The Asian oyster which some have hoped could withstand the Bay’s oyster diseases has turned out to succumb to a disease previously unknown in the region.
In a surprise development, researchers from the Virginia Institute of Marine Science found that large numbers of the sterile foreign oysters they had sent for experiments to Bogue Sound in North Carolina became infected with deadly parasites.
They strongly suspect it caused two large die-offs among Crassostrea ariakensis oysters which took place in North Carolina last summer.
“This is certainly not good news, said Gene Burreson, a VIMS shellfish disease expert. “We don’t know how bad of news it is yet.”
The disease was not brought in by C. ariakensis. Rather, the oyster appears to be susceptible to native parasites that already existed in the area but had never been noticed, apparently because they do not affect any commercial species.
Ironically, the native C. virginica oyster—whose populations have been devastated by other diseases—grows in the same area and is immune to the parasite, which appears to be an undescribed species in the genus Bonamia.
The discovery adds a new twist to the debate over the role C. ariakensis may have in the region. Until now, all studies had shown great promise for the foreign species. Previous tests in the Chesapeake had shown the oysters not only withstood diseases that devastated the native oyster population, but that they grew fast and tasted good.
That had spurred huge hopes among watermen and seafood industry officials, as native oyster populations have hit record lows.
The state of Maryland and Virginia have proposed quick action to introduce reproducing populations of the species in the Bay to rebuild oyster populations, although several federal agencies, environmental groups and scientists have called for significantly more research before any decision is made.
The new findings suggest C. ariakensis could have its own problems if introduced. While Bonamia has not been seen in the Chesapeake, that could change if C. ariakensis were introduced here and spread into coastal waters—in effect giving the parasite a living pathway through which it could move.
“If C. ariakensis gets spread up and down the coast, then this parasite could clearly move within C. ariakensis up into Virginia and into the Bay and limit its use,” Burreson said.
At the same time, he said, it’s possible the parasite will not survive in Bay conditions, where salinities are lower than those in Bogue Sound, which is located behind barrier islands along the Atlantic Coast. Infections have not been seen in oysters being tested in North Carolina’s Pamlico Sound, which has salinities similar to those of the Chesapeake, Burreson said.
The discovery of Bonamia is troubling because it is known to cause significant mortalities in some species of oysters. Two species of the parasite are known to exist; Bonamia ostreae on the U.S. West Coast and Bonamia exitiosa in New Zealand.
Bonamia ostreae is believed to have been accidentally introduced into France in the late 1970s, contributing to major die-offs of the native flat oyster. French officials recently considered bringing C. ariakensis to their waters as an alternate species for aquaculture until studies showed it was highly susceptible to Bonamia infections.
When the National Academy of Sciences issued a report last year about the potential of using C. ariakensis in the Bay, it specifically recommended that research be done to test its vulnerability to Bonamia because of the French experience.
At that time, the closest population of the parasite was thought to be in Maine, where it was associated with populations of European flat oysters used in aquaculture.
As a parasite, Bonamia has to live with a host species to complete its life cycle.
Burreson said it probably went undiscovered in Bogue Sound because its host was a largely unstudied species. “It might be in some little clam that is not commercially important and no one has done any pathology on it,” he said. “That is our best guess at the moment.”
That situation changed last summer when VIMS sent a half-million sterile C. ariakensis oysters to researchers in the state to study their growth and marketability. Nearly a million sterile C. ariakensis oysters are being reared by growers in Virginia as part of a similar study.
Two early batches of the North Carolina oysters suffered large numbers of mortalities. At the time, scientists there thought the deaths were caused by low dissolved oxygen levels in the nurseries where the oysters were being grown, or by infections from native worms.
Some samples were sent to VIMS for analysis, but they had been poorly preserved. Scientists noticed some cells that looked like Bonamia at that time, but there were only a few and, because of the poor condition of the specimens, a final determination was never made.
But in November, North Carolina officials sent fresh samples to VIMS for another examination before the oysters were to be distributed among aquaculture growers as part of an experiment.
In the new sample, VIMS scientists found that 50 percent of the oysters had distinctive, round protozoans infecting blood cells. “There was no question that this thing looked like Bonamia,” Burreson said. North Carolina officials sent another batch, and the scientists found that 60 percent were infected.
DNA analysis confirmed the identity, although Burreson said it appears to be a previously undescribed Bonamia species.
“I’m pretty convinced from what we know now that Bonamia was the cause of those two mortality events in Bogue Sound,” Burreson said. If so, the parasites can kill quickly. Burreson noted that the first batches of oysters died when they were only about an inch in diameter.
The parasites have not been detected in any of the oysters being used in Virginia, or those being sent to Maryland for the first studies in that state. In fact, VIMS still had some oysters from the same batch that was shipped to North Carolina. None of those were infected.
Burreson speculated that Bonamia is not found in the Bay or along the Virginia coast because its host species may be at the northern end of its range. Cape Hatteras, which elbows its way out into the Atlantic, is known by scientists to be a “faunal break” which blocks the distribution of many southern species from reaching farther north, and vice versa.
“It’s entirely plausible that there is some mollusk down in North Carolina that occurs south of Cape Hatteras and down the coast that has a Bonamia infection, and that host does not occur north of Cape Hatteras,” he said.
But if C. ariakensis were to become widespread—either through intensive aquaculture or through a reproducing population—it could become a new host that would likely allow the parasite to spread to new areas, Burreson said.
Burreson believes the distribution of the new Bonamia species is limited by salinity. What’s unclear, though, is whether that’s because the parasite does not tolerate low salinities, or because its host does not survive in those areas.
It’s a key difference: If the Bonamia itself does not tolerate low salinities, its potential distribution would be limited, and oysters in most of the Chesapeake would be safe. But if it is the host species which does not tolerate the lower salinities, then Bonamia could spread into those areas if it had a suitable host, such as C. ariakensis.
Salinities in Bogue Sound are about 30 parts per thousand. Salinities in oyster growing parts of the Chesapeake range from about 10 ppt to 25 ppt.
Burreson and colleagues were planning to begin work in early January to learn more about the organism’s salinity tolerance by exposing infected C. ariakensis oysters to different salinities under quarantined conditions. They will also try to identify the parasites’ host organism in North Carolina.
Those studies will shed light on the potential to use the oyster in the Chesapeake.
“It’s not necessarily a show stopper,” Burreson said. “It may be Bonamia just cannot survive in the Bay at all, and that C. ariakensis can be very useful in Chesapeake Bay.”
In fact, if the new Bonamia species only survives at high, ocean-like salinities, it could serve to limit the spread of C. ariakensis out of the Bay if there were an introduction—something that has been a concern voiced by other states.
On the other hand, if C. ariakensis were to allow Bonamia to spread into new areas, it’s possible that other mollusk species not currently exposed to the parasite could become infected as well.
Ironically, Burreson said, Bonamia was named for the scientist, J.R. Bonamia, who who first identified the parasite. In French, it translates to “good friend.”
“It’s not a good friend of oysters, though,” he added.