Horseshoe crab population drop worries scientists

Last year's count was 250,000, down from 1.2 million in 1990

July 14, 2002|By Stevenson Swanson | Stevenson Swanson,SPECIAL TO THE SUN

SOUTH BOWERS, Del. - In the pitch-darkness of a new-moon night, the couples on the beach are clinched in a tight embrace worthy of the wave-washed love scene in From Here to Eternity.

Bristling with claws and a spiny tail, these amorous pairs of horseshoe crabs are no Burt Lancaster and Deborah Kerr, that movie's passionate pair, yet they are obeying their eternal instincts, burying eggs in the sand as their ancestors did before the age of the dinosaurs.

But the number of crabs that scuttle out of Delaware Bay to spawn at this time of year has been dropping for more than a decade, raising troubling questions about the future of an extraordinary species that has become increasingly important to medicine as the source of a bacteria-detecting chemical.

In 1990, an annual horseshoe crab census counted about 1.2 million spawning crabs on the Delaware and New Jersey beaches of the bay, home to the largest concentration of horseshoe crabs in the world. Last year, the census mustered fewer than 250,000.

Similarly steep declines have been reported from Cape Cod, Mass., to Florida.

"Years ago, you couldn't walk here," said University of Delaware marine biologist Bill Hall, one of the census organizers, as his flashlight picked out clumps of horseshoe crabs scattered along the water's edge. "There would be solid crabs down the length of the beach. It was quite an orgy."

Web of relationships

Long dismissed as a "garbage" species with little value to mankind, the horseshoe crab is now at the center of a web of economic and ecological relationships.

Migrating shorebirds depend on horseshoe crab eggs as they fly north to their nesting grounds. The crab's medical and scientific uses have saved lives and resulted in a Nobel Prize. And commercial fishermen harvest hundreds of thousands of the horseshoes to use as bait, the likely cause of the crab's precipitous decline.

Today's four species of horseshoe crab trace their lineage back nearly 400 million years, roughly 150 million years before the first dinosaurs appeared. As other species came and went, horseshoe crabs changed little. Fossils, including specimens from Illinois coal veins, are remarkably similar to the helmets-with-tails specimens that are found along the Atlantic seaboard and in the western Pacific.

Carl Shuster, a professor at the Virginia Institute of Marine Science who has studied the species for 55 years, attributes the horseshoe crab's success to the fact that, in a world of specialization, it is a generalist. Worms, clams and dead fish all suit its diet, and it can live in a wide variety of conditions, including highly polluted water.

"If you think of life in terms of a game of chess, they've never put themselves in a checkmate position," Shuster said.

Not true crabs

Until recently, mankind had little use for these living fossils, which are not true crabs but are more closely related to spiders and scorpions. Indians used the stiff, spiny tails as lance points, and farmers once ground them up for fertilizer.

Medical uses for horseshoe crabs grew slowly starting in the 1950s, but demand exploded in the 1990s, when American commercial fishermen began to respond to an increased domestic and foreign appetite for eel and conch, both of which are irresistibly attracted to traps baited with horseshoe crabs.

Now, scientists and fisheries managers are getting serious about saving these ancient mariners from the kind of drastic population crash that devastated such prized fish as striped bass and cod. That possibility makes tracking the number of spawning horseshoe crabs an important part of the effort to protect them.

Answering a primordial call, the crabs come ashore to spawn when tides are highest and the sun is warm enough to incubate the eggs without baking them. That's when Hall and other marine researchers conduct the census at 24 beaches on both sides of the bay.

As a lumbering C-5A military cargo jet climbs into the night sky from nearby Dover Air Force Base, Hall quickly paces down the beach, counting the number of males and females at 10-meter intervals.

Females are larger

The sexes are easy to distinguish. Female crabs, which can produce as many as 90,000 eggs a year, are much larger than males. One male rides atop each female, spraying the eggs with sperm, while other males skitter to and fro nearby.

"This one's a real lady of the night," Hall said, pointing to the scratches at the bottom of one female's shell. The marks were made by males during previous matings.

The spawning season overlaps with the arrival of an estimated 1 million migratory birds, famished by the flight to northern nesting grounds.

The annual abundance of BB-size, pearly green horseshoe crab eggs has made Delaware Bay the avian equivalent of an interstate rest stop for a dozen kinds of birds, including sandpipers, sanderlings, ruddy turnstones and red knots, which fly a round trip of 20,000 miles from Tierra del Fuego in Argentina to the Arctic tundra and back every year.

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