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