Oysters have a solid reputation. They sit quietly on the bottom, pulling algae and excess nutrients out of the water, helping keep coastal bays from turning into green soup.

For years, that filtering job has been the main reason people care about bringing oysters back.

But oysters may be doing something else that matters just as much. They appear to be cleaning the water of disease, too.

And that could make a real difference for animals that live nearby, including one of the Chesapeake Bay’s most valuable residents, the blue crab.

A parasite with a deadly track record

The parasite is called Hematodinium perezi. It thrives in warm, salty coastal waters and hits juvenile blue crabs especially hard during summer.

In some bays, nearly every young crab can end up infected by the parasite during peak season. Once infected, many crabs don’t survive.

The study focused on what happens when crabs share the water with oysters.

Jeffrey Shields is a professor at William & Mary’s Batten School of Coastal & Marine Sciences & VIMS, who worked with graduate and undergraduate students on the study.

“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 Professor Shields.

Putting oysters to the test in the wild

To find out if oysters really affect disease spread, the researchers ran field experiments on Virginia’s Eastern Shore.

On hot summer days, when the parasite is most active, the team placed uninfected juvenile blue crabs in coastal bays where Hematodinium is common.

Some crabs were placed among live oysters. Others sat near empty oyster shells. A third group had no protection at all.

The results were striking. Juvenile crabs placed near live oysters were about one-third less likely to become infected than crabs deployed without oysters.

Empty shells did nothing. Only live oysters made a difference. That showed the effect came from active filter feeding, not just the physical structure of a reef.

What happens in the lab

The team also ran the same idea under controlled conditions at the Batten School & VIMS’ Seawater Research Lab. There, they exposed oysters to dinospores, the free-swimming infectious stage of the parasite.

The oysters filtered the parasites out of the water quickly. On average, the oysters eliminated more than 60% of the parasites within an hour. The removal rate matched how oysters filter other plankton they normally eat.

The scientists also noticed fewer crab deaths in the oyster treatments, though they stressed caution. Too many factors affect survival to credit oysters alone for that outcome.

Surprising patterns in young crabs

The study uncovered another unexpected result. While the smallest crabs were expected to be the most vulnerable, the researchers found more new infections over time in 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.”

That balance between fishing pressure, crab survival, and disease could shape future crab populations in ways managers haven’t fully accounted for yet.

Protection across entire fisheries

The work is part of a broader collaboration that blends field studies, lab experiments, and math-based modeling.

The goal is to understand how oyster filtering might affect disease across entire fisheries, not just in small test plots.

“This study is part of a larger collaboration with the eventual goal of modeling these parasite-host interactions at the fisheries scale,” said study lead author Xuqing Chen.

“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.”

Those models may help predict when oyster filtration can meaningfully slow disease spread, especially as coastal waters warm and parasites gain an edge.

Restoring oyster populations

“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 takeaway is simple. Oysters are not just water cleaners. They may also be quiet disease managers, protecting nearby species in ways scientists are only beginning to measure.

In a warming ocean, that hidden service could prove more valuable than anyone expected.

The full study was published in the journal Ecology.