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Living Resources: Horseshoe Crabs

More closely related to spiders than crabs, horseshoe crabs, Limulus polyphemus, are one of the oldest creatures on the planet, having been around for more than 300 million years.

The Delaware Bay is home to the largest population of spawning horseshoe crabs in the world.

Crabs & Shorebirds: A Vital Connection
 Horseshoe crabs line the shoreline as they prepare to lay their eggs in the spring. Photo by DRBC.

Every spring, usually starting in May, warming waters stir the crabs from the depths of the Delaware Bay and Atlantic Ocean to the sandy coastlines of Delaware and New Jersey.

At the high tides of the full and new moons, female horseshoe crabs come ashore to lay their eggs.

Waiting at the tide line, a male grabs hold of the female’s shell.

The female digs a shallow nest in the sand and lays up to 20,000 small, olive-green eggs. Next she drags the male over top to fertilize the eggs. Then they cover the eggs with sand and return to the water.

After two to four weeks, the 1/8 inch-long juvenile crabs hatch, dig to the surface, and head for the water. Feeding on tiny worms, clams and dead fish, the young crabs continue to grow and molt until they reach sexual maturity around 10 years of age. Females typically attain larger sizes than males. It is believed that they live up to approximately 20 years.

But, back on the beach, while many eggs hatch, others lay exposed, the waves at high tide having washed away much of the sand that covered them.

And, thousands of miles away another biological clock is ticking.

Red knots, ruddy turnstones, sanderlings, and semi-palmated sandpipers are already in flight, leaving behind their wintering grounds in Central and South America -- the mudflats of Surinam, the rocky nooks at Tierra del Fuego, the meadows on the Argentine Pampas.

They're winging some 7,000 miles towards the bay and the little green eggs which are now crucial to their survival. Depleted of fat reserves on arrival, many birds will almost double their body weight during their two-week stopover along the Delaware Bay.

They then depart on the next leg of their journey -- a 2,000-mile, non-stop flight to their Arctic breeding grounds.

By late June, the shorebirds will be nesting on the thawing tundra.

Because the Delaware Bay is the principal breeding grounds for American horseshoe crabs on the East Coast, it is also among the largest staging areas for shorebirds in North America.

And it is unique in that there's only one main course on the menu: the little green eggs.

Destroy the horseshoe crab's habitat and a vital link in the migratory chain would be broken, and thousands, perhaps millions, of shorebirds endangered.

Flyer: Little Green Eggs (pdf)

Importance to Humans

Much of what we know about the human eye and how we see began over 50 years ago with studies of the horseshoe crab’s large, compound eye.

Researchers have also studied the horseshoe crab’s chitin, a cellulose-like compound in its shell; chitin, when coated on sutures and burn dressings, increases healing time.

Most importantly, in the 1960s, it was discovered that the horseshoe crabs’ copper-based, blue blood contains a special clotting agent which attaches to bacterial toxins.

Today, Limulus Amebocyte Lysate (LAL), which is manufactured from horseshoe crab blood, has become the worldwide standard screening test for bacterial contamination.

Every intravenous drug, vaccine and surgical implant, such as pacemakers and prosthetic devices, must be tested using LAL.

Horseshoe crab blood has recently been found to be useful in cancer research as well.

Blood from horseshoe crabs is obtained by collecting adult crabs, extracting a portion of their blood, and releasing them alive. Following bleeding, most crabs are returned to near the location of their capture.

It is estimated that over 400,000 crabs are harvested annually for biomedical purposes, and the estimated crab mortality during and after the bleeding process is anywhere from 10-30%.

Scientists are researching whether a synthetic substitute can be used as a viable alternative.

Horseshoe crabs riding the waves to shore. Photo by DRBC. The life cycles of horseshoe crabs and red knots are closely connected. Photo by Mark Binder. Horseshoe crabs crowd a New Jersey beach in May. Photo by DRBC.
Horseshoe crabs riding the waves to
shore. Photo by DRBC.
The life cycles of horseshoe crabs
and red knots are closely connected.
Photo by Mark Binder.
Horseshoe crabs crowd a New Jersey
beach in May. Photo by DRBC.
Hey Kids - Get Crafty! Make a Horseshoe Crab Hat

Links to Learn More

About the Horseshoe Crab

Atlantic States Marine Fisheries Commission

Conserve Wildlife NJ: Horseshoe Crab

The Horseshoe Crab - Natural History, Anatomy, Conservation and Current Research

National Wildlife Federation: Horseshoe Crab

NOAA: The Remarkable Horseshoe Crab

NOAA: Are Horseshoe Crabs really Crabs?

Sea Grant Delaware: Horseshoe Crab Research

Crabs and Shorebirds

Birds and Crabs (National Public Radio, June 2002)

Delaware Birding Trail

Delaware Museum of Natural History: Shorebirds & Feast on the Beach

PBS NATURE: "Crash: A Tale of Two Species" (PBS, February 2008)

Delaware Division of Fish & Wildlife Shorebird Project

New Jersey Audubon Society's Cape May Bird Observatory

New Jersey Division of Fish & Wildlife Delaware Bay Shorebirds

Partnership for the Delaware Estuary's "Horseshoe Crab and Shorebirds" Fact Sheet (pdf)

U.S. Fish and Wildlife Service - Shorebirds, The Delaware Bay Connection (pdf)

Shorebird Sister Schools Program (A U.S. Fish and Wildlife Service program linking schools, biologists & shorebird enthusiasts along flyways)

Volunteer Opportunities

Horseshoe Crab Tagging

Wetlands Institute

Delaware Center for the Inland Bays

ReTurn the Favor

Delaware Bay Horseshoe Crab Survey

DNREC Delaware Bay Horseshoe Crab Spawning Survey