How Horseshoe Crabs Are Saving Human Lives

I spent spring break down at Slaughter Beach, Delaware. Away from the board-walk, outlet mall, bar-heavy WP_20140402_004Rehoboth, Slaughter is a little bit in-the-middle-of-nowhere for some people’s taste. While it was a pain to drive for 20 minutes to stop by the grocery store, a chance to commune with nature was just what I needed. The house we rented was right on the bay, and the tide in the bay goes way out this time of year. At low tide we could walk for probably 40 yards out into the bay, the water never reaching higher than our ankles, and look at what it had left behind. Snails, crabs, clams, and lots of sea weed were the main things, but we also saw sandpipers, seagulls, and blue herons. There are cool rocks, large chunks of coral, and big intact shells just sitting in the sand, waiting to be taken home as beach treasures. But the main thing we saw that week at Slaughter Beach was horseshoe crabs. It’s not the time of year that they come up on shore to breed, so I only saw one that was alive (I flipped him over and he ran back out to sea) but their shells and tails are scattered all over the beach – high tide or low.


Horseshoe crabs are a living fossil, a symbol of Delaware, an important link in the local ecosystem, and surprisingly they are a key element of the FDA’s screening process.

In the post I’ve linked to above, I talked about how ancient of an animal the horseshoe crab is. Horseshoe crabs evolved as early as 450 million years ago (pre-dinosaurs even!) and have changed very little over the years. This doesn’t make them primitive – it makes them well adapted. The horseshoe crab is so well suited to its environment, and its lifestyle that it hasn’t been under the pressures of natural selection and has remained nearly identical for 450 million years. If that doesn’t make these crabs badasses….just wait, the next paragraph will.

Horseshoe crabs evolved before most vertebrates did and are therefore very different than most animals you’ve learned about. Horseshoe crabs don’t have hemoglobin (or iron) in their blood like we do – they have hemocyanin (copper). Their blood isn’t red like ours, it’s blue, because of the copper. Their blood also contains Amebocytes, which are similar to our white blood cells and protect the crab from bacteria. Since the crabs are bottom crawlers, all of the chinks in their shells and armor allow a lot of dirty water to come into contact with their bloodstream, and they’ve had to evolve a very rigorous immune system. The blood is an extremely good detector of endotoxins (substances secreted by pathogens) and clots immediately when it comes in contact with them – stopping the fungi, bacteria, or virus from traveling any further in the blood stream.

The compound in their blood that does the clotting is called Limulus Amebocyte Lysate (or LAL) and has become a highly valued commodity in recent years. A quart of the blood is worth about $15,000 on the world market, and the LAL industry has a gross of $50 million annually. The FDA uses LAL to test every new drug for toxins, and surgical implants are also almost always screened with LAL. We haven’t figured out how to manufacture LAL on our own yet, so it still comes straight from horseshoe crabs (the $50 million dollar industry impacts about 250,000 horseshoe crabs a year).

The good news is, they don’t die for their contribution. LAL manufacturers report that the mortality rate for the crabs is less than 3%. The crabs are gathered up, bled for a few hours, allowed a small amount of time to recover, and then dumped back in the ocean. It’s about the same impact to them as giving blood is to you or I – we feel weird for a little while but then our bodies make up for the loss and we go back about our day.

As much as this seems like a low-impact, win-win situation, some scientists are concerned. