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March 2007

MSX (Haplosporidium nelsoni)

Contact: Jonathan A. McKnight, Maryland Department of Natural Resources | 410-260-8539 | jmcknight@dnr.state.md.us

diseased oyster healthy oyster

ANNAPOLIS, MD (March 9, 2007) - One of the hottest topics in mid-Atlantic conservation is the proposal to introduce a new species of oyster to the Chesapeake Bay. The Asian Oyster, it is argued, would be tougher that our native oyster, and could actually save the Bay by performing the critical function of filtering its waters as our native oysters did before they were almost wiped out in the second half of the twentieth century. What many people do not know however, is that at least one non-native species has already played an important part in the tragedy that has befallen the Chesapeake’s embattled bivalve. This species, MSX (Haplosporidium nelsoni), is the MISC Invader of the Month for March.

A few hundred years ago, the thought of running out of oysters in the Chesapeake Bay would have seemed ludicrous. Until fairly recently, there were so many oysters in the Chesapeake that the enormous reefs they created made it difficult to navigate shallow waters. Maryland watermen supplied a booming industry in Baltimore that fed the succulent bivalves to hungry customers across the nation. Today the native oyster, Crassostrea virginica, is at less than 2% of historic levels in the Bay.

Scientists have since discovered that oysters not only thrived in the Chesapeake ecosystem, but were vital to its upkeep. Without a living bottom of filter-feeders constantly straining the biological soup of the Bay, water quality suffers. And the loss of living oyster reefs removed a vital aquatic habitat.

What happened? We allowed water quality to deteriorate. We let sediment runoff smother oyster beds. We probably harvested too many oysters. But it was the introduction of an exotic biological pollutant; MSX; that dealt a lethal blow to our oyster population.

When oysters began to die off in devastating numbers in the late 1950’s – first in Delaware Bay and a year later in the Chesapeake, scientists had no idea what they were dealing with. The dying oysters were riddled with a parasite; a single-celled organism with many nuclei within it. They named it for these traits – Multi-nucleate Sphere X – X for unknown – or MSX.

It would be decades later that new DNA analysis technology would confirm that MSX was a parasite of the Japanese oyster, Crassostrea gigas. The Japanese oyster tolerates the MSX parasite, presumably because the two species co-evolved over thousands of years and developed an uneasy ecological truce. The devastating effect of this new pathogen on the biologically-unprepared native oysters of the Chesapeake has been likened to the effect of the smallpox virus on Native Americans after its introduction by Europeans.

Just as the identity of MSX remained unclear for forty years, so too the pathway that brought it to the Chesapeake and Delaware Bays remains a mystery. Scientists and aquaculturalists began introducing the Japanese oyster into local waters in the late 1940’s. At the same time, a fleet of military and support ships were returning from Asian waters in the demobilization of the Second World War. To further cloud the picture, we know that there is at least one important part of the life cycle of MSX that we do not yet understand: there may be an unidentified intermediate host for MSX. Oysters do not catch the disease from other oysters in the lab, but contract it readily from the open waters of the Chesapeake Bay. Despite years of searching, we do not know what this intermediary is, or what role it may have played in the introduction.

If it was indeed well-meaning scientists who introduced MSX to the east coast though the Japanese oyster, then the irony surely resonates in today’s controversy over the introduction of a third species, the Asian Oyster. Proponents have worked hard to ensure that they have a population of Asian oysters which do not carry with them a host of unknown viruses, bacteria, and protozoa from foreign waters. But the lessons of the unintended consequences taught by the demise of our native oyster to a foreign pathogen, and indeed by a long history of unwise introductions, surely call for extreme caution. One way or another, we cannot restore the Chesapeake Bay without a healthy oyster population.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.

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