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3. Are horseshoe crabs really crabs?

**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* **Did you know?** *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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  **Last updated:** 08/24/23 **Author:** NOAA How to cite this article Contact Us


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