Photograph by Joel Sartore, National Geographic Photo Ark
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Atlantic horseshoe crabs, which predate dinosaurs, are closely related to spiders and scorpions.

Photograph by Joel Sartore, National Geographic Photo Ark

Atlantic horseshoe crab

What is an Atlantic horseshoe crab?

Atlantic horseshoe crabs may appear alien, but their history as earthlings is pretty impressive. They’ve been around for 450 million years, predating the dinosaurs by more than 200 million years. They live on the Atlantic coast of North America, from Maine to down and around the Florida coast to Alabama and Mississippi. They also live around the northern Yucatán Peninsula in Mexico.

Despite their common name, horseshoe crabs are more closely related to spiders and scorpions than to “true crabs,” which are crustaceans. They’re in the class Merostomata, which means “legs attached to the mouth” because their mouths are literally in the center of all 10 legs. Horseshoe crabs also have 10 eyes—a pair of compound eyes on the front shell and more photo receptors along their tail.

Horseshoe crabs’ bodies are divided into three segments. The head, or prosoma, houses most vital organs, including the heart and the brain. The middle segment, the opisthosoma, contains the back gills and ridges that help with movement. The third segment is the tail, or telson, which looks like a big stinger but is used to help these animals flip if overturned and acts as a rudder as they move across the ocean floor. Horseshoe crabs molt their distinctive shells about 17 times before reaching maturity at age 10.

Blood harvest

Most of us probably owe our lives to horseshoe crabs. Their blue, copper-based blood contains lysate, which reacts to bacterial toxins by clotting. Horseshoe crab blood has long been harvested to test everything from water to intravenous drugs for contamination. It's also key to making vaccines for diseases such as COVID-19. Crabs are returned to the wild afterward but the process may have a negative effect on spawning.

A synthetic version of their blood has been produced that appears to be on course to eliminate the need to use animals in endotoxin detection. The pharmaceutical industry is beginning to accept this substitute but it has not yet ended the practice of harvesting these crabs for their blood.


Spawning season comes at different times of year depending on location, but the largest lovefest happens at Delaware Bay, where thousands of horseshoe crabs crawl onto the beaches in May and June. During spawning, several males gather around one female who lays her eggs in clusters of up to 5,000 apiece—ultimately laying about 20,000 eggs in one night and nearly 100,000 in a season. One or more males will fertilize the eggs, which then take two weeks or more to develop and hatch.

Threats to survival

Horseshoe Crabs Mate in Massive Beach "Orgy"

Horseshoe crabs have been on Earth an estimated 450 million years, pre-dating the dinosaurs by some 200 million years. They are one of the few living creatures with blue blood, which is used by the pharmaceutical industry for testing drugs. Horseshoe crabs live in the ocean year-round, but they make one annual visit to the shoreline to lay eggs in sandy, wet beaches. In the United States, the highest concentration of egg laying is along the Delaware Bay.

The biggest threat to horseshoe crabs is being harvested for human use, including for bait and biomedical materials. Habitat loss is also a significant threat. Coastal development, including structures like bulkheads and seawalls, alter the natural landscape and may get in the way of horseshoe crab spawning, which requires flat, sandy beaches. Climate change raises sea levels, which also leads to fewer spawning grounds for these ancient animals.