Unexplained Oyster Die-off

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Forbidden Sexual Development Suspected in Unexplained Oyster Die-off

It’s an abnormal sight, really.

Crowded around a makeshift table in the shallows of the Rappahannock River, four Virginia Institute of Marine Science (VIMS) researchers toss around oysters like they’re dealing cards. In waders and life vests, they trudge through calf-high water, bringing up submerged mesh bags, dumping out oysters, and counting them on the table.

Among them is Joey Matt, Virginia Sea Grant graduate research fellow at VIMS. He’s growing these oysters to unravel the mystery of a mass oyster mortality event that happened in the summer of 2014, in which some commercial farms lost up to 85% of their oyster crop.

“We still don’t have an answer to why that happened,” Matt says. “And that’s scary because these farmers—a lot of them¬ can’t afford for this to happen again.”

Matt’s trying to figure out what caused so many oysters to die at once, and he’s got a hunch. Despite being supposedly sterile—meaning they don’t produce eggs or sperm—an unusually high proportion of commercial oysters collected in 2014 were producing sex cells. And Matt thinks this forbidden sexual development may be what led the oysters to their demise.

“Sometimes when an unusual event happens the only way to make sense of it is to connect it to another unusual event,” Matt says.

During the 2014 mortality event, VIMS researchers collected surviving oysters from affected farms. When they popped them open, they expected to see signs of infection indicating that a disease caused the die-off. Instead, they found something much more surprising: some of the oysters were loaded with eggs—as though they were getting ready to spawn.

That’s not how it’s supposed to be. Growers in Virginia prefer to plant sterile oysters; an estimated 91% of oysters planted are sterile, according the 2014 Virginia Shellfish Aquaculture Situation and Outlook Report.

"For me, I don’t see challenges, except for limited time and money, and a lot of things I want to do."

A sterile oyster is a triploid oyster, which means that it has three sets of chromosomes instead of the typical two sets. Chromosomes are the structures that bundle your DNA into little sausage-like packages. You have two sets of chromosomes—one set from your mom and one set from your dad. So do wild, non-sterile oysters. An even number of chromosomes is what makes reproduction work, because you can evenly divide the genetic material in half.

But the odd number of chromosome sets in a triploid makes a mess out of the process that produces sex cells, rendering triploids sterile.

Growers prefer sterile oysters because they tend to be meatier year-round, faster-growing, and more resistant to disease than wild oysters. This might be because they don’t have to devote their energy to reproduction, which is energy-intensive.

“What does it mean when an oyster designed to be sterile produces lots of eggs? Is that a very stressful event that’s causing them to die?” Matt says. “We’re wading into some really unexplored waters of biology.”

And that means slipping into some real waders. Most of the affected farms reported growing a specific breed of triploid oysters, often referred to as the Northern strain. It’s a cross between Virginian and Maine oysters.

To investigate the possible connection between an oyster’s parents, their sterility, and their survival, Matt is growing the Northern strain and a bunch of other crosses of oysters, both triploids and diploids. His oysters reside at three of the farms on the Eastern Shore that were affected by the mortality event, as well as in the Rappahannock River, which didn’t have the extreme mortality. He checks on the oysters at all the sites monthly—except for May and June, when he’ll check them twice a month because he suspects that’s when the oysters will show the most sexual development.

When checking the oysters, Matt and other researchers dump them out on a table set up in the water, count how many are alive, and take some back to the lab to assess their condition and sexual development. They’ll keep at it until August.

“The industry will be interested in hearing from him and what he learns,” says Karen Hudson, VIMS extension staff affiliated with Virginia Sea Grant and Matt’s outreach mentor for this project. “It’s directly applicable.”

Matt says that knowing what caused the mortality event might help farms decide which breeds to grow in the future: “The more we know, the more we can do to prevent a mass mortality event like this from happening again.”

By Chris Patrick, staff writer
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