If you catch a female blue crab in the Chesapeake Bay — and know where to look — there’s a good chance you might find a tiny, parasitic worm embedded in its spongelike egg mass.
The worm, scientifically known as Carcinonemertes carcinophila, relies on crab eggs to grow and survive.
“They have a really interesting symbiotic relationship, where the worm cannot reach sexual maturity without consuming the eggs of its host, which is the blue crab,” said Alex Schneider, who recently earned a doctoral degree in marine science from William & Mary’s Virginia Institute of Marine Science.
Schneider and two colleagues at VIMS recently published research that gives more insight into the relationship between the two species – and potential implications for managing the blue crab fishery.
Blue crabs have been on a downward trend in the bay in recent years. The population in Virginia and Maryland dipped this year to its second-lowest number since local officials began their annual winter dredge survey in 1990.
Scientists are particularly interested in the number of spawning females in the bay, which can produce more babies to boost the species. Several decades ago, officials overseeing Virginia’s crab fishery started prioritizing the protection of those crabs.
“A lot of the management that we have today is focused on protecting female spawners,” Schneider said.
But, historically, a mix of biological factors has made it difficult to determine a female crab’s age or reproductive potential, she said.
“Because these things are very hard to measure in the field, it's really difficult to get that information from the population and to be able to apply it to management.”
Enter Carcinonemertes carcinophila.
The worms start their lives as larvae, swimming in the bay, seeking mature female crabs. Once they settle on a host and burrow into its egg mass, “they’ll be somewhat limited to that female for the rest of their life,” Schneider said.
In the beginning, the worm is tiny, thin and translucent, imperceptible to the human eye, she said. As it eats the crab’s eggs, it grows larger and a more vibrant pink or red in color, making it easier to spot.
After the crab’s eggs hatch, the worms recede into her gills and stay there until the next round of eggs.
Researchers became interested in the worm as a potential “biomarker” for the reproductive patterns of blue crabs. Its presence serves as a sort of living history, allowing officials to learn about the crabs’ past spawning and future potential.
“That helps focus management on protecting females that may produce disproportionately more offspring than other females,” Schneider said. “And in a management framework that focuses on protecting the spawning stock, that's really important.”
In some ecosystems, these worms are known to eat a large and potentially harmful proportion of the eggs of crabs or other crustaceans.
But Schneider said female blue crabs have disproportionately more eggs and faster embryonic development than most similarly sized crabs. The parasites don’t appear to affect their overall production.
To further confirm the worm as a reproductive indicator, the VIMS team wanted to ensure it could withstand a variety of water conditions, as crabs do.
Researchers carefully extracted the parasites from crabs and provided them with eggs to eat while exposing them to varying levels of water salinity, or saltiness.
Blue crabs can tolerate a wide range of salinity. The new study suggests that Carcinonemertes carcinophila has evolved to do so, too.
Schneider said VIMS is now assessing female crabs for worms as part of its portion of the annual dredge survey.
It’s “a really exciting opportunity to use unconventional research methods to learn more about a fisheries population,” she said.
And don’t worry, she said: These worms don’t make it into the crab parts people typically eat. “You won't encounter them in your crab cake.”
