Horseshoe Crabs Hit the Beach

Horseshoe crabs, named for the "U" shape of their shells, are more closely related to spiders and scorpions than to lobsters and true crabs.Their story just gets stranger from there.

A horseshoe crab would be the perfect monster of science fiction: it has ten eyes, blue blood, and it "chews" food with spiny legs before passing it into its mouth, which is located at the base of its legs.

And like any good monster, everything about horseshoe crabs is dramatic. They have grazed our muddy seafloors for over 500 million years and have survived all the major extinction events. Their blood, which turns blue when exposed to air, clots in the presence of certain bacterial toxins and is used in the pharmaceutical industry to test the sterility of intravenous solutions and antibiotics.

A back view of a Limulus polyphemus, distinctly showing the two body divisions.
Photo: B. R. Speer, Museum of Paleontology, University of California, Berkeley


In the United States, Limulus polyphemus ranges from Maine south to the Gulf of Mexico and the Yucatán. Horseshoe crabs can grow up to two feet long; they feed on small shellfish, worms, and algae on the seafloor up to 100 feet deep.
Photo: National Aquarium in Baltimore's Department of Education

Living up to 19 years, these arthropods reach sexual maturity at about 9 to 12 years old. During spring and summer full and new moons, the animals creep onto ocean beaches to mate. Females lay up to 30 thousand eggs, which males fertilize before burying them in the sand. These eggs provide a major food source for migrating birds along the Atlantic coast. Those that are not eaten hatch during the next high tide, and the tiny larvae are carried away to sea.

An underside view showing the many legs and gills. Photo: B. R. Speer, Museum of Paleontology, University of California, Berkeley