Bay Oysters Evolve Resistance to Diseases?

Oysters starting to show signs of resistance to Dermo, MSX

. . . on the heels of a major oyster die-off in the Chesapeake Bay attributed in large part to a mysterious new disease called MSX. Since then, the lab has been engaged in a long-running effort in both Maryland and Virginia to track and understand MSX and another disease, Dermo.

Both are single-celled parasites that target oysters, but uncharacteristically can kill their hosts — in a matter of weeks, if infections are high enough. Dermo has been in the Bay since the 1940s, and is believed to have been introduced by the importation of seed oysters from elsewhere. MSX first popped up in Delaware Bay in 1957 and the Lower Bay two years later — how it’s transmitted remains unknown.

Both have repeatedly ravaged Chesapeake oysters over the last five decades, and remain a major concern for the future of the estuary’s keystone shellfish, which is a source of income for watermen, a prolific water filterer and a builder of reef habitat for other fish and aquatic creatures.

Hundreds of oysters collected by Maryland Department of Natural Resources biologists during their annual reef survey are brought to the lab every fall. Once there, tissue samples are taken and processed, with cross-sections of the animal sliced thinner than a human hair, then stained purple, pink and blue-black and studied under a microscope.
. . .
Once a scourge of the Bay, especially its saltier waters, MSX has receded, and though still found infecting some oysters in some places — 11 percent on average in Maryland last year — it’s nothing like it used to be.

Dermo, on the other hand, remains widespread — nearly two-thirds of the oysters collected throughout Maryland’s part of the Bay in the fall of 2016 had it. But Dermo doesn’t appear to be as lethal as in the past, either.

“The trend that we’re really seeing is that the mortality consequences of those disease pressures seem to be fairly consistently less than they were in previous years,” said Chris Dungan, a DNR research scientist who oversees the Oxford lab’s oyster evaluations.

Although there’s no direct evidence, Dungan said he and others think that the decline in dead oysters found in surveys “reflects increasing resistance to those diseases… by the process of natural selection.”

Ryan Carnegie, Dungan’s counterpart at VIMS, said that although it’s largely circumstantial, he also sees evidence that oysters have developed resistance to becoming infected by MSX. And there are indications, he added, that they’ve developed an ability to tolerate Dermo without succumbing to it.

With oysters producing literally millions of eggs each spawning season, there is plenty of opportunity for natural selection to work on the oyster population. Being able to survive the presence of the diseases is a prerequisite to a successful restoration of oyster populations in the Bay. We're not going to be able the cure the diseases, but we might be able to encourage the evolution of disease resistance in the oysters.