The Question: I’ve heard that horseshoe crabs’ blood is used to make medicine. Is this true?

Submitted by: Rick, New York, USA

The Short Answer: Hundreds of thousands of horseshoe crabs (Limulus polyphemus) are bled each year to produce a substance called Limulus Amebocyte Lysate (LAL). LAL is used to test intravenous drugs and medical equipment for the presence of bacteria and endotoxin, a poison found in many bacteria. Endotoxin can survive heat sterilization and even very small amounts can cause serious and even deadly reactions in people when introduced into the blood stream with an intravenous drug. The LAL test is extremely sensitive, able to detect as little as one femtogram (a millionth of a billionth of a gram) of endotoxin in a milliliter of water.

How the Test Works: Horseshoe crabs, like all animals, have an immune system to protect them from infection. When a horseshoe crab is injured and bacteria are present, the immune system begins a clotting reaction to seal off the injury. The horseshoe crab’s immune system happens to use endotoxin as the major chemical signal that it is being infected, so even tiny amounts of endotoxin will trigger a dramatic clotting reaction. When this was discovered, scientists realized it could be used to create an effective test for the presence of endotoxin. Today, horseshoe crabs are harvested from the ocean, and taken to a lab where they are bled by sticking a needle into their heart and withdrawing the blood. Amebocytes, a type of immune system cell, are then separated from the harvested blood and broken open (lysated) to release the clotting factor. When you add the genus name of horseshoe crabs, Limulus, you get the Limulus Amebocyte Lysate (LAL) test. The LAL test solution is combined with a sample from a batch of medication. After a suitable wait, the presence or absence of the clotting reaction signifies whether endotoxin is present.

More Information: Prior to the development of the horseshoe crab LAL test, standard procedure was to inject a rabbit with the medication to be tested and see if the rabbit developed a fever. Using horseshoe crabs is presumed to be more humane … at least to the rabbits. Whether or not the bleeding process is humane to horseshoe crabs is another story. Very few horseshoe crabs die during the actual bleeding procedure. But the entire process of being captured, kept out of the water, shipped to the bleeding facility, cleaned of barnacles and other hitchhikers, sterilized, suspended upside down, and bled, before being trucked back to the ocean to be released, is probably enormously stressful on them. Gauging total mortality from capture to recovery, the most optimistic studies have shown a low of about 8% mortality, while one study showed a high of about 30%. Also, although we know horseshoe crabs can replace the blood fluid within a week or so, it takes a couple of months to replace the blood cells. It would be very difficult to do a study where the bled horseshoe crabs are tracked for a year or so in the wild, but that’s probably what it would take to get a real sense of how much they are affected by being bled.

If you are feeling badly for horseshoe crabs, it’s also important to remember that the LAL test has probably saved tens of thousands of human lives from death due to bacterial infection, and prevented illness in countless other people. In part because harvesting horseshoe crab blood is expensive, there is research underway to create a synthetic imitation to replace the use of wild horseshoe crabs.

Beyond the issue of whether bleeding is humane to horseshoe crabs, the mortality of bled horseshoe crabs matters because the number of horseshoe crabs is decreasing dramatically. Unfortunately, the medical industry is not the only one harvesting horseshoe crabs. In fact, fisherman use far more horseshoe crabs as bait in traps meant to catch American eel and conch. Eels and conch seem to be especially fond of horseshoe crab bait, so fisherman prefer horseshoe crabs to other possible baits. This results in the harvesting of millions of horseshoe crabs every year. By comparison, the medical industry uses a few hundred thousand horseshoe crabs each year, the vast majority of which are returned to the ocean and probably survive.

As the drop in horseshoe crab numbers continues, it is having an impact beyond the oceans. In the Delaware Bay and Chesapeake Bay, hundreds of thousands of shorebirds stop on their way north each year to feast on the eggs of horseshoe crabs, whose spawning peaks on nights of the full moon and new moon in May. As horseshoe crab numbers drop, shorebird populations may lose a critical migration food source. Some populations of Red Knots (Calidris canutus), a robin-sized shorebird, have been in sharp decline in the last decade and researchers believe the drop in food at beaches where horseshoe crabs spawn is probably the cause. The PBS series Nature did an excellent show on horseshoe crabs and Red Knots and how they are tied together. You can find the full episode online at: (Unfortunately, the video seems to become more and more pixilated as it goes on, at least on my computer.)

About horseshoe crabs: Horseshoe crabs are not actually crabs. In fact, they aren’t even crustaceans. They are members of the Chelicerata, a group of arthropods that includes spiders, scorpions, and mites. Horseshoe crabs are often called “living fossils” because the modern species look virtually identical to fossils of Horseshoe crabs from hundreds of millions of years ago. For more information has a wealth of information about these remarkable creatures.

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