Can Oysters Save the Crabs?

Probably not, but they might help a little on the margin. A study via the Bay Journal, Oysters reduce spread of disease among blue crabs, study shows.

With no wind to speak of, the water was calm along a pier on Virginia’s Eastern Shore. And with no clouds to speak of, the sun beat down on Xuqing Chen as she filled a mesh cage with oysters on a hot summer day in 2021. It was one of many days in the field for Chen, then a graduate student at the Virginia Institute for Marine Sciences (VIMS), as she took part in a study that concluded oysters likely do more than improve water quality. They can also save the lives of blue crabs — by feeding on a waterborne parasite that is often deadly to crabs in the saltier parts of the Chesapeake Bay and coastal Eastern Shore. “Our study shows that [oysters] actually have a hidden ecosystem service that hasn’t really been well studied,” said Jeff Shields, a VIMS professor and principal investigator for the study.

The researchers examined how oysters could filter out the parasite, Hematodinium perezi, which kills countless blue crabs every spring through fall. According to other VIMS research, the loss to the Virginia fishery may exceed $500,000 per year.

The H. perezi is most lethal to blue crabs during the summer and is found mostly in salty coastal bays, but the parasite is also present at the mouth of the Chesapeake. Crabs infect each other as they congregate in their underwater grass habitats.

 

The study, published in January in Ecology, a journal of the Ecological Society of America, was designed to test the hypothesis that oysters are reducing the parasites by feeding on them.

Researchers put parasite-free juvenile crabs in the middle compartment of a rectangular mesh cage, sandwiched between live oysters in the outer compartments. The bag was then lowered into the water in a modified crab pot for a prescribed period. Another bag, with no oysters in it, was simultaneously deployed. A third bag was deployed with the crabs sandwiched between oyster shells only — to rule out the possibility that shells alone might benefit the crabs.

The team placed the bags in Cobb Bay, South Bay and in Oyster, VA, for one to two weeks at a time. After nine trials between 2017 and 2023, the researchers tested almost 2,500 crabs.

The research team also replicated the experiment in a lab where they could control more variables and determine if the oysters were eating the parasites as they left the infected crabs.

The 10 trials in the lab showed that the oysters removed 60% of the parasites from the water per hour on average. In the field, where there are more variables, the crabs sandwiched between oysters were almost one-third less likely to become infected.

So, oysters can filter out and eat the Hematodinium parasite that causes disease in Blue Crabs. That is entirely reasonable since Hematodinium is actually an algae, and that's most of what oysters eat in the normal course of business. It is then, not surprising that in lab conditions, and field conditions manipulated to maximize the amount of oyster filtering around captive crabs, that oysters may decrease the infection rate. Unlike, many experiments where artificial conditions may not be fairly replicated in the field, this one may be in part because crabs tend to prefer oyster beds, because there are lots of things there for crabs to eat, including oysters, which they will crack open and eat. And unlike people, crabs don't 3 inch size limit, preferring the smaller spat as easier to open.