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Horseshoe Crabs: Ancient Wonders of the Seashore: Part I
By Don Riepe

Each spring, between mid–May and mid–June, thousands of fierce–looking sea creatures crawl from the sea to mate and lay eggs along the sandy shorelines and mudflats of New York City. These harmless animals called horseshoe crabs are not really crabs at all, but more closely related to arachnids (spiders and scorpions). A living fossil, the horseshoe crab evolved long before the dinosaurs with an ancestral heritage dating back to the Triassic Period two hundred million years ago. Currently, four species exist worldwide. One species populates the Atlantic coast from Maine to Mexico with the largest concentrations found in Delaware Bay. Named Limulus polyphemus after the one–eyed giant of Greek mythology, this horseshoe crab actually has nine eyes: one large compound eye on each side of its shell, two small ones in the front center and five light–receptive organs underneath. The other three species of horseshoe crabs are found in the Indian and Pacific Oceans.

During high tides, especially at new and full moons, these “crabs” emerge from the water to spawn. The larger females are usually accompanied by one or more smaller males that attach themselves to her back by specially adapted clasper claws. At the high tide line, the female will dig a nest in the wet sand and lay up to 4,000 tiny greenish eggs. The attached male fertilizes the eggs as they are laid and then both move back to deeper waters to feed on benthic animals such as marine worms, crustaceans and mollusks.

About a month later the eggs hatch out, each one containing a tiny, though tail–less replica of the adult crab. The little crabs will molt their shells several times yearly during the first few years of life and then once yearly thereafter. They reach adult size in about 10–13 years and may live another 7–10 years. Before molting takes place, a new skin forms under the existing shell. The old shell splits open along the front and the crab walks out. The crab then takes in water and digs into the sand. This new skin is stretched larger and hardens around the crab to form a new shell. The molted shells can be found along beaches at any time of year and make nice coffee table or shelf decorations.

The eggs provide a bonanza for migrating shorebirds arriving in New York City from their winter homes in Central and South American. Some birds such as black–bellied plovers and red knots may have traveled several thousand miles across the ocean, making their first landfall in the estuaries of New York and New Jersey. Peak shorebird migration coincides with the peak horseshoe crab egg–laying period. The horseshoe crab eggs provide critical nourishment for many shorebird species as they head to their Arctic breeding grounds. At the Jamaica Bay Wildlife Refuge in Broad Channel, Queens, I have observed these mating rituals and feeding frenzies for many years and still am amazed by the abundance and diversity of participants. Besides red knots, sanderlings, ruddy turnstones and about 20 other species of shorebirds, the egg feast attracts many laughing gulls, glossy ibis and even Canada geese, whose goslings feed on animal matter as well as vegetation.

Snowy egrets join in, not to feed on eggs, but the mummichogs, silversides and other small fish that are taking part in the bountiful melee. Larger predators, such as herring and black–backed gulls will frequently take advantage of overturned horseshoe crabs and peck out their gills, leaving a beach-front strewn with dead and dying crabs. Despite this heavy onslaught, the crabs keep coming ashore, determined to carry out the reproductive urge as they have done for millions of years, long before the advent of Homo sapiens. Horseshoe crab programs are becoming more popular and each spring both Urban and National Park Rangers as well as school groups visit NYC shorelines to tell the fascinating story of this prehistoric wonder, the ageless horseshoe crab. For information about horseshoe crabs, beach cleanup days, and other nature programs in the greater New York City area contact: The American Littoral Society, 28 West 9th Road, Broad Channel, N.Y. 11693. Telephone: 718–634-6467.

Don Riepe is a long–time resident of the Jamaica Bay area, Don is currently a supervisory park ranger for the National Park Service and Director of the Northeast Chapter of the American Littoral Society, a coastal conservation organization.

 

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