Unlike striped bass, the blue crab or canvasback duck, the horseshoe crab is not the first species that comes to mind when people think about the Chesapeake Bay.

It is not even a true crab; a survivor of a largely extinct fossil class dating back more than 360 million years, it is more closely related to spiders and mites.

Historically, the best use people could think of for the horseshoe crab was to grind it up for fertilizer.

In the late 1800s, as many as 4 million horseshoe crabs a year were pulled from the Bay and coastal waters. For decades, they were chopped up and put on fields as fertilizers or fed to chickens and cows. That stopped when people complained about the stench and synthetic fertilizers were developed.

After the 1940s, no one wanted the horseshoe crab for much of anything.

But lately, things are changing. In fact, the Maryland Department of Natural Resources is asking for the public's help in developing a new survey to determine whether there are enough horseshoe crabs to go around.

In recent years, the horseshoe crab has emerged as an important bait species, again boosting its economic value. At the same time, it's become clear that the horseshoe crab is important to migratory shorebirds, rare turtles - and even human health.

At least 20 species of migratory shorebirds, including turnstones, sandpipers, and plovers stop along the Bay and mid-Atlantic beaches for about two weeks during their northward journeys to eat crab eggs and build up their body fat before continuing. Delaware researchers have estimated the birds consume about 320 tons of horseshoe crab eggs.

Eggs from the horseshoe crab are also an important food for the juvenile loggerhead turtle, a threatened species that uses the Bay as a nursery area. A variety of finfish also eat their eggs.

Horseshoe crabs are particularly valuable for medical research as well, having been used in eye research and in the development of surgical sutures and wound dressings. Their blood is used to detect bacteria in drugs, a test that must be done on every manufactured drug. Blood from about 200,000 horseshoe crabs is drawn via hypodermic needle each year. The animals are released alive after the procedure, though it's estimated that their mortality is about 10 percent higher than for unbled animals.

And increasingly, horseshoe crabs are sought as bait for eels, conch and catfish. The bait harvest averages more than a million pounds a year - about double the annual catch in the late 1980s. In fact, last year summer flounder fishermen did not catch their quota because they found the horseshoe crabs to be more lucrative, said Tom O'Connell, a fisheries biologist with Maryland DNR.

Today's catch is less than what was taken in the early part of the century when the harvest may have been about 15-20 times greater. But, O'Connell said, "we kind of think the stocks were heavily fished back then, and it's taken 20 or 30 years to rebuild to where they're at now."

Concern that the stock may recently have begun another decline comes largely from Delaware where both a beach survey of spawning horseshoe crabs and a trawl survey of adults have shown drops in recent years.

Horseshoe crabs are found along the Atlantic Coast from southern Mexico to Maine, but about 90 percent of the population is located between Virginia and southern New Jersey.

Because horseshoe crabs take about a decade to mature, they are particularly vulnerable to overfishing. In addition, O'Connell said, egg-bearing females seem to be the most attractive bait, making them a prime target. "When they target females, that's an additional pressure," he said.

Also, there are concerns that the loss of beaches to development and bulkheading has reduce spawning habitat. The Japanese horseshoe crab became an endangered species largely because of loss of spawning habitat.

Because of concern about the crab, the Bay Program developed a fishery management plan for the species last year which called for gathering more information about the size of the population needed to support the horseshoe crab's role in the fishery, food web and medical research.

As part of that, the Maryland DNR last year began searching beaches in the Bay and along the coast that are used by spawning horseshoe crabs. When enough beaches are identified, biologists hope to develop an annual survey and index that will allow them to estimate the population size and health.

The extent of horseshoe crab spawning in the Chesapeake is largely unknown. They spawn on beaches at night during high tides, peaking on the new and full moons. Many small beaches in the Bay are only easily reached by boat, O'Connell said, making them difficult to check.

"So there may be a lot of spawning going on, but they're in small areas throughout the Bay," he said. "In Delaware, they have miles and miles of beaches with easily observable spawning aggregations."

Last year, horseshoe crabs were found spawning on beaches from just north of the Bay Bridge near Annapolis to the coastal bays of Maryland. Spawning took place from mid-May through early August, peaking during the full moon and evening high tide in June.

With the public's help, O'Connell hopes that more spawning beaches can be identified this year. "We're trying to encourage more volunteer people to go out and survey beaches because we're limited with staff," he said.

People who have seen horseshoe crabs spawning or have information that may help in developing the survey are asked to contact O'Connell at (410) 974-2241.