How horseshoe crabs may have saved your life

During my first year of grad school I conducted a jailbreak– a fellow grad student and I snuck into the Invertebrate Zoology lab and freed all the horseshoe crabs. I wish we’d know then that we dropped about $50,000 back into the sea. Of course, horseshoe crabs aren’t that expensive, but their blood is. One quart of horseshoe crab blood costs around fifteen thousand dollars [1]. And why would anyone pay for that? Well, in addition to coming in a shade of oh-so-chic blue, it also saves human lives.

Researcher prepares horseshoe crabs for bleeding. Screen cap from video at the end of this post.

Researcher prepares horseshoe crabs for bleeding. Screen cap from video at the end of this post.

Horseshoe crab’s superpower blue blood is a bacteria fighting machine, and scientists are literally borrowing this trick to help test medical injections for contamination. When a horseshoe crab is cut, the wound is immediately slathered in bacteria from its murky surroundings. To prevent a potentially lethal infection, certain blood cells carry compounds that cause the blood to clot up when exposed to bacteria fragments. Cut. Clot. Cut. Clot. The horseshoe crab is quickly sealed back up and ready to go. There it is, our horseshoe crab wallowing in a muddy pit, fighting infection and being awesome.

Now cut scene from our warm, cozy shoal to a cold, sterile lab. A scientist is cradling his head in distress. Even after sterilization, one of his vaccine batches is making people sick, causing serious complications. Little does he know that, thanks to molecules in the bacterial outer layer, these suckers can remain toxic even after death. The only way to test for the presence of bacteria pieces is to inject some adorable fluffy bunny with the vaccine and see if it gets sick.

At least, that was the only way. Bunny testing was the main way various injectables and implants were tested for contamination. Until folks realized that not only could you test with horseshoe crab blood and get near instant results, you didn’t even have to kill the crabs. Round ‘em up, wash ‘em off, give ‘em a good clean, and then extract about ⅓ of their blood. The crabs are then returned to the wild. The blood, on the other hand, is shipped off for processing, and the clotting factors isolated. If a vaccine is contaminated with bacteria, adding a bit of horseshoe crab powder will turn the solution into a gooey mess, if it’s safe, it remains liquid [2].

Do the crabs mind? A bit. Horseshoe crabs are roughly 10% more likely to kick the bucket in the year following bleeding, compared to their un-stuck comrades [3]. But this, scientists suggest, isn’t really a big threat to crab populations when stacked against all the other things they have to deal with- like being used for bait [3]. In fact, some of the same crabs may be collected for blood extraction year after year after year.

As for our escaped inmates? When we freed our Invertebrate Zoology horseshoe crabs, we thought for sure they’d return to their friends and tell them all about their terrible abduction to an alien lab. They would be shunned, the laughing stock of the horseshoe crab community. Clearly, we underestimated just how often horseshoe crabs visit our world.


Work Cited



[3]The effect of heavy bleeding on mortality of the horseshoe crab, Limulus polyphemus, in the natural environment

RR Helm (26 Posts)

I am a PhD candidate studying jellyfish development and evolution at Brown University. I've participated in numerous research expeditions, studying jellies all over the world, from Africa to the abyss. I am currently studying the beautiful mauve stinger jellies, found in the Mediterranean, and the ghostly Atlantic stinging nettles found on the US east coast.

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72 comments on “How horseshoe crabs may have saved your life
  1. You rebel :) Have recently become more and more convinced that a lot of well-intentioned “bunny huggers” are mostly misinformed…

  2. I’m glad that I don’t get rounded up and have a third of my blood taken, but that’s truly amazing and even better that the crabs live

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  4. Even the chiton, the stuff the exoskeleton of Limulus is made of, is ground up and is incorporated into wound dressings– cutting healing time in half! They are amazing creatures and should be protected everywhere! Although, we still need their blood!

  5. It is indeed a marvel that a creature can posses such awesome feature in its blood. We should indeed use this knoeledge that was gained.

