
A parasitic worm that lives on the eggs of female blue crabs might actually help scientists and fishery managers protect this iconic species in the Chesapeake Bay.
While the idea of egg-eating worms sounds alarming, new research shows these creatures could be used as a helpful tool to better understand crab reproduction—and possibly rebuild struggling populations.
Blue crab numbers in the Chesapeake Bay have hit record lows, according to the latest Winter Dredge Survey by the Virginia Institute of Marine Science (VIMS) and Maryland’s Department of Natural Resources.
This has caused concern for the many people who depend on the crab fishery for work and recreation.
Scientists are now looking for new ways to support crab populations while also balancing economic needs.
One unexpected source of insight is the parasitic worm Carcinonemertes carcinophila. This worm lives on female blue crabs and feeds on their egg clutches.
While it doesn’t eat enough to severely impact the crab’s ability to reproduce, it changes color and shape after feeding—signs that can help researchers determine if a crab has laid eggs.
This makes the worm a potential “biomarker,” a biological clue that shows whether or not a crab has spawned.
The idea was explored in a study by researchers at William & Mary’s Batten School and VIMS. Led by undergraduate student Alex Pomroy and Professor Jeffrey Shields, the team tested how well the worms could survive in different levels of salinity, which varies widely throughout the bay and its rivers.
While similar worm species can’t survive in waters with less than 20 parts per thousand of salt, C. carcinophila proved much more resilient. The worms did well in both high and mid-salinity water and even survived at low levels—down to 10 parts per thousand. They also recovered from short exposure to even lower salinity levels, like those found in crab migration zones.
That discovery is important because female blue crabs often move from low-salinity waters to saltier areas to lay eggs. The worms’ ability to survive throughout that journey suggests they’re likely present on crabs for much of their adult lives. This means they can serve as reliable indicators of a crab’s reproductive history.
Knowing whether a crab has reproduced is important for managing the fishery.
Crabs that are laying eggs for the first time, called “primiparous” crabs, produce more offspring than older, “multiparous” crabs that have already spawned. Identifying and protecting these first-time spawners could be key to increasing crab numbers in the bay.
Past research from Ph.D. student Alex Schneider and others showed that nearly all crabs spawning in June were primiparous, while later in the season, most were multiparous. Using the worm as a natural tag, scientists can now estimate how many crabs have already spawned—even in winter.
With more studies underway, VIMS is continuing to use this worm to help track crab reproduction.
The researchers hope this new approach will offer clearer guidance to policymakers and help support one of the Chesapeake Bay’s most valuable and beloved species.
Source: William & Mary.