Mass Appeal

For a million migratory birds, the annual spectacle of horseshoe crabs massing in Delaware Bay is a life-saving event

  • Leah Barash
  • Jun 01, 1993
Consider the mystery of it all: For thousands of years, triggered by an indistinct signal, throngs of horseshoe crabs have emerged like clockwork from the sea to breed and lay eggs on the shores of Delaware Bay. And with exacting regularity, hungry shorebirds that have flown from wintering grounds a continent away arrive just in time to feed on the crabs' bounty.

"Timing is everything." says Brian Harrington, an ornithologist at the Manomet Bird Observatory in Massachusetts. "And in this case, it means continued survival for several species."

Harrington is among a growing number of scientists who have witnessed one of the nation's greatest natural spectacles: For four weeks beginning in early May, the beaches around Cape May, New Jersey, and eastern Delaware teem with millions of spawning crabs and flight-weary birds. For the birds, the horseshoe massing provides a critical food source. "There's no question that Delaware Bay is the most important stopover for these birds on their way to the Arctic to nest," says Harrington, who has studied shorebird migrations from Latin America to Canada for ten years.

For the crabs, the bay provides the most ideal nesting conditions along the entire East Coast of the United States. Increasingly, scientists studying the creatures are finding reasons horseshoe crabs are important to people as well as birds. In recent years, the crabs inadvertently have helped researchers better understand a number of human health questions.

Often referred to as a living fossil, the horseshoe crab has existed essentially unchanged anatomically for at least 200 million years. Not a true crab (it forms its own order-Xiphosura, meaning "sword-tailed animal"), the horseshoe crab is an arthropod, more closely related to spiders, scorpions and ticks.

In the sixteenth century, a British naturalist described the creature as the "horsefoot" crab, a term which later was changed to "horseshoe." Early European settlers in North America found Native Americans using the hard carapace, or shell, of the horseshoe crab for a food and drink container and to bail out canoes. They used the animal's sharp pointed telson, or tail, to tip fish spears. Later, farmers ground up horseshoe crabs for fertilizer and hog feed.

Only four species of horseshoe crabs exist; three are Asiatic, ranging from Japan through Southeast Asia to India. The fourth species ranges intermittently on the Atlantic Coast from Maine to the Yucatan Peninsula on the Mexican Gulf Coast. Its largest spawning population-numbering possibly as high as two million-lives within Delaware Bay and ranges outside onto the nearby continental shelf.

The bay, with more than 140 miles of shoreline, has the longest, uninterrupted stretches of sandy beaches along the East Coast. Adjacent shallow areas provide critical nursery grounds and food supplies for hatchlings. "That's why the area is such an optimum place for the crabs to lay eggs," observes Carl Shuster, a biological oceanographer with the College of William and Mary's Virginia Institute of Marine Science, who has followed horseshoe migrations for nearly 50 years.

The spectacle begins in early spring, when mature horseshoes move en masse toward shore. "No one knows for sure what signals this migration," says Mark Botton, a Fordham University biologist who has been studying the crabs for 15 years. Scientists believe that increasing daylight hours during spring and certain phases of the moon are factors that trigger the animals' spawning activities.

Resembling miniature armored tanks, the 20-inch long, 2-pound males usually reach the shallow water at the foot of the beaches first and await the arrival of the females. As the larger females come ashore, the males jockey for position; at times, six or more males may surround one female. The successful suitor attaches himself to his mate with two front pincers, called pedipalps (appendages unique to males), and the female pulls him up the beach close to the high water mark. There she digs several nests about 6 inches deep and deposits some 4,000 eggs in each hole. The male then fertilizes the tiny eggs, which are each only about one sixteenth of an inch in diameter. Mission accomplished, the pair eventually returns to the water.

Horseshoe crabs are relatively slow-growing animals. Females do not reach sexual maturity until about ten years of age, and both sexes can live as long as 18 years. "In one season, a mature female can lay about 20 clusters, so that's roughly about 80,000 eggs per female," says Botton. No one knows how many of these eggs successfully hatch, but Shuster estimates that perhaps less than one out of 130,000 eggs produces an offspring that survives to adulthood. "From an evolutionary standpoint," adds Botton, "it makes sense to lay a lot of eggs if you come to the beach at all."

Often, a female deposits eggs on a beach already loaded with incubating eggs. In his studies, Botton collected core samples from several Delaware Bay beaches and found enormous densities of horseshoe crab eggs-as many as a half-million eggs under a square meter of beach in some places. As a result, a female's digging frequently forces older egg clusters to the surface. This activity, says Shuster, "is almost as if the horseshoe crabs are setting the table for the birds."

When looking for eggs, migrating birds seem to cue in on shoreline discontinuity-areas where there are jetties or sand spits located at the mouths of creeks. Eggs washed into the water tend to aggregate anywhere there is an obstruction to the longshore drift.