    But i cannot join the happy choir of people that, musing about this, justify the procedure.
    How woudl you like being rounded, cleaned up like they do in that factory (calling it a lab is a bit pretentious i think), being bled up to 1/3 of your blood via a metal needle popped in your only soft spot (as if you were a milk cow), then released……and if you’re lucky, folowing year, same treatment….. and people make apparently tons of money out of it…..
    I’d rather prefer to see scientists spending money and energy aiming at reproducing the marvel sinthetically without having to abuse animals, rather than just harvest, once again, nature wonders….. which may not survive the treatment….
    It’s just my opinion clearly….

    • I would have to say that if we were being harvested as humans, we had better start to evolve ourselves to stop it. It is the natural process of this world for every creature to use at its disposal every technic necessary for it’s own survival and continuation of its own species. I can accept that if we as a species continue to deplete our planet of all its resources for our own survival eventually leads to our own demise, so be it. The wonderful Earth will continue on without us, after all, much worse in the past has happened to this planet than what humans could ever do and the horseshoe crab is still here.

      • Evolution doesn’t work like that. It isn’t like Dragon Ball Z where the crab can be all “This isn’t even my final form, human!!!”

      • We’re actually quite capable of making even the most ancient species extinct, without noticing it.

        I’d agree, that’s a factory not a lab. “Most” of the crabs surviving is bad economics. It’s wastefully greedy as well as cruel. I’d like to see a fairer trade.

        Maybe knowing it saved a loved one’s life will make more people ask how the product was obtained.

    • Lara, spell check is a beautiful thing. People will take your discontent a lot more seriously if you have proper sentence structure, and your vocabulary is decent…. Just saying……….

    • Oh, forgot to mention – I worked as a dairy maid for 2 years. Not sure what needle you are referring to with the milk cow reference? For future reference, its spelt ‘synthetically’. You are welcome :)

    • Humans do this all the time Lara. It’s called giving blood/plasma. Duh?

      Said animals are invertebrates, and do not even possess the capacity to process things on the level of higher animals, let alone ourselves. Do you really think they go oh we’re dying/going to die on a level beyond instinct? No.

      PS Cloning exists. IF we can successfully clone a sheep, then it is only those ridiculous moral opponents (who need to disappear from the planet) who would oppose this, and after all I’m certain a crab is simpler to replicate than a sheep.

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  8. I’d just like to point out that you used the deprecated term “quart” in the 21st century. You may want to Google the word “litre” and come up to speed. Kudos on the rescue mission though, not everyone has the cojones to do what’s right.

  9. I may have missed it, but did anyone reading this acknowledge horseshoe crabs are severely endangered due to over-use as fishing bait, and especially for the miracle of blood work described above? Typically, as with all species, their impact on the ecosystem has ripple effects, each of their large clutches of thousands of eggs feed hundreds of shorebirds in a balanced system that has co-existed for millions of years. The severity of the threat deserves more than a footnote reference. I highly recommend Carl Safina’s excellent book “The View From Lazy Point” to gain a better understanding and appreciation for hundreds of bird and ocean species and the terrible crisis we need to confront before the ecosystems are utterly changed, vast numbers of species are gone, and possibly us with them.

  10. A correction–the crabs on the eastern US coast are those in severe trouble–I don’t know the status of the other species.

  11. Horseshoe crabs are roughly 10% more likely to kick the bucket in the year following bleeding, compared to their un-stuck comrades [3].

    When compared to all of the other threats, I highly doubt that stopping the bleeding is going to have a major effect when compared to farming them, using them for bait, accidental captures, etc.

    How about we use them for bleeding to save people, and not use them for bait……. That makes a little more sense, does it not.

  12. All created beings have limited time here on earth and each one has purpose why he/she/it was created. Be thankful that God The Creator gave wisdom to the scientists to discover the use of the blood of horseshoe crab.

    • Those crabs are alive because they deserve a life, just like everything else does. A life NOT being harvested for what ever humans find on it that they deem “helpful”. Your “god” is a cruel being if you seriously believe what you just said. Every living thing deserves to live a full and normal life. Newsflash. We humans can’t live for ever. We can’t cure everything. Why don’t we leave the animals out of our affairs for ONCE.

  13. If there’s some that are dying then reduce the blood taken from them to a fourth.. . And what is “most,” completely vague.
    Where there’s money, evil will follow.

