The Atlantic horseshoe crab is an ancient marine arthropod that has survived for hundreds of millions of years. Its eggs feed migrating birds, and its blood helps keep medicines safe.
Interesting facts
- Horseshoe crabs are not true crabs; they are more closely related to spiders and scorpions.
- Their ancestors lived in the oceans about 450 million years ago.
- They have ten eyes, including two large compound eyes.
- Crabs harvested for their blood are returned to the sea, although some die.
- Delaware Bay hosts the largest horseshoe crab spawning in the world, in May and June.
- Migrating red knots and other shorebirds depend on their eggs for fuel.
Behaviour
Horseshoe crabs live on shallow seafloors and come ashore to spawn on high tides around new and full moons in late spring.
Breeding and young
Females dig nests in the sand and lay thousands of eggs, which males fertilise. A female may lay about 80,000 eggs in a season.
Threats and conservation
Overharvesting for bait and biomedical use, loss of spawning beaches and coastal development.
Where to see it
- Delaware Bay beaches, such as Slaughter Beach and Reeds Beach (May and June)
- Cape Cod, Massachusetts
- Jamaica Bay, New York City
- Cedar Key, Florida
Status source: IUCN Red List. Profile last reviewed September 2026.
