The Horseshoe Crab, an Ancient Arthropod That Saves Lives

Horseshoe crab.
The Atlantic horseshoe crab, Limulus polyphemus, is more closely related to spiders, ticks, and scorpions than to crabs. Getty Images/Gallo Images/Danita Delimont

Horseshoe crabs are often called living fossils. These primitive arthropods have lived on earth for 360 million years, largely in the same form as they appear today. Despite their long history, the horseshoe crab's existence is now threatened by human activities, including harvesting for medical research.

How Horseshoe Crabs Save Lives

Any time a foreign object or substance enters the human body, there's a risk of introducing infection.

If you've had a vaccination, an intravenous treatment, a surgery of any kind, or had a medical device implanted in your body, you owe your very survival to the horseshoe crab.

Horseshoe crabs have copper-rich blood that appears to be striking blue in color. Proteins in the horseshoe crab's blood cells are released in response to even the smallest amount of bacterial endotoxin, such as E. coli. The presence of bacteria causes horseshoe crab blood to clot or gel, part of its hypersensitive immune response system.

In the 1960's, two researchers, Frederick Bang and Jack Levin, developed a method of using these coagulation factors to test for contamination of medical devices. By the 1970's, their Limulus amebocyte lysate (LAL) test was being used commercially to make sure everything from scalpels to artificial hips is safe for introduction in the human body.

While such testing is crucial to safe medical treatments, the practice takes a toll on horseshoe crab populations.

Horseshoe crab blood is in high demand, and the medical testing industry catches as many as 500,000 horseshoe crabs each year to drain them of their blood. The crabs aren't killed outright in the process; they're caught, bled, and released. But biologists suspect the stress results in a percentage of the released horseshoe crabs dying once back in the water.

The International Union on the Conservation of Nature and Natural Resources lists the Atlantic horseshoe crab as vulnerable, just one category below endangered in the extinction risk scale. Fortunately, management practices are in place now to protect the species.

Is a Horseshoe Crab Really a Crab?

Horseshoe crabs are marine arthropods, but they aren't crustaceans. They're more closely related to spiders and ticks than they are to true crabs. Horseshoe crabs belong to the Chelicerata, along with arachnids (spiders, scorpions, and ticks) and sea spiders. These arthropods all possess special appendages near their mouthparts called chelicerae.  Horseshoe crabs use their chelicerae to put food in their mouths.

Within the animal kingdom, horseshoe crabs are classified as follows:

Kingdom – Animalia (animals)
Phylum – Arthropoda (arthropods)
Subphylum – Chelicerata (chelicerates)
Class – Xiphosura
Order – Xiphosurida
Family – Limulidae (horseshoe crabs)

There are four living species in the horseshoe crab family. Three species, Tachypleus tridentatus, Tachypleus gigas, and Carcinoscorpius rotundicauda, live only in Asia. The Atlantic horseshoe crab (Limulus polyphemus) lives in the Gulf of Mexico and along the Atlantic coast of North America.

What Do Horseshoe Crabs Look Like?

The Atlantic horseshoe crab is named for its horseshoe-shaped shell, which helps protect it from predators. Horseshoe crabs are brown in color, and grow as large as 24 inches long at maturity. Females are considerably larger than males. Like all arthropods, horseshoe crabs grow by molting their exoskeletons.

People often believe the horseshoe crab's spine-like tail is a stinger, but it's actually no such thing. The tail functions as a rudder, helping the horseshoe crab navigate the bottom. If a wave washes the horseshoe crab ashore on its back, it will use its tail to right itself. Never lift a horseshoe crab by its tail. The tail is attached by a joint that works similar to a human hip socket. When dangled by its tail, the weight of the horseshoe crab's body can cause the tail to become dislocated, leaving the crab helpless the next time it's overturned.

On the underside of the shell, horseshoe crabs have a pair of chelicerae and five pairs of legs. In males, the first pair of legs is modified as claspers, for holding the female during mating. Horseshoe crabs breathe using book gills.

Why Are Horseshoe Crabs Important?

In addition to their value in medical research, horseshoe crabs fill important ecological roles. Their smooth, wide shells provide the perfect substrate for many other marine organisms to live on. As it moves along the ocean's bottom, a horseshoe crab may be carrying mussels, barnacles, tube worms, sea lettuce, sponges, and even oysters. Horseshoe crabs deposit their eggs by the thousands along sandy shorelines, and many migratory shorebirds, including red knots, rely on these eggs as a source of fuel during their long flights.

 

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