Oyster Foes East & West

Whether they are microbes like bacteria and protozoans or larger organisms such as oyster drills, parasitic worms and crabs, oyster predators are natural to any ecosystem. The daily warfare between predator and prey is one of attack and defense – over a long period, gains and losses will often average out. That has not been the case for the eastern oyster, Crassostrea virginica. In the mid-Atlantic, this species has been on the losing end for more than a decade to two protozoan parasites, MSX and Dermo. Its west coast cousin, the Pacific oyster, Crassostrea gigas – a species first brought from Japan in the early years of the century and now cultivated largely through hatchery production – can tolerate the impact of these two diseases; however, it often experiences high summer mortality. The following is a summary of some of the diseases that have been the focus of the Oyster Disease Research Program.

MSX. Its first appearance in mid-Atlantic waters was in Delaware Bay in 1956 where it ravaged oyster beds; the next year it arrived in the Chesapeake. Though the single-celled organism has been identified as a protozoan, Haplosporidium nelsoni, its life cycle and means of infecting oysters still remain as mysterious now as they did forty years ago. Unlike Dermo, MSX cannot be transmitted from oyster to oyster but new molecular tools are now making it possible to hunt for carriers of the parasite among estuarine organisms.

Dermo – Perkinsus marinus. First observed in Gulf Coast oysters in the 1940s and in the lower Chesapeake in the early 1950s, Dermo, like MSX, is now present on virtually every harvesting area in the Chesapeake Bay: this is largely the result of the widespread transfer of oysters from one location to another in an effort to renew unproductive bottom grounds. While studies have shown a higher virulence of Dermo in the mid-Atlantic than in the Gulf waters, genetic research has revealed differences between Perkinsus in mid-Atlantic locations and within Chesapeake Bay itself. Other bivalves host Perkinsus, but whether those are forms transmittable to oysters is currently under investigation.

JOD – Juvenile Oyster Disease. A disease that seems to affect only oysters cultured in hatcheries, it strikes only young oysters, typically less than 30 millimeters in size. In the Long Island area, JOD usually hits in late June to early July, while further north in Maine, oysters are not struck until mid-August or early September. The first indication of JOD is an abrupt stop in growth – within three to five weeks most oysters are dead. Though researchers suspect that JOD is a single celled organism, the cause and origin of JOD are not yet known. JOD is transmissible from one oyster to another and so far there is no simple cure. Commercial growers in the northeast have been working with researchers to develop successful techniques for managing around JOD's devastating effects.

Summer Mortality. Mass mortality of Pacific oysters occurs sporadically and inexplicably on west coast oyster grounds, especially when water temperature rises. Many shellfish growers are concerned that these mortalities pose a major threat to the industry. Researchers believe that the deaths may be the result of several factors, although there is little understanding of the cumulative effects of various environmental stresses on the oyster's disease tolerance. ODRP-supported research has been identifying bacteria populations such as Nocardia, which are thought to be a major cause of summer mortality. Molecular diagnostic methods for identifying and detecting Nocardia are under development and will enable growers to diagnose and screen spat that are infected by these bacteria.