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10 Hard-Shelled Facts About Horseshoe Crabs

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The plodding sea creatures have weird blood, weirder swimming habits, and a secret weapon that’s probably saved your life.  

1. HORSESHOE CRABS ARE INCREDIBLY OLD.

Discovered in 2008, the 25 millimeter-wide Lunataspis aurora crawled over Manitoba 445 million years ago. This makes it the world’s oldest-known horseshoe crab. Four species are with us today, all of which closely resemble their long-extinct ancestors.

Supposedly frozen in time, horseshoe crabs are often hailed as “living fossils” by the media. Yet, appearances can be misleading. Evolution didn’t really leave these invertebrates behind. They’ve transformed quite a bit over the past half-billion years. For instance, some prehistoric species had limbs that split out into two branches, but today's specimens have only one.

2. THEY'RE NOT CRABS.

In fact, they aren’t even crustaceans. Unlike real crabs and their kin, horseshoe “crabs” lack antennae. So, where do the strange ocean-dwellers belong on the arthropod family tree? 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 (hence the name).

3. EACH ONE HAS A HUGE ARRAY OF SIGHT ORGANS.

Rachel Oh via Wikimedia Commons // CC BY-SA 3.0

Large compound eyes rest on the sides of their shells. Come mating season, these bean-shaped units help amorous crabs locate a partner. 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 presumably help it navigate while swimming.

Most interesting to scientists are 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. BABIES CAN SWIM UPSIDE DOWN.

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 SPIKED TAIL HAS SEVERAL USES.


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

6. ADULTS MAINLY EAT BIVALVES.

Both larvae and fully grown horseshoes eat aquatic worms. Though adults will also devour algae and carrion, they predominantly consume clams and mussels. When feeding time comes, these low-profile predators mash food between the spiky upper regions of their legs before pushing it into the mouth. 

7. HUNDREDS OF THOUSANDS GATHER IN DELAWARE BAY FOR A MASSIVE ANNUAL ORGY.

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 these avians are scores of red knots, which use the crab fest as a final pit stop during their yearly migration from the Arctic to South America.

8. VERY FEW SURVIVE INTO ADULTHOOD.

A mother can lay as many as 90,000 eggs per clutch. Even so, it’s estimated that only about 10 of those individual embryos will ever become adults. Before they get a chance to hatch, fish, sea turtles, and birds gorge themselves on the eggs. With so many animals utterly dependent on this fodder, nesting horseshoe crabs are vital to the ecology of Delaware Bay and countless other 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, their counterparts don’t start breeding until age 10 or 11. 

10. IF YOU'RE UNDER 40 AND HAVE BEEN VACCINATED, THANK A HORSESHOE CRAB.

While human beings don’t have blue blood, horseshoe crabs do. Whereas our blood uses iron-based hemoglobin to transport oxygen throughout the body, horseshoe crabs rely on hemocyanin, which contains copper. Copper turns bluish-green when it oxidizes.

Strange coloration isn’t the only remarkable thing about horseshoe crab blood. Shorelines are downright squalid: a single gram of undersea sediment contains roughly 1 billion bacteria. Unlike us, the arthropods 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, thus halting the malady’s spread.

Ever since Johns Hopkins University physician Frederick Bang discovered this characteristic in 1956, medical scientists have been capitalizing on it. To ensure that a vaccine or injectable drug is safe, they introduce horseshoe crab amebocytes into a sample. If the cells start releasing their goo, it’s because they’ve encountered bacteria and, therefore, the product isn’t ready yet. In the 1970s, the FDA made this test mandatory for experimental drugs and surgical implants. Without these magnificent animals, thousands—perhaps even millions—of people might have died during the past four decades from unsanitary injections.

As you might expect, horseshoe crab blood is worth a pretty penny: sellers now command $15,000 per quart. The magic elixir is extracted from over 600,000 “donors” every year, each of whom parts with 30 percent of its blood before being released within 48 hours. Sadly, though, many don’t last that long. About 10–15 percent of captured crabs die somewhere in the process, and survivors can exhibit lethargic behavior down the road. Scientists aren’t insensitive to this problem. Researchers have been trying to develop synthetic amebocytes.

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