Scientists have identified a second parasite that may have been involved in massive die-offs of Asian oysters during experiments off the North Carolina coast last summer.

The discovery by Virginia Institute of Marine Science researchers further complicates the question of whether the parasites could hinder the prospect of using the foreign oyster, Crassostrea ariakensis, to revive the region’s seafood industry.

Last December, VIMS scientists found a previously unknown species of the parasite Bonamia infecting C. ariakensis oysters being grown in North Carolina’s Bogue Sound as part of an aquaculture experiment. A Bonamia species is known to have killed C. ariakensis in France, and the discovery was made after two massive oysters die-offs in Bogue Sound earlier in the year.

“We’re presuming that that particular Bonamia species is problematic for C. ariakensis,” said Ryan Carnegie, an assistant research scientist at VIMS. “But now we need to ask the question, is this second Bonamia species a problem? That does complicate our research.”

Scientists earlier this year discovered Bonamia in a small, native oyster, Ostrea equestris, which lives in high-salinity areas off the North Carolina coast. But a subsequent DNA analysis showed that the Bonamia in O. equestris was not the same species seen in C. ariakensis.

Scientists have had difficulty in obtaining new samples of the parasite because—as is the case with the parasites MSX and Dermo which infect native oysters—they became scarce as temperatures turned cold. “We are somewhat limited by the Bonamia we can find to work with at this point,” Carnegie said.

Like other parasites, Bonamia must live with a host species to complete its life cycle, and Carnegie said O. equestris is considered a likely host for both Bonamia species and that a closer examination may reveal that both Bonamia species are in C. ariakensis as well. Carnegie said the parasites can only be distinguished through DNA analysis, not through a microscope.

Scientists have focused on O. equestris as the potential host species for Bonamia because it is a flat oyster, and Bonamia is known to primarily infect other flat oyster species around the world.

Also, O. equestris is found only in high-salinity waters, like those in Bogue Sound. In North Carolina, Bonamia was only found in C. ariakensis in high-salinity areas.

North Carolina and Virginia last year began tests involving hundreds of thousands of sterile C. ariakensis oysters to determine whether they could be used for large-scale aquaculture in the future. The species’ ability to withstand diseases that have devastated the native oysters had attracted huge interest among watermen and the seafood industry, and the states of Maryland and Virginia have proposed introducing a reproducing population of the oysters into the Bay.

But all those plans could hinge on what studies reveal in the next few months. Specifically, scientists want to learn whether the parasites could spread to C. ariakensis in low-salinity areas. To do that, they need to answer two key questions:

  • Is the distribution of Bonamia found in high-salinity water only because its host species is restricted to those areas, or are the Bonamia species themselves unable to tolerate lower salinities?

    If the Bonamia itself is restricted to high salinities, then C. ariakensis oysters in the Chesapeake, Pamlico Sound and other lower salinity areas may be off-limits to the disease. But if only its host is restricted to high-salinity waters, then the Bonamia may spread to other low-salinity areas if the parasite finds another suitable host, such as C. ariakensis.

  • Can the parasite spread among C. ariakensis, or can it only spread from its current host to C. ariakensis? If the parasite can’t spread between C. ariakensis oysters, it would not be able to reach oysters living in low-salinity areas.

The answers to those questions could determine what the future of C. ariakensis in the region.

For example, even if Bonamia does survive in low salinities, aquaculture using C. ariakensis may still be possible if the oysters were geographically isolated from areas with Bonamia. But in that case, the introduction of a reproducing population may be more problematic if Bonamia is able to spread among C. ariakensis oysters, as their expanding population may eventually create a living pathway for Bonamia to reach low-salinity areas.

Carnegie said scientists would also like to learn more about O. equestris which, not being a commercial species, is poorly studied. The oyster is common in high-salinity areas off the Gulf Coast and the Southeast, but not in northern areas, although some reports have suggested it is in coastal waters off Virginia and Maryland.

If O. equestris turns out not to be the host for both Bonamia species, the research effort would become even more complex.

Ironically, C. ariakensis is being considered for use in the region because it appears to be immune to the diseases MSX and Dermo, which have devastated the native oyster, C. virginica.

But the natives species, like its Pacific oyster relative, C. gigas, are not affected by Bonamia, which is one of the reasons the presence of the parasites had previously gone undetected. C. ariakensis is an exception, as it has been shown to be highly susceptible to other species of Bonamia.