Famous for filtering their surrounding water, drawing out its algae and excess nutrients, oysters have long been heralded the metaphorical and literal cleaner of the ocean. It turns out – however – that their beneficial impact may have been underestimated. 

New research from William & Mary’s Batten School of Coastal & Marine Sciences shows that oysters can also reduce the spread of disease in nearby marine species, including among Chesapeake Bay’s prized blue crabs.

The study has found that these particular bivalve molluscs can significantly reduce the transmission of a lethal parasite called Hematodinium perezi, known for infecting juvenile blue crabs in the high-salinity waters of the Chesapeake Bay. The findings – published recently in the journal Ecology – point to a previously under-appreciated role for oyster reefs in disease dynamics within marine ecosystems.

In field experiments conducted along Virginia’s Eastern Shore, juvenile blue crabs placed near live oysters were approximately one-third less likely to become infected than crabs deployed in areas without oysters. The difference, researchers found, hinged on one critical factor: active filtration.

“Oysters and oyster reefs are known to provide a range of ecological benefits, and crabs are drawn to them for food and refuge,” said Jeffrey Shields, a professor at the Batten School and VIMS. “But their capacity to remove pathogens from the environment has received far less attention.”

To isolate that effect, the research team – led by doctoral researcher Xuqing Chen – designed experiments that compared infection rates among crabs placed near live oysters, empty oyster shells, or no reef structure at all. Only the presence of live oysters reduced infection risk, demonstrating that it was filter feeding, and not simply the physical structure of the reef, that curtailed parasite transmission.

The team replicated these findings in the controlled conditions of VIMS’ Seawater Research Lab. When exposed to dinospores ( the name given to the free-swimming, infectious stage of Hematodinium), oysters removed the parasites rapidly, at rates comparable to their filtration of other plankton. Within an hour, more than 60% of the parasites had been eliminated.

The researchers also observed lower mortality among crabs housed with oysters, though they caution that the system is complex and influenced by multiple interacting factors.

“This work is part of a larger effort to understand parasite–host interactions at the scale of fisheries,” said Chen, now a postdoctoral researcher at France’s Station Biologique de Roscoff. “Marine disease dynamics are often overlooked, despite their potential to reshape ecosystems and fisheries.”

That potential is particularly evident in the case of Hematodinium, which can cause near-total mortality among juvenile blue crabs during warm summer months. In some high-salinity bays, infection prevalence approaches 100%. While the smallest crabs were expected to be most vulnerable, the study revealed unexpectedly high rates of new infections among larger juveniles.

“This hasn’t been documented before, and it raises important questions,” said Shields. “The fishery removes roughly 40% of adult crabs each year. Juveniles must replace them, yet they’re highly susceptible to disease. Understanding how these pressures interact is critical.”

The research is part of a broader initiative that combines field ecology, laboratory experiments, and mathematical modelling. By working with mathematicians and biostatisticians, the team aims to explore how filter feeders such as oysters influence disease transmission at ecosystem scales.

Those models could help guide future fisheries management and oyster restoration strategies – particularly as warming waters intensify disease pressure in coastal systems.

“Despite major restoration efforts, oyster populations in the Bay remain far below historic levels,” said Shields. “That represents a substantial loss of filtering capacity. Modelling allows us to scale these effects up to ask whether restoring oysters can meaningfully suppress disease and deliver broader ecosystem benefits. That’s where we’re headed next.”