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Are horseshoe crabs really crabs?

Horseshoe crabs are "living fossils" more closely related to spiders and scorpions than they are to crabs.

Horsehoe crab mating season, Lewes, Delaware

Horsehoe crab mating season, Lewes, Delaware. Image credit: Jacqueline Bedell

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New Jersey and South Carolina currently forbid any harvesting of horseshoe crabs, and Delaware restricts harvesting to males only.

Despite their common name, horseshoe crabs are not really crabs (crustaceans), but are more closely related to spiders and scorpions. Atlantic horseshoe crabs (Limulus polyphemus), which swarm U.S. coastlines each summer from Maine to Mexico, have been called “living fossils” because they predate the dinosaurs by more than 200 million years. Their smooth, hard shell (exoskeleton) is shaped like a horseshoe (thus the name). They use their long tails as rudders in the water and to flip themselves if they get overturned on the beach. They are tremendous reworkers of sediments and feed on a variety of marine worms and invertebrates, including some commercially important fish.

Few people realize how important horseshoe crabs are to modern medicine. Because their blue, copper-based blood quickly clots in the presence of bacterial toxins, medical researchers use it to test intravenous drugs, vaccines, and medical devices, ensuring that they are free of bacterial contamination. Every year, many thousands of horseshoe crabs are harvested from their habitats, “bled” of about one-third of their blood, then returned to the beach relatively unharmed.

Horseshoe crabs also are important ecologically. More than 20 species of migratory birds, loggerhead sea turtles, and a myriad of commercial and recreation fish and crab species rely on the crabs’ eggs as an annual boon to their diets.

While Atlantic horseshoe crab numbers were declining in the 1990s, populations are recovering due to the regional management efforts of the states through the Atlantic States Marine Fisheries Commission. Delaware Bay has the largest population of horseshoe crabs in the world, and scientists from NOAA’s National Estuarine Research Reserves System help conduct annual horseshoe crab spawning surveys, which is a Delaware Bay-wide effort. However, habitat loss and high demand as commercial bait are still concerns for horseshoe crabs and migratory shorebirds.


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Author: NOAA

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