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January / February 2004

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Juvenile Onset

Juvenile Onset
UMaine researchers study a previously unknown species of bacteria that threatens oyster offspring

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Maine's Damariscotta River estuary is an ideal place to grow oysters. In the summer, they thrive in the algae-rich broth created by the mixing of a warm river with the upwelling sea. However, for oysters and oyster farmers, there is trouble in paradise.

An organism that causes juvenile oyster disease (JOD) also finds the estuary to its liking. With a touch of irony, it makes infected oysters starve in the midst of plenty. It has been the target of a concerted University of Maine research effort for more than a decade, one that is paying off.

Throughout much of the 1990s, researchers worked to identify a cause. Former UMaine shellfish pathologist Bruce Barber and his graduate-student-turned-oyster-farmer Chris Davis led that work. They showed that oyster growers could cut their losses by selecting for fast-growing stock.

In 1999, Kathy Boettcher, Barber and John Singer, all of the Department of Biochemistry, Microbiology and Molecular Biology, and the School of Marine Sciences, reported a milestone in the search for the cause of JOD. They demonstrated that the disease could be treated with antibiotics and thus must have a bacterial origin. Then they used new laboratory culture techniques and DNA analysis to discover the presence of a previously unknown species of bacteria on oysters that were sick. The bacteria were not found on healthy oysters. Additional studies with the bacteria, tentatively named Roseimarina crassostreae, have confirmed its primary role in JOD.

Boettcher's efforts to understand both the disease and what oyster farmers can do to minimize losses have earned praise from the industry. "Her work has given the industry an understanding of what causes JOD and what we can do to prevent it. She has shown a real determination to work with people in the oyster industry, and we appreciate it," says Dick Clime, one of the state's pioneer oyster farmers on the Damariscotta.

First recognized as a significant problem in Maine in 1988, and subsequently in New York and Massachusetts, JOD can kill more than 90 percent of the cultured young oysters in a farmer's stock. That's a tough loss in an industry just getting on its feet. Maine growers raise high-quality American oysters, Crassostrea virginica, for what they call the white tablecloth market. In 2002, market-size oysters brought in revenues of about $850,000.

The Maine Aquaculture Innovation Center (MAIC), a state-supported organization located at UMaine, is leading efforts to establish new oyster farms, but JOD is a significant barrier. "JOD is the one thing that has kept oyster culture in Maine from expanding," says Clime, MAIC board chair and a UMaine graduate.

Not a threat to people, the disease continues to affect oysters in other parts of the Northeast. In 2003, it was reported for the first time at Martha's Vineyard, where a major producer lost about half of his juvenile stock.

As its name implies, the disease kills young oysters. It generally strikes between July and October after they've been placed in open water to grow to adult size. The Damariscotta is both the heart of Maine's commercial oyster industry and the location of most JOD outbreaks in the state.

In 1996, Barber and Davis demonstrated a way to reduce losses. They showed that once young oysters grow to be at least an inch across, they are largely out of danger. Taking advantage of that result means growers need to select for fast-growing oysters. Such management practices can reduce, but not eliminate, the threat of JOD.

"One of the signs of JOD is that the oysters look like they've starved to death," says Boettcher. "The animals grow really well when they are first put out, and then the disease hits them, and they just stop growing.

"For a long time, people thought this was a starvation problem. We colonized oysters with bacteria in the lab and found that the oysters have a reduced capacity to filter algae. That's consistent with what we see in the river. They are starving because (bacteria cover) the tissue surfaces. This is not an invasive disease. Instead, it appears to grow as a film."

The species is not related to any human pathogens, she adds. Moreover, it is not seen on nearby mussels, clams or other marine organisms. Boettcher calls it an opportunist because it takes advantage of culture conditions and vulnerable young oysters.

Using DNA analysis, Boettcher has placed the bacteria in the Roseobacter group of microorganisms. Within the last decade, new detection methods have led to the conclusion that Roseobacter species are abundant in marine waters. They include some that live under extreme conditions in the deep sea, as well as those that inhabit near-shore areas.

For reasons that are not clear, JOD was in retreat last year, complicating Boettcher's efforts to test a potential treatment. It wasn't until October that she received a shipment of sick oysters. Opening a plastic bag with a few dozen animals, Boettcher held up a young oyster whose shell was less than an inch wide. "It looks as though it has an overbite," she says.

The mismatch between the two halves of the shell is a signature of JOD infection.

The oyster inside was dead. In its last days, it had built up a small dark internal ring of shell material around itself, as though trying to erect a defense. "JOD kills quickly. In a short period of time, it can kill half of a farmer's crop," Boettcher says.

With those samples, Boettcher and research assistant Aaron Maloy confirmed that Roseimarina was again present in overwhelming numbers on the sick animals. In addition, they worked with Clime to test an inexpensive probiotic oyster treatment to protect the young from JOD. Their approach uses a species of harmless, naturally occurring bacteria identified in oysters that had survived previous JOD outbreaks. By dipping the oysters in an enriched solution of the bacteria, the scientists hope to confer a protective coating that will keep Roseimarina at bay.

The low JOD incidence in 2003 made it difficult to determine if, in fact, the probiotic treatment worked. It's also possible that by selecting survivors of earlier outbreaks, growers produced animals with innate resistance.

With funding from MAIC, the U.S. Department of Agriculture and Maine Sea Grant, Boettcher and Maloy are focusing on methods to detect the troublesome bacteria. They also are looking at the strategy employed by Roseimarina to infect cultured oysters. Already they know that the species has the ability to swim freely, attach to tissues and create a colony that, in effect, smothers the animal inside its shell. Yet to be understood is the complex chemistry of that process.

Better understanding of how Roseimarina behaves could help to explain reports of oyster disease in other parts of the world, such as France and California, Boettcher notes. In addition, there is evidence that Roseimarina may be a factor in diseases of other organisms, notably some corals, but to date it has only been successfully cultured from American oysters affected by JOD.

by Nick Houtman
January-February, 2004

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