WASHINGTON — A parasite that is almost always deadly to blue crabs is affecting the Chesapeake Bay’s young crab population, says Jeff Shields, a scientist with the Virginia Institute of Marine Sciences in Gloucester Point, Virginia, who is studying its effects.
“One parasite in particular is called Hematodinium. It infects blue crabs in high salinity waters and within 30 to 40 days they die,” says Shields, a disease ecologist.
“In adults we don’t see it as much. It’s certainly there — 10 to 20 percent. But in juveniles we’ve found it as high as 80 percent,” Shields says.
The spawning-age female crab numbers in the bay are already lower than it’s been in more than a decade despite conservation efforts, according to the Chesapeake Bay Program.
The number of juvenile crabs last year was a record low, Maryland’s Department of Natural Resources says.
Little is known about the life cycle of Hematodinium and how it infects blue crabs and other crustaceans, but Shields has received grants from the National Science Foundation and the National Institutes of Health Ecology and Evolution of Infectious Diseases Program to study it.
“Over the years we’ve seen what looks to be good recruitment of juveniles, but then the recruitment pattern fades out,” Shields says.
The 2014 Chesapeake Bay Blue Crab Advisory Report says that the Chesapeake Bay blue crab stock is “currently depleted, but overfishing is not occurring.”
Shields believes managers working to improve blue crab populations should factor in deaths by disease when deciding catch limits.
Blue crabs that survive the Hematodinium infection have an altered taste and texture. Shields has eaten them and says they’re mushy.
“As far as we know, this particular parasite is not a problem for humans,” he says.
While Shields believes infected crabs are safe for humans to eat, he says there is a “real problem” with Hematodinium in snow crabs from Canada. Shields has been publishing research on that.
“In Canada it causes a disease called bitter crab disease and it’s like biting into an aspirin, it’s very unpalatable,” he says.