With an armored body and spiked tail, you might mistake the muddy-colored horseshoe crab for a prehistoric creature crawling from the ocean’s depths—and you wouldn’t be too far off.
Horseshoe crabs have more than 445 million years under their belts, so there is no doubt these ancient animals are resilient. To what do they owe this astonishing staying power? An incredible immune system.
Our iron-based blood is bright red, but a horseshoe crab’s veins are coursing with blood made pale blue by the presence of copper. The truly remarkable thing about a horseshoe crab’s blood, however, isn’t its color.
In fact, if you’ve ever had a shot, vaccine, implant or even stitches, you probably have a horseshoe crab to thank.
Unlike humans, horseshoe crabs don’t have white blood cells to stave off infection. When bacteria make their way past the crab’s exoskeleton and into the bloodstream, amoebocytes rush to the rescue. They release a chemical compound called limulus amoebocyte lysate, or LAL, which causes the blood to coagulate. The clot surrounds and traps the dangerous bacteria, preventing the spread of infection.
Scientists recognized the value of this unconventional defense. They began using LAL to detect bacterial endotoxins in pharmaceuticals and medical devices, and it’s since become an essential part of modern medicine.
The test can detect endotoxins at a staggering one part per trillion. In the presence of bacteria, it acts just as a horseshoe crab’s blood would, coagulating into a gel-like substance around the contaminant.
Approved by the Food and Drug Administration in 1970, the LAL test is used to detect bacterial contamination in everything from intravenous drugs and vaccines to prosthetics, implants, medical devices and more.
Each year, horseshoe crabs are harvested to donate blood for LAL test manufacturing. Up to 30 percent of their blood is collected, and the crabs are released back into the wild.
A Keystone Creature
The horseshoe crab’s importance is not limited to its role in human health. As a keystone species, its existence and numbers impact the entire ecosystem. Countless other species depend on the horseshoe crab to survive.
Like bulldozers, horseshoe crabs disturb the sand beneath them as they forage for food. Fish follow the crabs, pecking and feeding on what’s dragged up in their wake. “They’re like bioengineers,” says General Curator Jack Cover. “They sort of change the area they’re living in with their movement, so they’re benefitting a lot of other species.”
Horseshoe crabs also play host to a number of other organisms, including bivalves and slipper snails, which take advantage of the crab’s tough outer shell. Because other hard surfaces in the stretch of water they inhabit are limited, it’s an opportune place to attach and grow.
In addition, female horseshoe crabs lay an astonishing 80,000 eggs each year. Shorebirds, various fish and invertebrates feed on this bounty of eggs, while loggerhead sea turtles feed on both juvenile and adult horseshoe crabs.
The Horseshoe Crab Harvest
In recent years, the horseshoe crab has had its challenges. It requires a special kind of beach to breed—one where it won’t be overturned by harsh waves. These nestled coastal beaches, like those found at the mouth of the Delaware Bay, are few and far between.
As a result, horseshoe crabs tend to gather by the thousands on a beach to lay their eggs. This assembly is a spectacular sight, but it also makes them easy targets. In the past, the crabs were ground up and used as fertilizer. More recently, they’ve been taken as bait for eels, minnows and whelk.
In the early 1990s, horseshoe crabs were collected by the truckload from spawning beaches. And in just five years, from 1992 to 1997, their harvest increased from less than 100,000 to more than 2.5 million annually.
Luckily for horseshoe crabs and the species that depend on them, including humans, regulations have been set to limit their harvest. Fishery regulators are continually making adjustments for a better, more sustainable take. And in 2001, the waters off the mouth of the Delaware Bay became the first designated horseshoe crab sanctuary, helping to bolster the area’s principal spawning population.
Even the biomedical industry is doing its part. Scientists are already exploring synthetic solutions for the LAL test, so that one day the horseshoe crab may not have to be a blood donor.