Oysters play unexpected role in protecting blue crabs from disease
Oysters famously filter their surrounding water, but it turns out they are removing more than algae and excess nutrients. New research from William & Mary’s Batten School of Coastal & Marine Sciences & VIMS shows they can also reduce the spread of disease in nearby marine species, including Chesapeake Bay’s prized blue crabs.
Recently published in the journal Ecology, the study found that oyster filter feeding significantly reduced the transmission of Hematodinium perezi, a deadly parasite that commonly infects juvenile blue crabs in high-salinity coastal waters. In field experiments conducted on Virginia’s Eastern Shore, juvenile crabs placed near live oysters were about one-third less likely to become infected than crabs deployed without oysters.
“We know that oysters and oyster reefs provide a variety of ecological benefits, and that crabs are drawn to them for food and protection, but their ability to remove pathogens from the environment has not been well studied,” said Jeffrey Shields, a professor at the Batten School & VIMS who worked with his graduate student and several undergraduates on the study, including lead author Xuqing Chen, Ph.D. ’25.
The research team did experiments in both the lab and in the field. On sweltering summer days — when disease pressure from the parasite is at its greatest — they placed uninfected juvenile blue crabs in high-salinity coastal bays where the parasite is common. Some crabs were placed between live oysters, others between empty oyster shells and others were left unprotected. Only the presence of live oysters reduced infection risk, demonstrating that active filter feeding, not just the water’s interaction with the reef structure, was responsible for the effect.
The scientists replicated their experiments in the controlled setting of the Batten School & VIMS’ Seawater Research Lab. They found that when oysters were exposed to dinospores, an infectious, free-swimming stage of the parasite, they rapidly removed them from the water at rates similar to the removal of other plankton. On average, the oysters eliminated more than 60% of the parasites within an hour.
The researchers were also surprised to see a reduction in mortality among crabs in the treatment group, though they emphasized that there are too many variables to attribute the finding to the oysters alone.
“This study is part of a larger collaboration with the eventual goal of modeling these parasite-host interactions at the fisheries scale,” said Chen, who now works as a postdoctoral scientist at Station Biologique de Roscoff in France. “I would love to see more attention paid to disease dynamics in marine ecosystems, since they are complex and can have a huge impact on our fisheries.”
Scaling up the findings, with help from mathematics
Hematodinium infections can cause high mortality in juvenile blue crabs during warm summer months, with prevalence levels in some high-salinity bays approaching 100%. While the researchers expected the smallest crabs to be most susceptible to infection, they were surprised to document greater incidence, or new infections over time, among the larger juvenile crabs.
“This is something that had not been documented previously, and it has some interesting implications because the fishery removes approximately 40% of adult crabs from the system annually,” said Shields. “The juvenile crabs must fill that void, yet they are highly susceptible, so we need to think about how all of this comes together to increase or decrease the spread of disease.”
The study is part of a broader, interdisciplinary effort at William & Mary that combines field ecology, laboratory experiments and mathematical modeling, supported by a grant from the National Science Foundation. Shields and colleagues from the Batten School & VIMS are working with researchers in William & Mary’s applied mathematics and biostatistics community to explore how filter feeders like oysters influence disease dynamics at larger scales.
Those modeling efforts may help inform future fisheries management and oyster restoration strategies by clarifying when and where oyster filtration is most likely to suppress disease transmission, particularly in warming coastal systems.
“While we’ve made important strides in oyster restoration in the Bay, we know that populations are still far below historic levels. This represents a significant reduction in filtering capacity,” said Shields. “One of the beautiful features of mathematical modeling is that it allows us to scale this effect by orders of magnitude. That’s where we’re going next — trying to determine whether we can meaningfully influence this effect for overall ecosystem and fishery benefits.”
The full manuscript of the study is available on the Ecology website.