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Horseshoe Crabs Remain Mysteries to Biologists

Misty Edgecomb
Bangor Daily News (Maine)
June 21, 2002
 
When the new moon rises in June, thousands of prehistoric creatures
emerge from the depths of the sea, their heavy armor clanking as they
clamor over one another in the shallows of Taunton Bay, Maine. There
they mate, leaving hundreds of thousands of green eggs in the sand.
Then, they simply disappear.

Remarkably, this is all modern science can tell about the horseshoe crab in Taunton Bay or anywhere else.



How long horseshoe crabs live, whether they return to the beach of their birth for spawning, why their life cycles seem directed by the moon, where they disappear to for the other 10 months of the year—all these questions remain mysteries.

Somehow, the horseshoe crab has thrived for 500 million years, and Sue Schaller wants to know why.

"You've got an animal that predates dinosaurs by 200 million years, and it hasn't changed much at all. It hasn't had to evolve," said Schaller, a biologist who has studied Maine's horseshoe crabs for the past three years.

One rainy afternoon recently, Schaller and four local volunteers, dressed in slickers and hip waders, are hiking along the shore of Taunton Bay near Shipyard Point in Franklin.

It's the height of the spawning season, and Schaller's crew is peering into the cloudy water, looking for the green-brown, helmetlike shells of visiting horseshoe crabs.

"They vacation on the bay," Schaller jokes.

The biologist finds a mating pair, a small male grasping a larger female in hopes of being nearby when she begins to release her eggs. As Schaller turns the crabs over, they gesture threateningly with long spike-like tails, known as telsons. They flap their hinged shells, and wave the pincher-tipped walking legs and daisylike swimmerets that surround their mouths.

The horseshoe crab is the sort of fanciful, frightening creature that appears more frequently in science fiction stories than in zoology texts.

"I love 'em," Schaller said. "Once you scratch the surface and get over the fear thing, they really are intriguing."

Horseshoe crabs were misnamed centuries ago, when mariners thought the odd creatures' sloped shells resembled horse's feet.

Related to Long-Extinct Trilobites

But horseshoe crabs aren't actually crabs. They're chelicerates, the zoological classification that includes spiders, scorpions and mites. In fact, the 500 million-year-old animal is more closely related to long-extinct trilobites than to any living creature.

Horseshoe crabs can walk along the sea bottom on six pairs of legs, or swim upside down or right side up depending on where they're trying to go.

Schaller is here because Shipyard Point boasts the world's most northerly breeding population of the creatures. The Taunton Bay group is tiny compared to the hundreds of thousands of horseshoe crabs massing on the beaches of Cape Cod and Delaware Bay this month, but that's what makes it an ideal research site. The small population creates the possibility that every individual animal can be tagged and studied.

Horseshoe crabs are rare in Maine because the Atlantic is so cold and the rocky coastline provides few of the shallow, sandy estuaries in which they prefer to spawn, Schaller said.

Last summer, she visited 15 such pockets of horseshoe crab habitat scattered throughout the central and south coast of Maine. With the help of more than 70 volunteers, she began to count individuals and was surprised at the size and health of Maine's population. At the Damariscotta Mills site, 275 crabs were spotted on a single day.

"As you walk along, you don't necessarily see lots of animals, but there are more than we thought," she said.

Crabs Tagged With Plastic Markers

Taunton Bay is the only site where survey animals are being tagged, by drilling a small hole in the shell's outer edge and attaching a plastic marker. More than 1,300 crabs were so identified last summer.

Schaller believes the population could be much larger. Only a quarter of the animals counted this summer sported the bright yellow research tags.

"We're not getting a lot of returns, and we don't know where they're going," she said.

This year's survey was reduced from 15 to seven key sites, because funding from the Maine Outdoor Heritage Program was not renewed. Funded or not, Schaller hopes to continue the survey indefinitely, and answer some of the big questions about horseshoe crabs that have confounded researchers for decades.

Schaller, a former U.S. Fish and Wildlife biologist, has already proved that horseshoe crabs possess the rare ability to regenerate lost body parts in a manner similar to starfish.

In Massachusetts, Delaware and even southern Maine, horseshoe crabs are in high demand as bait in eel pot fisheries—a U.S. $70 million business according to the Atlantic Marine Fisheries Commission.

The crabs are also harvested for their blood, a valuable commodity for medical researchers. A substance extracted from horseshoe crab blood is used in hospitals to detect the presence of bacteria in biomedical equipment.

Commercial harvesters have generally overlooked eastern Maine's small, scattered populations.

Fishing regulations are virtually unheard of, and federal research suggests that larger horseshoe crab populations may be in decline. However, the knowledge of a horseshoe crab's life history, which would be essential for drafting good regulations, just doesn't exist.

With her living laboratory in Taunton Bay, and the ongoing efforts of local volunteers, Schaller hopes to provide that key information.

"It's a backwater kind of place that people don't value, but for wildlife and for fisheries, it's one of the treasures of the coast of Maine," said Steve Perrin, a Bar Harbor naturalist and president of the Friends of Taunton Bay conservation group. "It's kind of an honor to have such an ancient being come here and choose this bay."

Copyright 2002 Bangor Daily News
 

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