A Blue View: The True Blue Bloods

The horseshoe crab may look like a simple creature, but its protective armor and incredible immune system have made it a beacon of survival for millennia.

Published August 18, 2015

In fact, very little about this living fossil has changed in the more than 445 million years since it first roamed the ocean.

A hard, external shell protects it from most predators, and six pairs of appendages on its underside make bulldozing through the sand in search for food simple. A few of them even help grind and deliver food straight to its mouth.

Horseshoe crab

But don’t be fooled by the horseshoe crab’s appearance (or its common name). This “crab” is not a true crab at all. It’s actually more closely related to arachnids. That’s right…spiders, scorpions and other things that go bump in the night. Don’t be alarmed—they are hardly threatening.

What appears to be the most menacing part of a horseshoe crab, it’s telson, or tail, is really just a rudder. The horseshoe crab uses its telson to steer and flip itself over when toppled by a wave.

These strange creatures also have unique blood, made pale blue by the presence of copper—and that special blood plays a critical role in modern medicine.

To learn more about horseshoe crabs, check out this week’s podcast:

Episode Transcript

At $15,000 a quart the blood of the lowly horseshoe crab is plenty valuable, but the riches to be derived from the blood of this ancient arthropod go well beyond its dollar value. For in its unique blue blood is a compound that has changed medical science.

With their armored helmet-like shells and spiked tails, you’d be forgiven for describing horseshoe crabs as something from the days of dinosaurs—and they are. In one form or another, they have inhabited this planet for at least 445 million years. And they’re still going strong. To what do they owe this amazing staying power? An incredible immune system for starters.

Our iron-based blood is bright red, but a horseshoe crab’s veins course with blood made baby blue by the presence of copper. But the truly remarkable thing about this animal’s blood isn’t even its hue.

A horseshoe crab’s blood could save your life…and in all likelihood it already has.

First, some anatomy and physiology: Our human body is armed with white blood cells that stave off unwanted intruders, but horseshoe crabs don’t have these defenders. When their immune system is under attack, specialized cells called amoebocytes rush to the rescue.

They release a chemical compound called limulus amoebocyte lysate, LAL for short, which causes the blood to coagulate. The resulting clot surrounds and traps the dangerous bacteria, preventing the spread of infection.

In the 1970s, the miracle of LAL met modern medicine. At that time, the FDA approved the use of LAL to test for bacterial contamination in pharmaceuticals and medical devices. Since then, its use has become ubiquitous. All IVs and medical equipment coming in contact with the blood supply have to pass the LAL test.

Think about it. Have you ever received a shot or vaccine? Had a cut that required stitches? Donated or received blood?

Thank a horseshoe crab.

The LAL test can detect endotoxins at a staggering one part per trillion. To put that it in perspective, it’s equivalent to isolating a single drop of ink in an Olympic-sized swimming pool. The test is simple, in the presence of bacteria, the LAL coagulates and that means contamination.

More than 600,000 horseshoe crabs are collected every year for an involuntary blood donation, conducted by a handful of companies that manufacture the LAL test. They carefully collect about 30 percent of the crab’s blood and then release it back into the wild. Fortunately, it’s in everyone’s best interest for the crabs to survive this encounter, and mortality is generally low.

As a keystone species, horseshoe crabs impact the survival of countless other animals. They are bioengineers. Diminutive bulldozers that alter the landscape as they forage for food in the sand. Fish follow, pecking and feeding on whatever is dredged up in their wake.

Even their eggs are valuable. Females lay an astonishing 80,000 each year. During spawning season, the crabs gather by the thousands on coastal bays and on beaches.

The assembly is a spectacular sight but it also makes them easy targets to predators, like the Red Knot, a migratory shorebird that travels north to nest making a critical stopover at the very same time the horseshoe crabs are spawning. These circumpolar navigators gorge on excess eggs, then leave recharged for the next leg of their 9,000-mile trek.

Luckily for horseshoe crabs, harvest regulations have been set to help bolster principal spawning populations, and the biomedical industry is exploring synthetic alternatives to LAL.

But for now, let’s all say a silent thanks to this lowly arthropod for its inadvertent contributions to a better world.

To learn more about the amazing horseshoe crab, visit aqua.org/ablueview. A Blue View is produced by the National Aquarium for WYPR. For the National Aquarium, I’m John Racanelli.

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