On The Trail Of Robo-crabs

Device: Scientists outfit female crustaceans with tiny computers to learn more about their migrations in a effort to boost their numbers.

Medicine & Science

December 08, 2003|By Howard Libit | Howard Libit,SUN STAFF

What do you call an adult female crustacean with a tiny computer strapped to its back?


Scientists hope the backpack-toting animals will provide crucial insights into the life cycle of the Chesapeake Bay's female blue crabs and a boost to a population struggling to rebound after decades of overharvesting.

Specifically, the robo-crabs are answering what seem like two simple questions: After female crabs mate in the upper Chesapeake Bay, when do they start heading south? And how do they travel?

Finding that information - which could lead to adjustments in fishing sanctuaries for spawning females - has required more than a bit of creativity: The live, adult females are transformed into robo-crabs by strapping miniature computers to their backs. The 4-centimeter devices, built in a lab at North Carolina State University, contain instruments that record such data as water temperature, salinity and depth.

That's what happens every six minutes as the females make the long journey from the mating waters of Maryland to the spawning grounds of Virginia.

"With just $30 of parts, we can collect a wealth of information," said Thomas G. Wolcott, a professor of marine, earth and atmospheric sciences at North Carolina State. "We can take the data and basically re-create the path they take as they migrate down the bay."

The robo-crabs - a term Wolcott disavows but was coined by other scientists at the Smithsonian Environmental Research Center and National Oceanic & Atmospheric Administration - represent something of a technical compromise between the two most common methods of tracking animals.

Most of the time, scientists fix simple identifying tags to hundreds of animals, and the tags are returned when the animals are caught. Scientists rely on fishermen to record where they tags were recovered, and usually offer a few bucks for each tag to make it worthwhile for the fishermen. It's a low-cost, low-tech way to track population movement and size with three pieces of information: where the animal was released, where it was caught and how long it was loose.

For minute-by-minute records of animal activities, scientists use radio tracking devices, a process known as biotelemetry. But the gadgets are expensive, and for small marine animals, scientists often have to stay nearby on boats to keep a running log of data. Larger, more powerful transmitters that send data through satellites aren't feasible for most crab experiments.

Enter Wolcott, who fancies himself a "gadgeteer" and studies animal behavior and interaction with his wife, Donna, also a professor at North Carolina State. He decided to invent something between those extremes specifically for blue crabs.

"It's call the focal animal approach," Wolcott said. "We got lots of information from a few individuals to find out what they're actually doing."

Scientists knew that after reaching maturity, crabs typically mate in the upper bay. Then the females travel south into the spawning grounds along Virginia. Having only one partner, they store the sperm and use it over and over to produce batches of fertilized eggs, sometimes for three years or more.

But researchers wanted to know more about the migration. Do the crabs make the journey all in one year, or stop to "winter" in the bay by burying themselves in the mud? How do crabs travel - by crawling along the bottom or rising up during ebb tides to catch an easy, floating ride toward the mouth of the bay?

And, perhaps most significantly, what path do the crabs tend to follow? Advances in technology during the past two decades have enabled watermen to target more females in deeper waters. Knowing where the crabs travel is crucial infomration for establishing sanctuaries to protect the migrating females from harvest just as they're about to reproduce.

About 150 mature female crabs were collected each year. Walcott's devices - which he calls dataloggers - were then attached to the crabs' backs. Last year, Wolcott used wire to bind the dataloggers to the crabs' lateral spines, but he switched to nylon cable ties this year because they were easier to manipulate. The crabs were then released along the east and west sides of the bay near Annapolis.

While it's under water, the datalogger - which has a real-time clock - takes a reading every six minutes. First, it determines whether the water is warm enough for the crab to be active - at least 10 degrees Celsius, or about 50 degrees Fahrenheit.

Whenever the temperature is right, the datalogger polls its other measuring devices. For salinity, it relies on a small circuit to check electrical conductivity - water with higher salinity conducts more easily. Depth is checked by testing water pressure.

Because global satellite positioning equipment doesn't work underwater, Wolcott says, he uses the other data as a rough substitute to track a crab's path.

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