Breeding

Disease

Resistance

in the

Hatchery

Fast-track research has led to eastern oysters bred for resistance to MSX and Dermo

A spawning oyster releases thousands of eggs which will be fertilized by sperm from male oysters.

If disease-devastated oyster populations were left to themselves for a long enough time, chances are that eventually enough would survive from one year to the next to serve as foundation stocks for natural, ongoing replenishment. This is because individual oysters, like all organisms, vary with regard to growth, reproduction, and susceptibility to disease. Even under the deadliest attack of Haplosporidium nelsoni (MSX) and Perkinsus marinus (Dermo), some eastern oysters survive.

Is this tolerance to the impacts of disease due to heredity or environmental influences? Or both? Scientists don't yet know.

How long would oysters need to be left? Ten years, twenty, a lifetime, two lifetimes? There is no basis for predictions.

Such a waiting game of unknown duration is impractical if not untenable, whether for aquaculturists, public harvesters, or resource managers working to bring oysters back for ecosystem restoration. That is why the Oyster Disease Research Program has supported fast-track research on developing strains of the eastern oyster that are hardier and more tolerant of disease. That work has taken advantage of hatchery technology to help oysters do what they might do otherwise, if left alone. Researchers have been speeding up that process -- they use traditional breeding practices to spawn adult survivors of disease. These survivors are then reared to maturity and spawned to produce the next generation.

"The results of our work are coming in," says Standish Allen, "and over the next several years they could begin to have an impact on the success of oyster restoration and commercial farming." Allen, formerly at Rutgers University and now director of the Aquaculture Genetics and Breeding Technology Center at the Virginia Institute of Marine Science, has been a catalyst in a region-wide effort that has brought researchers throughout the mid-Atlantic together to work with resource managers and growers to begin testing hatchery-developed oysters under varying conditions of disease pressure.

"We have gone from what was originally my own project," says Allen, "to a truly regional mandate. With support from the Oyster Disease Research Program," he adds, "we have evolved from a grant to a collective regional mission."

Researchers have bred eastern oysters for resistance to MSX and Dermo -- these CROSBreed oysters are now growing in Chesapeake and Delaware bays.

Selecting for Disease Resistance

Beginning in the years after MSX invaded Delaware Bay in 1956, nearly eliminating the oyster industry there, Rutgers scientist Hal Haskin began spawning survivors of MSX in the hatchery. The young oysters that survived to maturity were then spawned; those that reached maturity were then spawned again. For the past several decades, scientists at the Rutgers Shellfish Lab continued this breeding program, rearing pedigreed lines of oysters that would tolerate MSX, while working to avoid the potential problems of inbreeding.Those lines were used for both research and for commercial aquaculture.

Then in 1992, Perkinsus marinus invaded Delaware Bay; the specially-bred stocks for MSX had little resistance against this new parasite and were hard hit. Though many died, still there were some survivors - and it is those survivors and their progeny that have been serving as the foundation stocks for developing oysters resistant to both MSX and Dermo.

Allen and his collaborators are using these dual disease-resistant oysters in a regional project to evaluate the growth and survival of different oyster lines in the mid-Atlantic. Called CROSBreed for Cooperative Regional Oyster Selective Breeding, the program involves Allen, Susan Ford of Rutgers University, Kennedy Paynter of the University of Maryland Department of Zoology and Don Meritt of the University of Maryland Center for Environmental Science, Mark Luckenbach and Eugene Burreson of the Virginia Institute of Marine Science, and Pat Gaffney of the University of Delaware College of Marine Studies.

CROSBreed researchers have been testing the specially-bred Rutgers oysters at three sites -- one in Delaware Bay and two in the Chesapeake, in Maryland's Choptank River and Virginia's Mobjack Bay.

Selectively-bred oysters are demonstrating better performance than wild unselected oysters.

