Horseshoe Crabs: Weird, Wonderful & Amazing

Lemulus polyphemus, the North American species of horseshoe crab, photographed on Cape Ann, Mass. 

WEIRD AND WONDERFUL CREATURES, HORSESHOE CRABS are survivors from life’s earliest times who almost certainly have saved your life. And, they’re almost spiders. For the record, they’re not crabs in any way.

If all that sounds overblown, consider the following horseshoe crab facts:


Beneath their dome-like carapaces, horseshoe crabs walk around the seafloor on 10 legs and they view the world with 10 eyes spread around their bodies, some on their shells, some on their tails. They mostly crawl on the sea bottom, but sometimes they swim – upside down – using their legs as paddles.

Pick one up – carefully, please, the carapaces are sensitive – and you’ll find not a bottom plate but the exposed body, gills and legs. Their blood is copper-based and appears blue when exposed to the air.

horseshoe crab facts
On their undersides, horseshoe crabs are legs, gills and body. The flexible spines along the edges of their rear carapace plates help them feel their way and detect changes in temperature and current.

Horseshoe crabs came onto the world stage as long ago as 445 million years ago, making them among the most ancient of living animals. They were around more than 200 million years before the dinosaurs came along. They’ve survived four major mass extinctions, including the Permian Extinction that killed off 90 percent of species on the planet and the Cretaceous Extinction that killed off the dinosaurs.


Horseshoe crabs’ blue blood is blessed with a substance called “Limulus Amebocyte Lystate,” (or, “LAL”) a compound that coagulates in the presence of even small quantities of bacterial toxins. Harvested safely in laboratories, It’s used to test for the safety of medical applications as diverse as surgical devices, contact lens solutions and vaccines and other injectable drugs. It’s not stretching our horseshoe crab facts to say that horseshoe crab blood has almost certainly saved your life. More than once.

horseshoe crab facts
If properly handled, horseshoes can yield up to 30 percent of their blue blood and be returned to the ocean without harm.

Scientists have long assumed that horseshoe crabs were somehow related to spiders and scorpions, possibly as ancestors, but their exact role has been difficult to pin down. New research by geneticists at the University of Wisconsin-Madison have placed them squarely in the middle of the arachnid family tree. Which leaves a great many unanswered new questions about arachnid evolution.


Horseshoe crabs were almost certainly tagged as “crabs” because of their broad, chitinous carapaces, vaguely resembling those of crabs. But, while arachnids, crustaceans and insects are all members of the arthropod phylum, they’re different classes.

Horseshoe crab carapaces consist of two parts, a front plate called the prosoma and a rear plate called the opisthosoma. Look closely and you’ll see that the prosomas are, in fact, horseshoe shaped, giving them the other part of their name.

  • There are four species of horseshoe crabs living in the world’s oceans. The one most familiar to Americans is Limulus polyphemus, found along the Atlantic and Gulf coasts from Canada to Mexico.  The other three species – Carcinoscorpius rotundicauda, Tachypleus gigas and Tachypleus tridentatus –  are all found in in Indo-Pacific basin, in Indonesia and Southeast Asia.
  • Horseshoe crabs are nocturnal, largely hunting their prey of worms, mollusks and crustaceans at night.
  • Horseshoe crabs don’t have teeth. They mash their prey up with their legs in order to eat it.
horseshoe crab facts
Horseshoes come ashore en mass to spawn.
  • Most famously, horseshoe crabs often mass together on mudflats and shorelines during high tides and full or new moons to spawn. A female can lay more than 100,000 eggs in the sediment, usually in batches of several thousand at a time, to be fertilized by males.
  • Most of the eggs are eaten by shorebirds and fish. Migrating birds like the red knot have come to time their travels through the Delaware and Chesapeake Bay areas to match peak horseshoe crab spawning.
  • Despite having 10 eyes around its body, horseshoe crab visual capabilities aren’t very impressive. Many are more photoreceptors for distinguishing light from dark, which they are good at. On the other hand, they have provided researchers with key clues to the evolution of human vision.
  • Horseshoe crab tails look intimidating, but actually they are harmless. They’re important in helping the animals right themselves if they find themselves on their backs.
  • Some 500,000 crabs have their blood harvested annually. If done right, some 30 percent of a crab’s blood can be drawn without harm to the animal, which is returned back to the waters from which it came. The dollar value of LAL obtained this way amounts to some $50 million per year. Scientists still haven’t figured out how to produce it synthetically.
  • Major threats to horseshoe crabs include loss of prime spawning areas to shoreline development, and also people who go out to watch them and disturb their nests.

PRINCIPAL SOURCES:  “This creature has 10 eyes, legs that chew and blood that saved your life,” Washington Post; “Forget Dinos: Horseshoe Crabs Are Stranger, More Ancient—And Still Alive Today,”; “Facts About Horseshoe Crabs and FAQ,” Florida Fish and Wildlife Conservation Commission; “Horseshoe Crab,” National Wildlife Federation; “Bizarre horseshoe crabs are actually spider relatives,” National Geographic; “Study confirms horseshoe crabs are really relatives of spiders, scorpions,” University of Wisconsin-Madison News Office; “Horseshoe Crab,” Wikipedia.