What has an armored body; five pairs of legs; a long, pointed tail; is related to spiders; and has changed very little over the last 360 million years? The horseshoe crab!

Named for its distinctive horseshoe-shaped body, the horseshoe crab (Limulus polyphemus) is found along the Atlantic Coast from Maine to the Yucatan peninsula. This fierce-looking yet harmless creature is valuable both ecologically and economically.

Each spring, shorebirds migrate from wintering grounds in South America to breeding grounds in the Arctic. The birds must stop along the way to feed, refueling before they can complete their journey north.

Delaware Bay is the prime stopover site and the birds' stop coincides with horseshoe crab spawning. Shorebirds like the red knot, ruddy turnstone and semipalmated sandpiper, as well as many others, rely on horseshoe crab eggs to replenish their energy reserves before heading to their Arctic nesting grounds.

Spawning along the coast begins in late April and runs through mid-August, although peak spawning in the mid-Atlantic is from May 1 through the first week of June.

During high tide, horseshoe crabs migrate from deep water to beaches to spawn. The female digs a nest in the sand and deposits 4,000 to 30,000 eggs that the male will fertilize with sperm. At low tide, adult crabs go back into the water but may return again at the next high tide. Horseshoe crab spawning increases on nights with a full or new moon, when gravity is stronger and makes high tides higher.

At the same time, migrating shorebirds arrive to rest and feed along Delaware Bay. Horseshoe crab spawning activity is so high that many nests are disturbed and the eggs surface. Although the beaks of most shorebirds are too short to penetrate horseshoe crab nests, the birds can easily feed on those eggs that have surfaced prematurely.

In the past, horseshoe crabs were harvested for fertilizers and even as food for chickens and livestock. Currently, horseshoe crabs are used as bait for eels, whelks and catfish.

They have also become an integral part of the medical industry. Horseshoe crab blood clots when exposed to endotoxins, poisons released by certain bacteria. A byproduct of horseshoe crab blood, Limulus Amoebocyte Lysate (LAL), is used to test the sterility of injectable medicines, dentistry instruments, needles and even heart valves. About 20 percent of a crab's blood is collected in six to seven minutes and then the animal is returned to the water. About 250,000 horseshoe crabs are bled each year. Mortality averages about 10 percent. LAL is used internationally, and is more accurate, more simple and less expensive than similar tests.

Loss of shoreline habitat affects horseshoe crab populations as well as shorebirds. Sandy beaches are essential spawning habitat for horseshoe crabs and nearshore shallow water habitats (i.e., mud and sand flats) are important nursery grounds for juvenile crabs.

Many activities can reduce or disturb the shoreline areas that horseshoe crabs need for reproduction. Some types of shoreline erosion control structures used to protect property reduce available spawning grounds. These include structures like bulkheads and rip rap. They block access to spawning beaches, eliminate sandy beach habitat or entrap and strand spawning crabs during times of high wave energy.

Coastal states have taken measures to ensure that horseshoe crabs remain an integral part of this fragile ecosystem. In 2001, the National Marine Fisheries Service established the Dr. Carl N. Shuster, Jr. Horseshoe Crab sanctuary in federal waters off the mouth of the Delaware Bay estuary. The sanctuary was created to protect the large spawning population of horseshoe crabs in the Delaware Bay and maintain the superabundance of crab eggs available to migratory shorebirds. The rectangular sanctuary is bounded by Delaware waters to the west and extends 30 miles to the east, to just south of Atlantic City, NJ, and just north of Ocean City, MD.

No commercial harvesting of horseshoe crabs is allowed within sanctuary waters. In addition, the harvest or landing of all horseshoe crabs in Delaware is prohibited between Jan. 1 and June 7. In Maryland, the same rule applies from Dec. 1 to June 7. In Virginia, no horseshoe crab can be harvested within 1,000 feet of any tidal shoreline from May 1 to June 7. Additional regulations apply for other times in the year; harvesters must contact the appropriate state natural resource agency.

No other event illustrates so clearly the interdependence of life on this earth. Hundreds of horseshoes crabs crawl slowly and surely up into the sand to lay eggs. At the same time, thousands of shorebirds alight along the beach to fuel their exhausted bodies, readying themselves for another leg of flight to their nesting areas.

It is a sight to behold and one we should never let ourselves lose.