NEWPORT NEWS, Va. (AP) — Marine researchers have spent 15 years trying to nail down the life cycle of a single-celled parasite fatal to blue crabs.
On Oct. 4, they announced they cracked the case.
"Describing the entire life cycle of the Hematodinium was an important breakthrough for us," said Jeff Shields at the Virginia Institute of Marine Science (VIMS) in Gloucester Point. "Having all stages in culture means we can now really start picking the life cycle apart to learn what the organism does and how it functions."
Armed with this new knowledge, researchers hope to stem infections that are a growing concern to wild fisheries and aquaculture operations. The parasite is harmless to people.
The parasite is highly prevalent in crabs in the high-salinity waters of the Eastern Shore, particularly in newly settled juveniles, and far less prevalent in the lower-salinity waters of the Chesapeake Bay, said Shields. Still, all of the crabs that enter the bay must first come through the high-salinity waters at its mouth.
"It would be premature to say much more than that," Shields said. "But we definitely see it in newly settled juveniles, so there's a possibility for it to affect crabs in a wider area."
At the Virginia Marine Resources Commission, spokesman John Bull said they've heard no complaints from crabbers about the mortality rate of the crabs they catch, and no complaints about die-offs in the shedding tanks where they're placed until they molt and become soft-shell crabs.
"Which leaves us to believe this parasite isn't a harvest issue," Bull said. "This is a stock issue."
Shields agreed, but said the parasite may still be causing losses of $500,000 a year to Virginia's blue crab fishery. In Virginia's seaside bays, he said, they found prevalence levels of 50 percent to 70 percent among juvenile blue crabs.
"Imagine a harvest with 50 percent more crabs, and the effect of the parasite becomes quite clear," said Shields.
Blue crabs have long been an iconic feature of the Chesapeake Bay, and efforts to restore them in the estuary after decades of overfishing and degraded habitat have largely been successful.
In April, VIMS announced that the total population of blue crabs in the bay had reached 764 million — an increase of 66 percent since 1993 and a 20-year high. VIMS conducted its annual bayside survey with the Maryland Department of Natural Resources.
According to Bull, the juvenile crab stock in particular "has done fabulously" recently.
"The juvenile stock at the beginning of this year was the highest level recorded in 40 years," said Bull. "So if we're seeing an impact, we're not seeing an impact on the juvenile stock. It is booming. Whether or not it would be booming more without this parasite, that's impossible for us to say."
The Hematodinium was first reported in Virginia's blue crabs in the early '90s, according to VIMS. During seasonal outbreaks, crab mortality can reach 50 percent in crab pots and 75 percent in shedding facilities for soft-shell crabs on the Eastern Shore.
"In late June and early July," Shields said, "we often get comments from watermen about dead crabs in pots. It's kind of ephemeral. Short-lived."
Infections are increasing in frequency in Virginia and worldwide, researchers said. The parasite is also turning up in new hosts, such as shrimp, sand fleas and other swimming crab species.
In China, it was reported by crab growers in 2004 and in aquaculture shrimp operations in 2008. China is the world's largest aquaculture grower.
The VIMS study was funded in part by a grant from the National Science Foundation. More research will be conducted to better understand the parasite's genetics to more readily understand its virulence, transmission routes and other potential hosts.
"We would very much like to see this parasite controlled," said Bull. "That would certainly make our rebuilding job a lot easier."
Information from: Daily Press, http://www.dailypress.com