  14. May all of you bleeding heart be stricken by a disease that can only be cured by horseshoe crab extracts. How soon we would see you hypocritically change your tune.

  15. I would imagine that these crabs are feeling a lot of pain with being harshly cleaned, strapped and held down. Then those needles being inserted and kept in them, being bled. While scientists can take samples, I think there must be a kinder, gentler way of doing it!

    • I agree, if we must do it for the good of our species, and we appear to agree that our apathy extends to these horseshoe crabs (following on the logic that they do not have the emotional or intellectual capacity to realise their predicament), then surely we are “intelligent” and responsible enough to do this in a kinder, less-hazardous manner? And if no method that is kinder than this brutal extracting (which is obviously not very safe if ~10% of crabs are not surviving through it) exists, then are we not intelligent enough to create a better way?

      The good of us should never be built on the expense of others. Whatever happened to the morals and ethics that we preach everywhere?

  16. Nowdays the horseshoe crab protein that binds to the toxic molecules is cloned in bacteria and produced recombinant. This means that you can get all the benefits of the amazing blood without touching the crab. If only the conservative microbiologists would swap analysis assays we wouldn’t have to bleed the crabs.

  17. This is so inhumane. It’s an example of human self-importance. We still view ourselves as being above animals, and still think we can do whatever we like to them.

  18. one thing that is not mentioned is that this process weakens the horseshoe crabs to the point that their reproduction numbers have gone down, which means that their survival rate as a species has gone down. This has a ripple effect such as the survival rate of migratory birds that land on the east coast after traveling thousands of miles, feed on this particular species as their main staple, and then fly thousands more miles north. If their main food source is depleted, then so will they. I’m all for science and progress, but we must not forget that everything is connected.

  19. 2 comments actually, 1 technical, one having to do with policy. (1) you initially say that the blood Factor kills bacteria, but then say that it is a powerful clotting agent. Is direct bacteriocidal action actually involved? (2) How long does it take from the egg stage to reach “bleedability”? Is artificial culturing of horseshoe crabs practical?

    • (1) good catch! I assume clotting also kills bacteria by trapping them in a bloody goo, but I don’t actually know this for sure. I changed “killing” to “fighting” to reflect this. Thanks for pointing it out! (2) this is a really good question, I did a bit of digging and found the following quote “juveniles took up to 11 yr and 17 instars to reach sexual maturity, at a prosomal width of 159.7 ± 5.1 mm”. I’m not sure how large they have to be before bleeding, though, but it sounds like it takes them a while to get large. The quote is from “Abundance and population structure of the Atlantic horseshoe crab Limulus polyphemus in Pleasant Bay, Cape Cod” (

      • Fighting still seems to suggest a bacteriocidal or bacteriostatic effect. This matters because it leads the reader in the direction of thinking the discovery of a new clinically viable anti-bacterial mechanism is a plausible outcome. Introducing a clotting agent into the human bloodstream is not usually a good idea though (this seems to have more embolism with infections written all over it). The remaining part of the article on the other hand seems to refer only to a use as an indicator.

        I understand it fights infections only in horseshoe crabs, but without that nuance it’s still misleading.

        • Interesting. I see your point and for some readers this may be misleading. I’ve thought about it, and have decided I will keep the original wording for two reasons: 1) because the original statement is still true in the context of horseshoe crabs, and 2) because in the same sentence I state its specific medical use, which I hope will clarify any uncertainty for most readers. In the future I’ll try to keep an eye out for these types of innuendoes. Thank you for pointing it out.

  20. The test that comes from the compound derived from horseshoe crab blood (endotoxin and LAL) is a fundamental test to water quality. Yes, we should work toward making a synthetic version, but using their blood (mostly a renewable source) over killing rabbits is a step in the right direction. Not to mention all of the human lives that have been saved as well.

  21. Folks get rounded up every eight weeks and drained of 1/8 of their blood on a regular basis and return to do so again and again.

    It isn’t quite the same but then again neither are the brains.

