Killer from across the Sea


The Bellevue wreak - photo by Michael W. Fincham

SKEPTICS QUESTION whether an invasive species has ever really caused significant ecological damage in the Chesapeake Bay. The answer is yes. We called it MSX.

It was like the plague for oysters, and it came from somewhere else. In 1959, when MSX showed up in Virginia's Mobjack Bay, one million oysters died. In a single year. If oysters were once hailed as "white gold," then MSX was the White Death.

More than a nuisance, when this exotic parasite appeared in Chesapeake and Delaware bays in the late 1950s, it destroyed the richest oyster fisheries in the world. It also killed the region's best reef builder and filter feeder.

What about Overharvesting?

Some might argue that the Bay's oystermen had — through decades of aggressive harvesting — already destroyed their golden goose. And there is some truth in that. By knocking down the old bars, watermen not only removed bargeloads of oysters from the Bay, but they left the remaining oysters lying on the bottom. Increasing clouds of sediment covered them. Boring sponges and other predators attacked them.

But it's also true that during the middle of the 20th century the Chesapeake oyster fishery had reached something of an equilibrium. Though down from the reckless white gold rush of the 19th century, for much of the mid-20th century Baywide oyster harvests fluctuated roughly between 20 to 30 to 40 million pounds, year after year. In 1980, in the Maryland portion of the Bay, watermen entering the fishery saw about the same oyster harvest their parents had seen when World War II ended in 1945. About the same harvest Maryland watermen had pulled in as far back as the late 1920s.

Baywide, the oyster harvest — though at a reduced level and constrained by gear restrictions and other rules — found a kind of sustainability. In the 1950s, the Bay was in decent shape. Underwater grasses still lined the shallows. Oysters, crabs, and fish were still in good supply.

Then one day as the 1950s came to a close, a foreign parasite showed up. And that changed everything.

Where Did It Come From?

When oysters began dying in droves in the Delaware and Chesapeake bays, oyster scientists rushed in from around the country. It was not white gold that brought them, but the White Death. They looked at slides beneath their microscopes and found a profusion of round cells (plasmodia), filled with multiple nuclei. They compared these with oyster parasites they'd seen before — but they found no match.

Stumped, they named the unfamiliar organism "Multinucleated Sphere Unknown," or MSX.

Forty years passed before tools turned up that could pull MSX from a global lineup of oyster parasites. Thanks to genetic detective work by researcher Eugene Burreson and his colleague Nancy Stokes at the Virginia Institute of Marine Science (VIMS), we now know that this oyster killer came from Asia, almost certainly from Japan and Korea, after World War II and the Korean War.

The parasite lives in the Japanese oyster, Crassostrea gigas, used for aquaculture in many places around the world, including the northwest U.S. Records show that scientists and oyster growers also brought the Japanese oyster to the east coast, to see how it would do. Ships returning from duty in Japan and Korea may also have unwittingly brought back nonnative organisms, including the oyster parasite. Exactly how MSX got here makes for a fascinating story and some intriguing speculation (see Who Killed Crassostrea virginica?).

But wouldn't Bay oysters have fought off this foreign invader if decades of overharvesting and habitat destruction hadn't already weakened them?

Burreson doesn't think so. Based on the way MSX swept through healthy oyster bars when it first showed up here, he thinks that oysters would have succumbed anyway. Their immune systems simply had not evolved to handle this invading parasite. Like other diseases — the chestnut blight, Dutch elm disease, small pox — when this parasite from the Old World reached America it found little resistance.

After MSX invaded Delaware, Virginia, and Maryland, it marched up and down the Atlantic coast. In the 1980s, it appeared as far south as Florida and as far north as Maine. In 2000, MSX killed a large number of oysters in Nova Scotia, Canada.

graph of rise of MSX and Dermo
Ghosts of a bygone fishery, abandoned Bay workboats (top of article) bear witness to the passing of what was once the world’s richest oyster industry. On the oyster landings graph above, the high harvests of the nineteenth century trace the dismantling of the Bay’s virgin oyster reefs. Only after the 1920s did Chesapeake oyster harvests stabilize, shoring up a substantial fishery. That changed in 1959 when an exotic parasite called MSX hit. The Virginia oyster harvest plummeted first. Maryland’s fishery hung on until the early 1980s, then several years of drought brought disease riding up the Bay on a wedge of saltwater. The Bay’s oyster populations have still not recovered. Credits: photo by Michael W. Fincham and graph adapted from a NOAA Chesapeake Bay Office Graph.
Who Killed
Crassostrea virginica?
oyster shell

The remarkable story of how scientists found the origins of MSX and their speculations about how the parasite ended up in the Chesapeake and Delaware bays is now the subject of a new documentary by Maryland Sea Grant, Who Killed Crassostrea virginica? The film traces the decline of the Bay’s native oyster and the fatal blow brought by MSX and another prevalent disease, called Dermo.