One such place is the stone jetty at the north end of Reeds Beach, about 14 miles north of Cape May Point on the New Jersey side of the bay. "The jetty tends to trap sand," says Botton, "and it also acts as a trap for eggs."

The shorebirds' journey also begins in early May, when hundreds of thousands of red knots, ruddy turnstones, sanderlings, semipalmated sandpipers, dunlins, short-billed dowitchers and at least a dozen other migratory species fly nonstop from their winter feeding grounds on the southern shores of South America to the northern coast of Brazil. After a short stop to rest and feed, they again fly nonstop to the eastern U.S. coast-a trip which takes 60 to 80 hours.

In a phenomenon barely studied until recent years, the birds-seriously depleted by their long journey-stop at Delaware Bay and take advantage of the horseshoe crab eggs. Early in May, as many as one million migratory shorebirds gorge themselves on some 150 tons of crab eggs. Bird numbers in the bay peak the third week in May, then rapidly fall. A month later, the migrants have all left, having refueled and added considerably to their fat reserves. Many have even doubled their weight.

Time is of the utmost importance, and each bird spends no more than 10 to 14 days "tanking up" in Delaware Bay. Breeding grounds in the Canadian Arctic beckon, and migratory shorebirds face keen competition for territory in which to mate, nest and raise their young. Birds that arrive too late in the area do not breed successfully because all the nesting sites have been taken already.

In addition to providing fuel reserves for flight, says Brian Harrington, "The resources in Delaware Bay are crucial to the beginning of the nesting period." A male faces high energy demands in terms of establishing and defending territory. A female has to form eggs, and a clutch of eggs can be 50 or 60 percent of her body weight. "A good chunk of that energy needed to form the eggs probably comes from the fat she picked up in Delaware Bay," notes the ornithologist.

As the birds head north to Canada, the crabs leave the beaches after completing spawning and move into areas of deeper water. Though the crabs retreat as the tide recedes, wave action often turns the creatures over, which can mean eventual death for a horseshoe crab. If the animal escapes having its book gills ripped out by gulls and other kinds of predators, the gills may still dry out if exposed to the air for too long, ensuring the crab's demise.

During the spawning season a few years ago, Botton and a colleague, biologist Bob Loveland of Rutgers University, conducted a study on the probability of horseshoe crabs being overturned and stranded on the beach. They found the chances were considerable and estimated that nearly 200,000 of the crabs died on the New Jersey and Delaware shores. "That's 10 percent of the estimated spawning population," says Botton. "It's a risky proposition to come ashore."

The animal originally was named Limulus polyphemus after the mythological Greek cyclops. In fact, the horseshoe crab has nine eyes and a number of additional photoreceptors along its telson. The two oval lateral eyes (one on either side of its shell) are the largest and have a somewhat simple construction.

For several years now, neurophysiologists have been examining these lateral eyes of Limulus, which function much like most vertebrate eyes. "We're trying to understand how the human eye and the brain work together to give us vision," says Robert Barlow, a Syracuse University neuroscientist. "And we feel there is a good chance of answering this question with the visual system of the horseshoe crab."

Probably the most important medical use of the horseshoe crab involves its blood. The animal's white blood cells contain a coagulating agent called lysate. This material is used to manufacture Limulus Amoebocyte Lysate (LAL), which detects fever-inducing substances called pyrogens. "All pharmaceutical companies must test any drug that is injected into the human blood stream for pyrogens, so they use LAL taken from the crabs to determine if their products are potentially dangerous," says Benjie Swan, a Cape May biologist who bleeds horseshoe crabs for a lysate-producing company.

Bleeding horseshoe crabs is a process that does not harm the animals. Technicians take about 12 ounces of blood, which turns royal blue when exposed to the air due to a copper binding agent. The crabs are returned to the water after a single collection.

Though horseshoe numbers seem to be stable in Delaware Bay, some experts worry about increasing demands for the animals and potential threats to their habitat. Fishermen use the crabs, especially females with eggs, as eel bait. In recent years, the demand for eels on the dining tables of Europe and Japan has spurred the development of the eel industry in this country to new heights.

"Last summer I asked a Waterman at the Cape May ferry if he had found any horseshoe crabs in New Jersey," says Carl Shuster. "He took me to his truck and the whole back of it was packed with horseshoe crabs, mostly females."

Bulkheads and other shoreline stabilization structures destroy horseshoe crab spawning areas. In addition, Delaware Bay has one of the highest levels of oil-tanker traffic on the East Coast. A single accident during crab spawning season, while the birds are feeding in the area, could be devastating.

Yet for now, the spectacle continues each spring with remarkable regularity, much as it has for thousands of years. And as it unfolds, scientists will be there, exploring the mystery of it all.

Associate Editor Leah Barash visited the Cape May area of Delaware Bay last spring and is certain that the no-see-urns outnumber both the crabs and the shorebirds. Underwater photographer Fred Bavendam lives in New Hampshire.

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