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Keeping Skinny Oysters off the Menu

Louisiana is an oyster epicenter. More eastern oysters (Crassostrea virginica) are harvested here than any other state. Their economic value in the last decade has been over half a billion dollars. And oysters provide more than commercial benefit by providing ecosystem services like water filtration, shoreline stabilization and enhancing other fisheries.

Oysters are particular about where they grow, they have a sweet spot.

They need some salt to grow, struggling in areas with lots of freshwater input. However, when the salinity gets too high, oysters are susceptible to parasites and predation. They need it just right.

That is why many in the state are concerned about the status of Louisiana oysters. Coastal habitat is rapidly changing, through both natural and manmade efforts. These shifting environmental conditions threaten that oyster sweet spot. When an oyster becomes stressed – from changes in temperature or salinity – they expend more energy, but don’t eat as much. “And skinny oysters taste bad. You don’t see people marketing low-fat oysters,” said Morgan Kelly, Louisiana State University (LSU) assistant professor of biology.

In an attempt to keep skinny oysters off the menu, Kelly is studying how a region’s unique environment can influence an oyster’s health and growth. There are persistent differences in salinity across Louisiana. Some places in Barataria approach freshwater conditions (0 ppt salinity). Other regions are saltier. “We’re trying to figure out if oysters are locally adapted to salinity,” said Kelly.

Graduate student Scott Riley, post-doctoral fellow Kevin Johnson and Kelly have been comparing oysters from Calcasieu Lake (highest salinity), Sister Lake and Vermilion Bay (lowest salinity). The researchers were particularly interested in exploring the combined effects of temperature and salinity had on the oysters.

Oysters from each region were collected, brought to the Grand Isle Oyster Research Lab, spawned and reared to juvenile size. Then the juveniles were transplanted back into the field, but to a different site than their parents’ origin to test whether the environment was impacting the oysters’ genetics.

And it appears that environment is influencing oyster genetics. While growth was fairly uniform across the different sites, there was a difference in disease susceptibility, in particular Dermo. Oysters whose parents came from Calcasieu Lake, showed greater resistance to disease, even when grown in Sister Lake or Vermilion Bay. And this difference in Dermo susceptibility
became more pronounced as temperatures grew.

Now the challenge is finding out how an oysters’ genes correlate with this better defense, which is where Johnson’s work comes in. He is measuring the genetic difference between the stocks, looking for genetic hotspots. These hotspots result from the addition of a chemical element (a methyl group) to the DNA. Depending on where it is added, the genetic expression changes result in added benefit or harm.

A better understanding of oyster genetics provides insight into the oyster’s habitat suitability. Research from Kelly and her lab can tell us which oysters are best suited for commercial or restoration plans. “Given Louisiana’s changing environment, the ideal oyster would be able to stand long periods of freshwater inflow and be disease resistant,” said Riley.

Like most Louisianans, oysters are really tough. They have already survived some big freshwater events – the floods of 2016 and Hurricane Harvey. It remains to be seen how they will respond to proposed diversions. And Kelly is optimistic; there will be oysters in Louisiana’s future. “Humans will move them, or they will move themselves, but they will still be there,” she said.