Produced, written, and directed by veteran filmmaker Michael W. Fincham, the film captures both the poignant destruction of a fabled fishery and the prolonged scientific inquiry into the origins of the killer parasite. Painting a fair picture of the Bay’s oyster heritage is a tough assignment. On the one hand the Bay’s watermen are its iconographic characters, as native to this region as bullfighters are to Spain. On the other hand, their graceful skipjacks pulled dredges that brought down the Bay’s virgin oyster reefs.

The film asks whether we can save both the oyster and the oystermen. And it peers toward a future where the Bay’s historic oyster grounds may shrink to areas where disease does not dominate. The film premiered this spring at the Smithsonian Museum of Natural History, as part of the annual Environmental Film Festival, and it will be released for broadcast in September 2009. For more information, visit the web at

MSX Today

As the Bay moves into the 21st century, MSX is "everywhere" in the saltier reaches, according to Burreson.

But even so, it may be that the shadow of MSX is finally lifting. At least in the lower Bay.

"It's still killing firstyear spat [baby oysters]," says Burreson, "but not so much adults." In Virginia, he's seen that once oysters reach their second year, they seem to do okay. It's a similar story in Delaware Bay. Veteran oyster researcher Susan Ford at Rutgers University tells him she's seeing the same pattern.

In both Virginia and Delaware, where MSX has hammered away relentlessly for decades, oysters have developed an apparent resistance.

Says Burreson, "It's taken fifty years."

But the saga of MSX is far from over. In Maryland, where it takes a drought to bring salty water up the Bay, oysters see MSX only at intervals. Burreson thinks they haven't had time to develop the same tolerance as oysters in the lower Bay.

Researcher Chris Dungan agrees. He's at the Cooperative Oxford Laboratory, a research facility on the Eastern Shore created in 1960 expressly to take on the mystery of the new oyster killers. He says that a second oyster disease, known as Dermo (Perkinsus marinus), is now more prevalent in Maryland. But ongoing sampling shows that MSX still crops up in dry years when salinity rises.

And it still kills.

And yet Dungan, too, sees a possible ray of hope. The diseases are still out there, but despite dry weather over the last couple of years they haven't killed as many oysters as in the past. Are Maryland's oysters developing some natural resistance? Maybe, he says. We won't know until we have "empirical evidence." The best thing we can do now, he says, is to leave the large survivors in the Bay. He argues that we need sanctuary areas where surviving oysters can reproduce and pass on their ability to resist disease. And then we have to wait.

Still a Mystery

Despite all we've learned about this devastating parasite, there's good reason to keep the X in MSX.

Though scientists long ago characterized the singlecelled organism — first naming it Minchina nelsoni in 1966 and then Haplosporidium nelsoni in 1980 — they still haven't figured out its lifecycle. Or even how it infects its oyster victims.

Burreson still puzzles over this and has devoted endless hours in the search for an intermediate host — an organism that he believes plays a key role in how oysters become infected and how MSX manages to move around so fast and so far. He says it's hard to find continued funding for this work, though, and next year he plans to retire.

"Someone will get lucky one day and just stumble on it," he says. "They'll say, ‘Gee, what's this doing here?'" He goes on to say, "I hope this happens before I drop dead. I want to know what it is."

What's the future for oysters and MSX in the Bay? Burreson actually feels "some optimism"— since oysters in high salinity areas now appear to show signs of resistance. The bigger problem now, in Virginia as in Maryland, is that other oyster parasite, Dermo.

This leads Burreson to ask another question. Is there some as yet undiscovered connection between Dermo and MSX? Some hidden relationship? We know that Dermo was already in the Bay before MSX showed up. Researchers found it in 1949, the first time they went looking for it. But it wasn't until MSX appeared ten years later that Dermo became a real problem. Cause and effect? Or just coincidence?

That's a hot topic for his younger colleagues to take on, Burreson says. For now he'll have to be satisfied with being part of the team that figured out where MSX came from. That will go down as a pivotal discovery — especially for those who want to know just what kind of damage a nonnative parasite can do to a major fishery, and to a national treasure like the Chesapeake Bay.

June 2009
vol. 8, no. 2
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