The Blood Harvest

Each year, half a million horseshoe crabs are captured and bled alive to create an unparalleled biomedical technology.
A still from the PBS Nature documentary Crash ( PBS )

The thing about the blood that everyone notices first: It's blue, baby blue. 

The marvelous thing about horseshoe crab blood, though, isn't the color. It's a chemical found only in the amoebocytes of its blood cells that can detect mere traces of bacterial presence and trap them in inescapable clots. 

To take advantage of this biological idiosyncrasy, pharmaceutical companies burst the cells that contain the chemical, called coagulogen. Then, they can use the coagulogen to detect contamination in any solution that might come into contact with blood. If there are dangerous bacterial endotoxins in the liquid—even at a concentration of one part per trillion—the horseshoe crab blood extract will go to work, turning the solution into what scientist Fred Bang, who co-discovered the substance, called a "gel."

"This gel immobilized the bacteria but did not kill them," Bang wrote in the 1956 paper announcing the substance. "The gel or clot was stable and tough and remained so for several weeks at room temperature."

If there is no bacterial contamination, then the coagulation does not occur, and the solution can be considered free of bacteria. It's a simple, nearly instantaneous test that goes by the name of the LAL, or Limulus amebocyte lysate, test (after the species name of the crab, Limulus polyphemus).

The LAL test replaced the rather horrifying prospect of possibly contaminated substances being tested on "large colonies of rabbits." Pharma companies didn't like the rabbit process, either, because it was slow and expensive. 

So, now, the horseshoe blood test is a big business. "Every drug certified by the FDA must be tested using LAL," PBS's Nature documentary noted, "as do surgical implants such as pacemakers and prosthetic devices." 

I don't know about you, but the idea that every single person in America who has ever had an injection has been protected because we harvest the blood of a forgettable sea creature with a hidden chemical superpower makes me feel a little bit crazy. This scenario is not even sci-fi, it's postmodern technology. 

The only problem is that the companies need a large supply of the blood of live crabs. Horseshoe crabs live on the seafloor, near the shore. When they want to mate, they swim into very shallow water, and horseshoe crab collectors wade along, snatching the crabs out of their habitat.

The biomedical collectors are not the first to make use of the crabs' bodies. As far back as colonial times, "cancerine fertilizer" was used to enrich fields. In the 20th century, though, this became an organized industry around the Delaware Bay. The crabs were steamed and then ground into meal for the fields. Others were fed to hogs. Millions of crabs were harvested.

Horseshoe crab harvest for fertilizer production, 1928 (Delaware Public Archives)

As we slowly killed off the horseshoe crab population, by the 1970s, the fertilizer industry declined and had died off. But harvests picked back up in the 1990s, when fishermen realized they could use the crabs as bait for catching large snails called whelk (aka conchs).

That is to say, these animals have not been treated kindly by humans. They don't inspire the kind of affection we have for, say, bunny rabbits. In the eyes of people before Fred Bang, the only merit attached to the horseshoe crab was its proximity. They like the shoreline, as do we. 

And that bacteria-rich habitat is why, Bang speculated, the crabs evolved their marvelous chemical defense. Their circulatory systems work more like a spider's than like ours. If we inhale something bad, that thing has to find its way through our bodies and into our bloodstreams, fighting its way through our white blood cells along the way. But if bacteria find their way under a horseshoe crab's exoskeleton, they can roam free to do damage. 

"Large sinuses exist that allow blood direct contact with tissues," the Woods Hole Marine Biological Laboratory's history of the crab explains. "There are many wide open spaces and bacteria entering a crack in the shell of a horseshoe crab have easy access to large internal areas of the crab, a potentially deadly scenario."

The coagulogen changes the wide-open terrain of the horseshoe crab's circulatory system. When the crab blood cells sense invaders, they release granules of the chemical, which becomes a gooey physical barrier to the movement of the bacteria, preventing the spread of infection. The best metaphor might be the superpower of the X-Men's Iceman, but instead of using cold to encase enemies, the horseshoe crab instead uses its remarkable chemistry.

This trick, perhaps unfortunately for the horseshoe crab, does not work on humans. 

After the biomedical horseshoe crab collectors get them back to a lab, they pierce the tissue around the animals' hearts and drain up to 30 percent of the animals' blood. The LAL is extracted from the blood, and can go for $15,000 per quart. Only five companies bleed the crabs: Associates of Cape CodLonza, Wako ChemicalsCharles River Endosafe, and Limuli Labs (which does not have a website).

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