  22. And this is different than Chinese-backed poachers of rhinos for hard-ons and black bears for “anti-inflamatory” gall bladders because…

  23. LAL production is not the basis for declining horseshoe crab numbers but collecting for bait is. Crabs used for bait are typically chopped into quarters by fishermen and then put into eel and conch traps. Sadly, it’s really only the females that are useful for bait as the primary attractant are developing eggs. Males just get chopped up because no one bothers to check. I worked as a “crab bleeder” for a college summer job at a LAL lab. The crabs do get bled of approx. 30% of their blood volume. Their handling, however, was quite delicate. After the procedure the crabs are placed in barrels covered with damp canvas and kept in a air-conditioned truck. The next morning they are returned to the sea. While there may be some pushback by some zero-tolerance concerns, the fact is it is a renewable resource and economics plays a role in conservation efforts. Sad but true. Collecting for bait, particularly when done on spawning grounds, is simply a recipe for the destruction of the resource. Limulus are among my favorite animals and I wish to see them flourish.

  24. The fact that they are able to suffer is reason enough not to do this. Other lives are here for their own reasons, not ours. We have no right to just use other life-forms for our own selfish gain. Yet another epic fail.

  25. Boom – In the larger scheme of things, the use of Limulus for LAL is hardly ‘epic’ in any sense of the word. I’m not sure what you know about Limulus or the capacity for them to suffer in LAL production or any other circumstance, but whatever it is, it doesn’t even budge the needle on the scale of the use of life-forms or their suffering. It doesn’t make it right, but it’s very blindered. Suffering is a primary characteristic of all nature. Human-caused suffering is hardly unique or noteworthy in regard to its quality and perhaps even to its scale. I would reckon that most non-photosynthetic life forms cause suffering of one sort or another. Whether a plant plucked from the ground suffers I leave up to you. The food you eat, however, is the product of direct or indirect suffering whether you are a carnivore or a local-only vegan. The range of purely recreational forms of life-form-suffering activities are a bit more worthy than griping about this. If it was your child suffering from meningitis or some other disease either prevented by or diagnosed by LAL, I bet you would feel differently. I’m all for synthesizing alternatives (harder than you think) or developing protocols and conservation policies for minimizing impact on Limulus but get real. Complain about the wholesale and irreversible slaughter incumbent to bait harvesting before you bark up this tree. And think twice about what you call “epic.”

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  27. Really? When profit becomes involved then they will exploit and kill the animals like all humans do. As humans we even sell babies and you say this a good thing? Not when profit is involved.

  28. Truc – As noted earlier, humans do give blood. Proportions are indeed more significant in Limulus but it’s difficult to make quantifiable comparisons. Certainly I have seen Limulus with injuries that would appear lethal from which they recover. The degree to which they suffer in the LAL production process is hard to ascertain. They do survive. Whether it’s cruel or not isn’t an empirical assertion. It’s a matter of opinion. Some people may think it’s cruel to leave a Limulus out of the water for 24 hours but they don’t appear to suffer and can survive for many days if kept damp. Chopping them in quarters for bait seems pretty cruel but I don’t think they suffer very much. I’m more concerned about the health of the species as a whole and quantifiable benefit is a more effective tool to counter bait harvesting then any unsupported sentiments which may even be taken too far. I saw this case of private trophy farms in Texas that maintain the largest existing populations of some critically endangered oryx or some other antelope. A woman objected to the cruelty of the hunting of a tiny fraction of the group, from which the proceeds supported the entire breeding program. She sued to stop the hunt and the private parks no longer handle the species. Her actions were, in my opinion, wholly misguided; the species equivalent of cutting off ones nose to spite their face.

  29. I’m wondering about your fluffy bunny argument. You mention that the vaccine was previously tested on bunnies. This implies that testing on bunnies has become unneccesary due to this process. Does this mean medical ethics commissions allow phase I clinical trials without prior animal testing if this test is performed? If they do not, and animal testing still occurs but only the probabillity of the bunnies dying during the experiment is reduced, would that really count as saving the rabbit? I’m not sure how this works for rabbits, but I know in the Netherlands rats are generally killed after experimentation (even the control group) because nobody really cares enough to create a shelter for non-simian lab animals and re-use for experimention is considered inethical, I’m not sure how this works for rabbits or elsewhere in the world at all though.

    My second question is similar: what is the reduction in mortality for the lab-mammals? I do not neccessarilly think it needs to be similar to the 10% mortality for the horse-shoe crabs, but in my opinion a decent reduction in mortality is needed to compensate for the mortality and suffering of the horse-shoe crabs.

