Horseshoe Crabs on the Beach: Living Fossils, Blue Blood and Why They Matter
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "Are horseshoe crabs really crabs?", checked October 29, 2026. Harvest rules vary by state; check your state wildlife agency.
Not really crabs
Despite the name, horseshoe crabs aren't crustaceans. They're more closely related to spiders and scorpions. Atlantic horseshoe crabs (Limulus polyphemus) gather on U.S. coasts from Maine to Mexico each summer. They're called "living fossils" because they predate the dinosaurs by more than 200 million years.
What you'll see on the beach
- A smooth, hard, horseshoe-shaped shell.
- A long tail they use as a rudder and to flip themselves over if they land upside down on the beach.
- Spawning crowds in season. Delaware Bay has the largest population in the world.
They stir up sediment as they feed on marine worms and other invertebrates.
Why they matter
- Medicine: their blue, copper-based blood clots when it meets bacterial toxins. Researchers use it to check that IV drugs, vaccines and medical devices are free of contamination. Harvested crabs are bled of about one-third of their blood and returned to the beach.
- Wildlife: more than 20 species of migratory birds, loggerhead sea turtles and many fish and crabs eat their eggs.
Status and rules
Atlantic populations declined in the 1990s and are recovering under state management through the Atlantic States Marine Fisheries Commission. Habitat loss and demand for bait are still concerns. NOAA's page notes that New Jersey and South Carolina forbid harvesting and Delaware allows males only. Rules can change, so check with your state.
Related: sea turtle nesting beaches, jellyfish and Portuguese man o' war, beach cleanups.