10 Fascinating Facts About Horseshoe Crabs

A horseshoe crab takes a breather.
A horseshoe crab takes a breather. / TanawatPontchour/iStock via Getty Images

Horseshoe crabs have weird blood, mate en masse in May and June, and harbor a secret weapon that’s probably saved your life. Here are some more facts about these ancient arthropods.

1. Horseshoe crabs are incredibly old.

The 25 millimeter-wide Lunataspis aurora crawled over Manitoba 445 million years ago, making these fossils the world’s oldest-known horseshoe crabs. Four species in the family Limulidae are with us today. Horseshoe crabs are often called “living fossils,” but evolution didn’t really leave these invertebrates behind. For instance, some prehistoric species had limbs that split out into two branches, but today's specimens have only one.

2. Horseshoe crabs are not actual crabs.

In fact, they aren’t even crustaceans. Unlike real crabs and their kin, horseshoe crabs lack antennae. Biologists classify them as chelicerates, a subphylum that also includes arachnids. Members possess two main body segments and a pair of unique, pincer-like feeding appendages called chelicerae.

3. Horseshoe crabs have multiple eyes.

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Large compound eyes rest on the sides of their shells, allowing horseshoe crabs to locate partners during mating season. Behind each one, there’s a small, primitive photoreceptor called a lateral eye. Towards the front of the shell are two tiny median eyes and a single endoparietal eye. On its underside, a horseshoe has two “ventral eyes,” which may help it navigate while swimming.

Most interesting to scientists is the compound pair. By virtue of their relatively simple wiring, they’re easy to study and have taught us a great deal about how our own eyes function.

4. Horseshoe crab babies can swim upside down.

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Walking around on the ocean floor is generally how horseshoe crabs get from point A to point B. Nevertheless, young ones will often flip over and start propelling themselves through the water, using their gills as extra paddles. With age, they do this less frequently.

5. The horseshoe crab's spiked tail has several uses.

Stinging isn't one of them, despite what many believe. Among its uses are assuming rudder duties and helping the arthropod right itself after getting stuck on its back.

6. Adult horseshoe crabs mainly eat bivalves.

Both larvae and fully grown horseshoe crabs eat aquatic worms. Though adults will also devour algae and carrion, they predominantly consume clams and mussels. These low-profile predators mash food between the spiky upper regions of their legs before pushing it into their mouths.

7. Horseshoe crabs gather in Delaware Bay for a massive annual mate-a-thon.

Every year in May and June, the bay becomes the largest Atlantic horseshoe crab spawning zone on Earth. During the night, a female will climb ashore with a male (or several) in hot pursuit. After she digs a hole and deposits her eggs, the males fertilize them. Migratory shore birds descend upon the bay in huge numbers, fattening themselves on the nutrient-rich eggs. Among them are hundreds of red knots, which use the crab fest as a pit stop during their yearly migration from the Arctic to South America.

8. Very few horseshoe crabs survive to adulthood.

A female horseshoe crab can lay as many as 90,000 eggs per clutch. Only about 10 of those individual embryos will ever become adults. Fish, sea turtles, and birds gorge themselves on the eggs. With so many animals dependent on this fodder, nesting horseshoe crabs are vital to the ecology of Delaware Bay and other coastal regions around the world.

9. Atlantic horseshoe crab females are 25 to 30 percent bigger than males.

When it comes to reproduction, females also mature more slowly. While males are ready to mate by age 8 or 9, females don’t start breeding until age 10 or 11.

10. If you've been vaccinated, thank a horseshoe crab.

Horseshoe crabs have blue blood containing hemocyanin, a copper-rich protein that transports oxygen. Copper turns bluish-green when it oxidizes.

Horseshoe crabs also lack infection-fighting white blood cells. Instead, special cells called amebocytes attack pathogens in the horseshoe crab's body by sealing them inside a gooey physical barrier and halting their spread.

This characteristic was discovered by Johns Hopkins University physician Frederick Bang in 1956, and since then, medical researchers have studied it for use in keeping injectable drugs free of pathogens. To ensure that a vaccine or injectable drug is safe, a technician will introduce horseshoe crab amebocytes into a sample to test for pathogens. In the 1970s, the FDA made this test mandatory for experimental drugs and surgical implants. Horseshoe crabs literally saved people's lives.

Today, about 600,000 horseshoe crabs "donate" their blood every year, which can sell for more than $15,000 a quart. Soon after the amebocyte-rich blood is extracted, the crabs are released.