Oysters - Evolution in Action

Discovery of Disease-Resistant Oysters Brings Call for Shift in Preservation Strategies 

Development of disease resistance among Chesapeake Bay oysters calls for a shift in oyster-restoration strategies within the Bay and its tributaries. That’s according to a new study by researchers at the Virginia Institute of Marine Science (VIMS).

The study, by professors Ryan Carnegie and Eugene Burreson, is the feature article in the most recent issue of Marine Ecology Progress Series. It is based on 50 years of research into the prevalence of MSX disease among the native eastern oyster Crassostrea virginica.

Carnegie, a research assistant professor in the shellfish pathology lab at VIMS, said, “Our results point to substantial reproduction by disease-resistant oysters in the high-salinity areas where the parasite causing MSX thrives. We thus argue that reefs in areas of higher salinity should be the focus of conservation and restoration efforts, not just those in disease-free lower salinity areas.”

To date, restoration strategies have rested on the idea of protecting these “low-salinity refugia” as sources of larvae for replenishment of disease-ravaged populations in saltier areas of the bay. These strategies are based on the high levels of mortality traditionally seen among oyster populations in saltier waters (initially more than 90 percent), and computer models showing that tidal currents can indeed carry oyster larvae downriver from fresher to saltier areas.

The MSX parasite is believed to have been introduced to Eastern Oyster populations early in the 20th centuryfrom attempted transplants of oysters from the west coast.  Thus, the Eastern Oyster was not well equipped by evolution to resist the parasite.

Scientists have been predicting that in the long run, oysters would evolve resistance to the MSX parasite. Really, the only question was how long it would take.  It appears that the answer was on the order of a centruy (while resistant, I doubt the oyster have achieved as much resistance as they will eventually acquire, assuming they survive).

Using these results as evidence, scientists can now reasonably call for more oyster sanctuaries in higher salinity water, which will encourage the evolution of more resistance in the oysters.

Carnegie and Burreson’s research, however, paints a different picture. They’ve found clear evidence that oysters in the bay’s saltier waters are developing resistance to both MSX and Dermo, despite the increasing prevalence in the bay of the parasites responsible for the two diseases. This is possible only through reproduction by resistant oysters in high-disease areas—thus their call for a focusing of restoration efforts onto these disease-resistant areas and populations.

Carnegie says "We basically need to confront the diseases head-on where they are most active, rather than avoiding them by working in low salinities. It’s in the high-disease areas that resistance is developing most rapidly, so restoration efforts should be focused there."

And now that oysters are resisting the disease better, watermen will have less rationale for taking every  oyster in the bay before it dies of disease.