Horseshoe Crabs | Research & Encyclopedia Articles

The horseshoe crab (Limulus polyphemus) is the American species of a marine animal that is only a distant relation of crustaceans like crabs and lobsters. Horseshoe crabs are more closely related to spiders and scorpions. The crabs have been called "living fossils" because the genus dates back millions of years, and Limulus evolved very little over the years.

Fossils found in British Columbia indicate that the ancestors of horseshoe crabs were in North America about 520 million years ago. During the late twentieth century, the declining horseshoe crab population concerned environmentalists. Horseshoe crabs are a vital food source for dozens of species of birds that migrate from South America to the Arctic Circle. Furthermore, crabs are collected for medical research. After blood is taken from the crabs, they are returned to the ocean.

American horseshoe crabs live along the Atlantic Ocean coastline. Crab habitat extends south from Maine to the Yucatán in the Gulf of Mexico. Several other crab species are found in Southeast Asia and Japan.

The American crab is named for its helmet-like shell that is shaped like a horseshoe. Limulus has a sharp tail shaped like a spike. The tail helps the crab move through the sand. If the crab tips over, the tail serves as a rudder so the crab can get back on its feet. The horseshoe crab is unique; its blood is blue and contains copper. The blood of other animals is red and contains iron.

Mature female crabs measure up to 24 inches in length. Males are about two-thirds smaller. Horseshoe crabs can live for 19 years, and they reach sexual maturity in 10 years. The crabs come to shore to spawn in late May and early June. They spawn the during the phases of the full and new moon. The female digs nests in the sand and deposits from 200 to 300 eggs in each pit. The male crab fertilizes the eggs with sperm, and the egg clutch is covered with sand.

During the spawning season, a female crab could deposit as many as 90,000 eggs. This spawning process coincides with the migration of shorebirds. Flocks of birds like the red knot and the sandpiper eat their fill of crab eggs before continuing their northbound migration.

Through the years, people found a variety of uses for horseshoe crabs. During the sixteenth century, Native Americans in South Carolina attached the tails to the spears that they used to catch fish. In the nineteenth century, people ground the crabs up for use as fertilizer or food for chickens and hogs.

During the twentieth century, researchers learned much about the human eye by studying the horseshoe crab's compound eye. Furthermore, researchers discovered that the crab's blood contained a special clotting agent that could be used to test the purity of new drugs and intravenous solutions. The agent called Limulus Amoebocyte Lysate is obtained by collecting horseshoe crabs during the spawning season. Crabs are bled and then returned to the beach.

Horseshoe crabs are also used as bait. The harvesting of crabs increased sharply during the 1990s when people in the fishing industry used crabs as bait to catch eels and conch. The annual numbers of crabs harvested jumped from the thousands to the millions during the 1990s, according to environmental groups and organizations like the National Audubon Society.

The declining horseshoe crab population could affect millions of migrating birds. The Audubon Society reported seeing fewer birds at the Atlantic beaches where horseshoecrabs spawn. In the spring of 2000, scientists said that birds appeared undernourished. Observers doubted that they would complete their journey to the Arctic Circle.

The Audubon Society and environmental groups have campaigned for state and federal regulations to protect horseshoe crabs. By 2002, coastal states and the Atlantic States Marine Fisheries Commission had set limits on the amount of crabs that could be harvested. The state of Virginia made bait bags mandatory when fishing with horseshoe crab bait. The mesh bag made of hard plastic holds the crab. That made it more difficult for predators to eat the crab so fewer Limuluscrabs were needed as bait.

Furthermore, the federal government created a 1,500-square-mile refuge for horseshoe crabs in Delaware Bay. The refuge extends from Ocean City, New Jersey to north of Ocean City, Maryland. As of March of 2002, harvesting was banned in the refuge. People who took crabs from the area faced a fine of up to $100,000, according to the National Marine Fisheries Service.

As measures like those were enacted, marine biologists said that it could be several decades before the crab population increased. One reason for slow population growth was that it takes crabs 10 years to reach maturity.

Fortey, Richard. Trilobite!: Eyewitness to Evolution. New York: Alfred A. Knopf, 2000.

Tanacredi, John, Ed. Limulus in the Limelight: 350 Million Years in the Making and in Peril? New York: Kluwer Academic Publishers, 2001.

Atlantic States Marine Fisheries Commission., 1444 Eye Street, NW, Sixth Floor, Washington, D.C. 20005 (202) 289-6400, Fax: (202) 289-6051, Email: comments@asmfc.org, http://www.asmfc.org

National Audubon Society Horseshoe Crab Campaign., 1901 Pennsylvania Avenue NW, Suite 1100, Washington, D.C. 20006 (202) 861-2242, Fax: (202) 861-4290, Email: pplumart@audubon.org, http://www.audubon.org/campaign/horsesh oe/contacts.htm

National Marine Fisheries Service., 1315 East West Highway, SSMC3, Silver Spring, MD 20910 (301) 713-2334, Fax: (301) 713-0596, Email: cyber.fish@noaa.gov, hhttp://www.nmfs.noaa.gov

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