CROSBreed: Offering Hope

In August 1995, nearly 75,000 young oysters (called spat) were divided among three test sites, the Cape Shore Laboratory in Delaware Bay, the Choptank River in Maryland, and at Wachapreague in Virginia. These sites had 4,000 oysters from each of the five strains (20,000 total) of the Rutgers-bred oysters, as well as 4,000 wild oysters from Delaware Bay; in addition, each site deployed about 4,000 local spat. In total, some 100,000 oysters were planted for comparing the response of disease-resistant lines with native oysters to the challenges of disease.

How are these CROSBreed oysters doing?

"At regular intervals from fall 1995 through fall 1997, samples from all groups at all sites were examined for MSX and Dermo, as well as growth and survival," says Pat Gaffney. "In general, the CROSBreed lines have had better overall performance than the unselected controls from New Jersey wild stocks."

Analyzing the combined effects of both diseases over these last two years has been more difficult, largely because MSX was hardly a factor at any of the oyster sites - probably because of unusually low salinities in these areas. MSX was not observed in the Maryland and Virginia sites, while infections in New Jersey's Delaware Bay were at a low frequency in the fall of 1995 (about two percent) and below five percent until summer 1997, when they reached 14 percent.

Among the findings, says Gaffney, are these:

New Jersey, Delaware Bay. By the fall of 1996, Dermo was prevalent in 40-50 percent of the sampled CROSBreed oysters and there were significant differences among the five lines. For example, 71 percent of sampled oysters were infected in one line, while the other showed lower rates of infection, 15-35 percent. Though this pattern persisted through the summer of 1997, by August 98 percent of the sampled oysters were infected, though the intensity of parasitic cells varied.

Virginia, Chesapeake Bay. By fall of 1996, Dermo was present in 9 percent of sampled oysters, but by summer 1997, disease had spread to 79 percent. All of the CROSBreed lines fared better than the local Virginia controls, especially line 1, which was the worst performer at the New Jersey site. By fall 1997, 95 percent of sampled oysters were infected and, again, significant differences existed among lines.

Maryland, Chesapeake Bay. Scientists found little Dermo infection on this Choptank River bar -- less than 2 percent. The few light infections appeared to be randomly scattered among the five different lines.

Because of the low levels of MSX infection, says Gaffney, it is difficult to evaluate the MSX resistance of the selected lines. Dermo infection, on the other hand, was high. Does this heavy Dermo infection mean that the CROSBreed oysters are ineffective for resisting disease? We have to remember, Gaffney says, that the CROSBreed lines were derived from MSX-resistant lines that have been under selection for resistance to Dermo for only one to two generations at most. "That these the lines contracted Dermo," he says, "is not surprising. It may be that resistance to Dermo involves resisting death at a high infection level rather than resisting infection per se."

Gaffney speculates that if MSX had been present at the three sites, the overall performance of the CROSBreed lines would have been more remarkable. To get a better indication of how oysters do under high MSX pressure, the researchers are now moving the Maryland site from the lower salinity Choptank River site to the higher salinity waters off Deal Island. Because the Virginia and New Jersey sites were similar in salinities, the Virginia site is being moved to even higher salinity waters near the sea. "We have shown that the CROSBreed lines were better or equal to local controls," says Allen. "The stock seems genetically robust. Our testing over these next couple of years will tell."

With signs pointing to the success of CROSBreed oysters, Allen is now working with Sea Grant Extension agents on the east coast to get disease-resistant oysters into the hands of hatchery operators.

In an ODRP funded project, researchers will conduct workshops for commercial hatcheries on the principles of managing CROSBreed oysters. "We are offering these oysters to hatchery operators," says Allen, "with the provision that they attend the workshop. They'll learn about principles of genetics, especially as they relate to maintaining these and other strains of selectively bred oysters."

These workshops could catalyze a change in the relationship between commercial hatcheries and scientists like the consortium of ODRP researchers. Allen is enthusiastic. "The benefits for oyster aquaculture on the east coast," he says, "could be considerable."