vasta speculates that "excessive iron accumulation in the oyster in summer promotes proliferation of Dermo, which may inhibit the oyster from producing the oxygen compounds it needs to defend itself. By better understanding environmental factors such as iron, it may be possible, he says, "to design strategies for blocking their proliferation."

Both of these investigations, while they hold promise of practical applications, suggest the complexity of uncovering interrelationships between parasitic disease and the oyster immune system. There are other complicating factors. For example, Robert Anderson and Eugene Burreson have shown in lab studies how a pollutant such as tributyltin, a bottom paint for protecting boat hulls, upped the susceptibility of oysters to Dermo. Fu-Lin Chu, also at VIMS, has done comparable studies with poly-aromatic hydrocarbons (PAHs), which, once released from the combustion of fossil fuels, gather in Bay sediments.

The interrelated effects of multiple pollutants and the environment remain a complex and tangled web.

While resource managers await the results of such work in the Chesapeake Bay, the Oyster Disease Research Program has already laid the groundwork for addressing problems in other parts of the country, according to Jim McVey, National Sea Grant Program Director for Aquaculture. "The techniques we have developed for studying Dermo and MSX are being employed to study as yet unidentified microbial diseases that have had severe impacts on oysters in other regions, juvenile oyster disease in the northeast, and summer mortality in Pacific oysters in the northwest."

[Map of Dermo and MSX]

Often inadvertently spread from place to place, oyster diseases have plagued stocks of popular molluscs through this country and abroad, and have largely ravaged the fames Chesapeake oyster grounds. Maps reprinted courtesy of Susan Ford, Rutgers University.

Breeding for Disease Resistance

"The most important thing we're doing that could make a difference in the relatively near term is a cross breeding program," says Stan Allen of Rutgers University's Haskins Shellfish Laboratory on Delaware Bay. For some years, Hal Haskins and Sue Ford, also at Rutgers, have employed traditional genetic breeding techniques to rear strains of oysters in Delaware Bay that are able to resist the devastating impact of MSX. It is these MSX-resistant stocks, says Allen, which have also gone through one-and-a-half years of Dermo exposure, that will be used in a region-wide planting effort to select for broodstock oysters resistant to MSX and Dermo.

Working with Ken Paynter of the University of Maryland College Park and Don Meritt, Maryland Sea Grant Shellfish Specialist and UMCES scientist, and with Eugene Burreson and Mark Luckenback of VIMS, these researchers are deploying the specially bred oysters in floating trays in the Choptank River on the Eastern Shore of Maryland and in Mobjack Bay, Virginia, and comparing their growth and resistance to disease with local oysters. Surviving oysters will then be sent to the shellfish lab for breeding. "We'll at least get a first read-out on survival," says Allen. "If successful, we'll go from there."

While Dermo did not appear in strength in the upper Bay until the late 1980s, it has been an inhabitant of the Gulf of Mexico and other southern waters since about 1950, and was spotted in the Chesapeake as early as 1954. Because southern strains of oysters have been subjected to Dermo constantly for so long, these oysters may have developed natural immunities that Bay stocks, which are terribly susceptible, do not have. "The basic assumption," says Paynter, "is that geographically separated oyster populations behave differently with regard to Perkinsus. Some are simply less susceptible."

To test that assumption, Paynter is working with Don Meritt and Pat Gaffney from the University of Delaware to try to identify those populations that are less susceptible to Dermo and disease progression. Using southern oysters from Texas, Louisiana and Florida and strains from the Carolinas and Delaware Bay, they are placing oysters in floating trays at different sites in the Bay, to begin with, in the Wye River, Choptank River and Mobjack Bay. "Our hope," says Paynter, "is to identify popula-tions that are less susceptible to Dermo and disease progression." Pat Gaffney is doing DNA analysis. "If we do find differences," says Paynter, "then we may have a genetic marker to identify resistance."

As one scientist said, it has taken more than a century to deplete the Chesapeake Bay - it will take consid-erably more than a few years to try to replenish it.

What's Ahead

Until the Oyster Disease Research Program, research support on oyster disease had been spotty, largely because funding support had been spotty. It waxed and waned like MSX, which first showed up in 1957 ravaging oyster beds throughout Virginia, moved up into Maryland, then retreated. Only over the last decade have Dermo and MSX dug in for what seems like the long haul. "These diseases are not going away," says Eugene Burreson.

It is this near-elimination of the oyster fishery that moved Congress to fund the Oyster Disease Research Program. Most scientists and managers agree that in five years it has been a critical factor in spurring rapid progress, not only in expanding our understanding of oyster-parasite interactions and in developing molecular tools to better study such interactions, but also in less heralded advances. "For example," says Steve Jordan, director of the Oxford Cooperative Research Laboratory, a State of Maryland and National Marine Fisheries Service lab, "we now know that Dermo abundance is greatest in the Chesapeake in June, with lower peaks throughout the summer. This is totally new information." Such knowledge could be important for raising and planting seed oysters. "We may be able to develop strategies for better determining where and when we move seed oysters for planting," he says.

While Stan Allen tempers his enthusiasm because practical applications of research may seem slow in coming, the coordinated oyster disease effort, he believes, has been important in many practical and subtle ways. "Everyone is using the Dermo culture now as the way to deal with handling practices," he says. "There is a unity that has been brought about by the funding source - it represents a core of people who have to function together because the funding and the region are small. It will make a difference," Allen says. "We hope it will make a practical difference."

Top of Page

[Maryland Marine Notes]

Contents of this issue
Other Issues

[Maryland Marine Notes]
[Maryland Sea Grant][NOAA]