Researchers discover viruses responsible for peeler crab deaths
New studies are looking for method to identify the sick crabs and separate them from the well ones
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For decades, many who tended to soft crabs around the Chesapeake Bay expected about a quarter of their animals to die before they could be sold.
Some assumed the mortality was the result of the stressful shedding process. Others figured it was because the animals were in close proximity to each other in the tanks during a time of physical weakness - a phenomenon seen in shrimp and other aquaculture operations. And still others thought that something in the enclosed system's water made the crabs sick.
But no one really knew, and no one pushed to find out. A soft crab fetches about three times as much as a hard one. As long as crabbers could net big profits from the remaining animals, they could still make a good living.
It took a crabber with a Ph.D. in sociology and a molecular biologist who spent part of his career studying yeast to discover that a compromised immune system and close quarters were only part of the story. Crabs in the shedding tanks were dying in a most conventional way; they were succumbing to a virus.
The partnership between biologist Eric Schott, a Harvard-trained geneticist, and crab-tender Lee Carrion, a University of Chicago trained sociologist and former special education teacher, has the potential to change the industry. If scientists can help crabbers reduce mortality, they will salvage more of their catch and keep more profits. They also may reach a point where they choose not to catch crabs that won't peel for a week or so, leaving them in the Chesapeake longer.
"What Eric's done has tremendous potential to help this industry, if there's a resulting method to identify the sick crabs and separate them from the well ones," Carrion said.
Schott and his team were not the first to suspect that crabs were dying of a virus. Phyllis Johnson, who worked at the Cooperative Oxford Laboratory in the 1980s, identified several kinds of viruses present in blue crabs. Johnson's work, though, pointed out how much was unknown, and she suggested further research to narrow down which viruses were affecting the Chesapeake's population. But that research wasn't done, and the trail was left cold. That was partly because soft-shell crabs represent only 5 percent of the crab catch, and partly because they're not viewed as sustainable aquaculture.
Schott and his fellow researchers picked up on the hints Johnson left and published a paper on their findings a few months ago in the Diseases of Aquatic Organisms journal. Since then, they have been fielding calls from researchers all over the country. This summer, Schott and his graduate student, Ammar Hanif, will be running a project funded by Maryland's Department of Natural Resources to screen soft crabs for the virus before they enter a shedding system. Schott is in the process of writing grants to secure more funding to continue the research; so many questions, he said, remain unanswered.
Yet for all its major implications, Schott's research had much more humble beginnings. He simply wanted to know why the crabs growing in his lab were dying.
In 2002, the University of Maryland's Center for Marine Biotechnology received $2 million to conduct a pioneering research project. Eventually, it would grow more than 200,000 crabs in its downtown Baltimore laboratory and investigate how they live, where they migrate and how fast they grow. The project eventually cost $15 million and employed a couple dozen scientists at universities around Maryland as well as in Mississippi, Virginia and North Carolina.
Schott was one of the researchers. He had been studying Dermo, a disease that affected oysters. Schott came on board to investigate potential crab diseases, and he began to notice many of the crabs were dying. Water quality wasn't an issue in the hatchery. And the stress of the molting process didn't explain the massive die-offs.
"Over time, we would have half of them die. We didn't know why. We didn't have a clue what was killing them," Schott said. "But in the absence of water quality issues, it's always viruses. And viruses are really hard to figure out."
Once a crab has died, Schott said, its tissues degrade too quickly for study. But if he could identify a sick crab, he thought he might have a chance at figuring out how it got that way. He thought, too bad there are no crabs in aquaculture in the Chesapeake. Then, it hit him; soft crab shedding tanks were aquaculture.
About the same time Schott made his discovery in the lab, Carrion was seeing crabs die in her Dundalk peeler business, Coveside Crabs. Carrion had left a career as a special-education teacher in Fairfax, VA, when she fell in love with Richard Young, a lifelong Baltimore County waterman. Young's dream was to have his own crab business; they bought the Dundalk property in 2005.
Carrion tended to the crabs every two or three hours as they shed to prevent them from hardening again. But no matter what she did, about half of them still died. Young told her that peelers had always died, but Carrion refused to accept that.
"These crabs were in my charge," said Carrion in her thick North Carolina drawl. "I didn't want to be taking care of these animals that were dying. I said, 'what in the world could be killing these crabs?' I just didn't get it."
Carrion and Young added aerators to the tanks. They lessened the population in each one. They moved away from bringing in the "white-line" peelers, which are several days from shedding, so the animals would spend less time in the tanks. The mortality lessened. But, Carrion recalls, "I still couldn't get it fixed."
In the summer of 2007, she saw a posting that Schott was looking for a crab business to help him research crab mortality in shedding operations. Carrion called him right away. Schott recalls that his new partner was not only willing to offer samples and monitor the animals, but even seemed eager to dissect the crabs.
Carrion collected 10 healthy animals as well as 10 that had died. Schott extracted tissue and looked for a double strand of RNA, which would be present in a dead or dying crab if it was infected with a virus. In the animals that had the virus, it was everywhere, and unmistakable. Retroviruses, Schott says, have a distinctive banding pattern. Nothing else looks like them. Of the 10 healthy crabs, six of them had the banding pattern and soon died. Then, Schott injected five healthy animals with the virus, and they all died.
Schott and his colleagues at the Oxford lab then tested crabs in the Bay for the virus. Only about 4 percent had it. The researchers assumed it was more prevalent in the shedding tanks because the crabs are in such close quarters and because the shed compromised their immune system.
The next step will be figuring out what crabbers can do about the problem. Crabbers look at a fin to see when a crab will molt. The color changes from black to white to yellow to pink to red. White crabs will shed in about 10 days; reds will shed within 48 hours. Because of Schott's research, Carrion and Young will only put the pink and red ones in their tanks. They say they will catch the white ones when they're red.
The Center of Marine Biotechnology became the Institute of Marine and Environmental Technology and the faculty was split among three campuses. Schott is now with the University of Maryland Center for Environmental Sciences.
Schott wants to get to a point where crabbers will be able to look at an animal and tell whether it is infected. In the fast-paced world of peeler pens, that will be a challenge.
"One infected crab in your tank of 200 can affect your survivorship," he said. "I think a lot of what we're looking at is more careful screening. But it has to be something you can spot in a second. Even a second may be too long."
Not all peeler operations have high mortality; it depends on the location and the river system.
On Taylor's Island, for example, Mike Willey of Willey's Seafood runs a peeler pen using water pumped in from the Little Choptank and hasn't seen large-scale die-offs. The key, he said, is "we pump in fresh water, all the time."
Even so, Maryland Department of Natural Resources blue crab manager Brenda Davis said she believes Schott's research will be beneficial if crabbers get the message to leave the white peelers alone and wait for them to turn pink.
"The longer the crabs are held, the more likely you are to have problems," she said.
Schott's work is happening simultaneously with other work aimed at solving the problem. The University of Maryland Eastern Shore is looking at multiple unit protein skimmers in the tanks, which remove dissolved organics from the system. Eventually, Schott wants to examine whether crabs caught in the Bay can bring the virus back through the shedding tanks' circulation to creeks where it didn't previously exist.
Jonathan Kramer, the executive director of Maryland Sea Grant, said Schott's work couldn't have happened without both advances in molecular technology and the researcher's own approach to the problem.
"It gives you an entry point to ask the difficult questions," Kramer said of Schott's work. "The fact that he's looking at it is a really good thing."
- Category: Fisheries
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