Horseshoe crab

Looking at the shape of its outside shell, you can easily see why a horseshoe crab (or Limulus polyphemus) got the "horseshoe" part of its name, but what about the "crab"? People once thought the horseshoe crab was a crab, but the animal is, in fact, more closely related to the common garden spider than to a blue crab.

Horseshoe crabs, which feed upon worms, small mollusks, and algae, are a living fossil; they have been literally unchanged for the past 360 million years!

Picture: Young naturalist inspecting a horseshoe crab shell. The carapace was empty. If this was a live animal, picking up by tail could cause injury to the crab. Picture courtesy of Mary Hollinger, NODC biologist, NOAA. To the original photo.

Full-moon romance

When it is time to mate, sometime in the early summer during high spring tides, which are influenced by the full moon, the female horseshoe crabs will come ashore. The males follow, often even hanging on to the females' backs with special hooks in their first pair of legs. The female horseshoe crab digs a hole in the sand and deposits her strands of dark greenish eggs. The male horseshoe crab then releases his sperm to fertilize the eggs.

Picture: Mating pair of horseshoe crabs. Picture courtesy of Mary Hollinger, NODC biologist, NOAA. To the original photo.

The importance of the horseshoe crab

Many animals depend on the horseshoe crab. For example, they are an important part of the diet of juvenile loggerhead turtles, which summer in and around the Chesapeake Bay. The crab's eggs and larvae are also a significant food source for commercially important finfish and shellfish.

As mentioned in the Watershed Radio program, many migrating shore birds time their migrations so that after flying from South America, they arrive at Delaware Bay at the same time as the horseshoe crabs are laying their eggs. The birds depend on the eggs and larvae to complete the last part of their journey to their northern nesting areas.

Picture: Red Knot, a migratory bird. Migratory birds, like the Red Knot in this photo, depend on the eggs and larvae of the horseshoe crab. After eating the eggs and larvae, the birds are ready for the last part of their long trip to their northern nesting grounds. Picture courtesy of Peter S. Weber.

Humans also rely on the horseshoe crab and have found many commercial uses for the animal. Lysate, an extract from their blood, is used worldwide in cancer research and to test for bacterial contamination. Horseshoe crabs are also commercially used as bait in eel and conch fisheries. The high demand for conch and eel, however, has led to an overharvest of the horseshoe crabs in recent years.

References and further reading

The horseshoe crab. All you ever wanted to know about the horseshoe crab. Excellent drawings and explanations.

The National Audubon Society has been working to stop the overfishing of the horseshoe crab and achieve sustainable management practices to protect the horseshoe crabs and the migratory shorebirds. Read more about their horseshoe crab campaign.

"Horseshoes anyone?" (a .pdf document) by Tom Barnard and Lyle Varnell in The Virginia Wetlands Report, Winter/Spring 1999, Volume 14, Number 1. The Virginia Wetlands Report is a quarterly publication of the Wetlands Program at the Virginia Institute of Marine Science of the College of William and Mary.

Visit the Assateague Naturalist for more pictures and information on the horseshoe crab.

Life in the Chesapeake Bay, by Alice Jane Lippson & Robert L. Lippson. The Johns Hopkins University Press, Baltimore and London. 1984. ISBN: 0-8018-3012-5. This illustrated guide to fishes, invertebrates, and plants is an excellent resource for anyone who wants to learn more about life in the Chesapeake Bay.

The Seaside Naturalist, A Guide to Study at the Seashore, by Deborah A. Coulombe. Prentice Hall Press, New York. 1984. ISBN: 0-13-797101-X. Another great guide to learn more about wonderful seaside creatures.