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This is pretty rapid for evolution to be at work, but there is tremendous selective pressure, and lots of scope for selection to operate. Disease runs through the oyster populations like a plague, often killing the majority of them, leaving the disease survivors (some of them resistant) to breed. Oysters have short generations times. They start to breed in their first year (mostly as males), and rarely live longer than a few years before watermen, disease or predators take them. Thus, there are frequent extreme selection events.
For decades now, Chesapeake Bay oysters have been ravaged by two diseases that nearly wiped them out. Dermo and MSX and the parasites that cause them are still rampant in high-salinity waters of the bay and its tributaries, but research from the Virginia Institute of Marine Science (VIMS) suggests native oysters are developing a resistance to them.
"They're organisms living sort of in an evolutionary space, and they had to develop some resistance or disappear," said Ryan Carnegie, research associate professor at VIMS in Gloucester Point. "We still see some disease, and they still die. But it's just not as bad in the wild oysters that have been exposed as it used to be."
However, not all are convinced that natural oysters are evolving disease resistance on their own:
Oyster expert Jim Wesson with the Virginia Marine Resources Commission (VMRC), calls it a "double-edged sword" that oysters grow best in saltier waters that likewise have the highest disease mortality. Wesson is not persuaded by Carnegie's conclusions.I'll reiterate my plan for oyster restoration. Ignore them for five years. No fishing, no planting. If they're not making a comeback, they won't, at least under the current state of the bay.
"I'm not totally convinced that there's any significant resistance," Wesson said. "I think we're managing around it better."
...Wesson is in the midst of VMRC's annual fall oyster survey and said he's finding it hard to find big, healthy oysters in the high-salinity, high-disease areas he's targeting.
"It's not black and white, what's going on," Wesson said. "If resistance was really out there, we ought to be seeing a lot of 4- and 5-year-old oysters."