A tiny, egg-eating worm that lives on blue crabs might seem like a nuisance, but new research suggests it could be a powerful new tool for managing the Chesapeake Bay’s struggling crab population. A recent study from William & Mary’s Virginia Institute of Marine Science, published in the journal PLOS One, confirms the worm’s potential as a reliable biomarker, or a living indicator, that can tell scientists about a female crab’s reproductive history. This comes at a critical time, as recent surveys have shown historically low numbers of blue crabs, increasing the need for innovative management strategies.

The worm, Carcinonemertes carcinophila, attaches to a female crab’s egg mass to feed. While it does not harm the crab’s overall reproductive success, the worm’s presence and appearance can reveal if a crab has spawned before. However, scientists were unsure if the worm could survive the bay’s wide-ranging salt levels. Blue crabs migrate from low-salinity, or less salty, tributaries to high-salinity waters to spawn. The study sought to determine if the worms could withstand these changes. Researchers, led by then-undergraduate student Alex Pomroy, conducted lab experiments in which the worms were subjected to various salinities typical throughout the bay.

The results showed the worms are surprisingly resilient. As expected, they thrived in the high-salinity water, ranging from 20 to 30 parts per thousand (psu), where crabs typically spawn. However, the study also revealed that the worms can easily acclimate to a mid-range salinity of 10 psu and even survive short periods of up to 39 hours in very low salinity of 5 psu. This toughness means the worms can likely remain on their hosts throughout the crabs’ adult lives, from their low-salinity homes to their high-salinity spawning grounds, making them a consistent and dependable tracker.

This confirmation has significant implications for fishery managers. By using the worm to distinguish between first-time spawners, known as primiparous crabs, and repeat spawners, called multiparous crabs, scientists can better understand the bay’s spawning population. Previous VIMS research showed that first-time spawners produce more eggs. By identifying and potentially protecting these more productive crabs, managers can make more informed decisions to help rebuild the blue crab population, ensuring the health of one of the Chesapeake Bay’s most vital economic and ecological species.productive crabs, managers can make more informed decisions to help rebuild the blue crab population, ensuring the health of one of the Chesapeake Bay’s most vital economic and ecological species.

References

• Pomroy, A. K., Schneider, A. K., & Shields, J. D. (2025). Salinity tolerance, hyposaline stress recovery, and survival of the nemertean worm, Carcinonemertes carcinophila (Nemertea) in relation to its host, the Atlantic blue crab, Callinectes sapidus. PLOS One, 20(7), e0326493. https://doi.org/10.1371/journal.pone.0326493

• Wallace, J. & William & Mary. (2025, July 11). A parasitic worm may help rebuild blue crab populations in the Chesapeake Bay. Phys.Org; William & Mary. https://phys.org/news/2025-07-parasitic-worm-rebuild-blue-crab.html