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

Despite their common name, horseshoe crabs are not really crabs (crustaceans). 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 — not as weapons, which is commonly assumed.

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.

Horseshoe crabs also are important ecologically. Loggerhead sea turtles and more than 20 species of migratory birds rely on the crabs’ eggs as an annual boon to their diets.

Atlantic horseshoe crab numbers have been declining since at least the 1990s. Delaware has the largest population of horseshoe crabs in the world, and scientists from NOAA’s National Estuarine Research Reserves System are studying the species there. Reasons for the crabs’ reduced numbers include habitat loss and high demand as commercial bait. Scientists are currently working to develop artificial bait that may potentially reduce the number of horseshoe crabs taken for bait.


Search Our Facts
Get Social

More Information

Did you know?

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

Last updated: 06/25/18
Author: NOAA
How to cite this article

Contact Us