    My last question is different: Why is the mortality 10%? Is it mostly caused by the catching, handling around and during the draining and the release? Or is it strongly related to the amount of blood that is drained?

    From what I read in your article, the blood draining does not seem to save humans, but mostly rabbits…

    • Companies involved in the synthesis and manufacture of approved medical devices used to batch-test on rabbits. Now they use horseshoe crab (Limulus amebocyte lysate: LAL) extract, giving faster, cheaper results. This is where the number of rabbits being used has been reduced as a result of LAL discovery. For clinical trials, I’m not sure. Also remember this is just being used to test for gram-negative bacterial membrane contamination, not drug efficacy. So for clinical trials I’d imagine they’d make use of both LAL and rabbits. But again, I don’t have personal experience with this so I’m not sure.

      For the reduction in mortality question, I’m not sure, nor do I imagine this would be easy to quantify. Because the potential decrease in lab animals is mostly seen on the manufacturing side, you’d have to collect data from individual companies.

      For your last question: control crabs were caught, handled and brought into the lab for a few days, but were not drained of blood. So the 10% mortality is related to blood draining.

      As for your last point, the use of rabbits for testing prevented toxic medical fluids and equipment from being used on humans, now LAL is used to ensure no contaminations reach patient. So both rabbits and humans are saved. It used to be rabbits saving people (with their own lives), now it’s crabs :)

  30. PinkShinyRose, Firstly, I apologize to the author for repeatedly responding but some clarification is in order. The product derived from Limulus blood is not a vaccine, nor is it for rabbits. Limulus amoebocyte lysate (LAL) is an assay. it is used to detect the implied presence of gram-negative bacteria through the detection of the lipopolysaccharide molecules that coat them. Lipopolysaccharide is also known as endotoxin and is in a class of compounds called pyrogens, because they cause fever in rabbits and people. Endotoxin, or rather the bodies reaction to the presence of endotoxin, is what kills people who die from toxic shock syndrome. LAL is used to test vaccine solutions, surgical irrigation saline, swaps from dialysis tubing, or any other solution or object that will be placed in the body or that comes from the body and should be bacteria free (urine, cerebrospinal fluid, etc.). They want to make sure the ‘cure’ won’t end up making you more sick due to the presence of endotoxin. The benefit to the huge rabbit colonies used to support the earlier rabbit tests for endotoxin is clear and complete. They aren’t needed.

    I can’t comment directly on the 10% mortality in Limulus. I believe that number is high based on my knowledge of the process. I also doubt whether an accurate control was used to account for natural mortality, weak individuals, etc., which would affect this number.

    As noted earlier, I did spend a summer working in one of these labs (in 1983) but I have had no affiliation with them since. I do know quite a bit about Limulus and am very much a champion of their conservation. Like most species, threatened or otherwise, I generally don’t have the luxury or resources to advocate on the survival of individuals. Stabilization and conservation of the species comes first. In this case, bait-collecting is the clear threat and countering it is actually supported by sustainable use in LAL production, which provides an economic counter-argument that provides significant weight to otherwise purely personal objections. Without LAL, rabbit testing would be a competitive alternative to the endotoxin assay.

    Here is a link to a Limulus web site I made about 15 years ago.

    • Please don’t apologize! As with your past comments, I found this explanation to be succinct and informative. I’ve learned a great deal and am grateful for your contributions to this discussion :)

  31. Are the horseshoe crabs marked in any way that would identify them in case they are caught again before they could recover? It would seem simple enough to use a color-coded plastic tag fastened to the shell someplace to denote the year they were bled and avoid over-utilizing them.

  32. Sarah, It’s a good question and the inadvertent bleeding of individuals could be a factor in the current mortalities. My experience was that you could identify at least some specimens that had been previously bled because there was still an indication of the needle puncture on the dorsal membrane. This probably is not a completely reliable method though. First, it may not be the case that it’s easy to see in all cases and second, the crabs do molt and I’m not sure if this clears that membrane entirely. Marking them in some unobtrusive way that would include some means to ascertain the marking date, would be a worthwhile idea.

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