Getting the dirt on blue crabs Biology: New research is revealing some of the most intimate details in the life of the crustacean. The findings are coming at a critical time.

July 27, 1998|By Kirsten Scharnberg | Kirsten Scharnberg,SUN STAFF

Armed with high-tech gadgets and waterproof gizmos, scientists are working to expose the smutty and salacious side of the Chesapeake Bay.

Their discoveries are X-rated: Sex, murder and gluttony. Cannibalism. Violent duels.

It's a jungle out there.

"We basically track the feeding, fighting and fornicating," said marine ecologist Anson Hines. "We call it 'Sex and Violence in the Life of a Blue Crab.' "

The groundbreaking -- albeit slightly unconventional -- research at the Smithsonian Environmental Research Center in southern Anne Arundel County is revealing some of the most intimate details about blue crab biology, including population dynamics, sources of mortality, and feeding, fighting and mating habits.

A sample of scientific findings reads like comprehensive crustacean trivia:

How many bites does it take a crab to chew up a Baltic clam? About 200.

How many times do males molt (shed their shells) in a lifetime? Ten to 12.

True or false? Crabs are cannibals. True.

The Smithsonian study has tracked hundreds of blue crabs by attaching tiny, battery-operated sensors to their every part -- from their claws to their mouths to their "unmentionables." After returning them to the wild -- mostly in the Rhode River in southern Anne Arundel County -- the scientists can tell when a particular crab is chewing, fighting, swimming and, well, enjoying the company of the opposite sex.

"The '40s and '50s was when scientists figured out the basic life cycle of the crabs," Hines said. "What we are doing now is determining what it is that a crab does on a daily basis to complete that life cycle."

The research comes at a critical time.

As overfishing and habitat deterioration lead to shortages of bay oysters and finfish, there is an ever-increasing interest in protecting the multimillion dollar crabbing industry.

After more than two decades of government and privately funded research, the Smithsonian scientists may be on the brink of discoveries that could have implications for development practices and fishery management in Maryland.

"One specific little piece of information is just that -- one specific piece of information," said Brenda Davis, a fisheries biologist with the state Department of Natural Resources. "But piece a bunch of them together, and that's when we get the kind of discoveries that change fishing management."

Specific information is just what these scientists -- under the direction of Hines and several North Carolina researchers -- are gathering.

Here are a few of their findings on the creature often dubbed the "emblem of the bay":

On fighting: Beneath the water's smooth surface, a savage civil war rages between the cutthroat crustaceans. Ninety percent of young blue crab deaths come not from rampant pollution or overfishing, but from deadly infighting.

"Basically, we're finding they are cannibals," Hines said. The scientist said the older, larger male crabs prey on younger crabs that have molted and are still soft. In their experiments, Smithsonian scientists anchor soft crabs at different water depths and sit back to observe where -- and by whom -- the small crabs get eaten.

On feeding: As crabs feed on clams, their primary food, they tear into the animal, inadvertently releasing the strong scent of clam juice into the water.

"That releases a plume of odor, and the other crabs come a'swimming," Hines said. As more and more crabs arrive, they become more violent. "They fight to the point where they are no longer efficiently eating."

On molting: When crabs shed their shell, their bodies become soft and vulnerable. The scientists have learned that crabs tend to molt in shallow grassy or wooded areas so they can hide from predators -- mainly other crabs -- until their new shell hardens.

"This means that as we continue to develop our shorelines, tear- ing out the trees and grass, we put more crabs at risk of being killed long before they can mature, get caught and be sold," Hines said. "This could have a very damaging effect on the blue crab population."

On reproduction: Larger crabs appear to release more sperm than smaller crabs, thus allowing them to fertilize more of a female crab's eggs.

With the growing demand to harvest the largest crabs, this could lead to a long-term decrease in the size and number of the male crab population.

This week at the Smithsonian "crab lab" -- a room cluttered with clawed creatures, living and dead -- Hines and Heather Turner, a North Carolina State graduate student, are tracking a female crab as she migrates to the mouth of the bay.

"There's next to nothing known about the migratory habits of female blue crabs," Turner said. "That's our new project.

"It's amazing if you think about it. For decades, the management plan of the bay depended on very little knowledge of the fundamentals of blue crab biology. We're working to change that."

Though it has long been known that female crabs migrate to the mouth of the bay to complete fertilization, the Smithsonian scientists are trying to pinpoint how long the trip takes, how soon the female starts migrating after she has mated, what her primary sources of food are during the trip and whether she travels by riding the high tides.

As Hines plucks a crab from an aquarium, he is quick to see why an outsider would laugh at the image of blue crabs wired with little backpack tracking devices.

"It may seem unconventional," he said, "but it's allowing us to learn things that have always remained unknown and unexplored."

Hines pauses a moment to consider his profession.

"What would we do without these tasty treats?" he joked. "The way I see it, the rule of the profession is this: You must eat your research. So you pick your animal of study very, very carefully."

Pub Date: 7/27/98

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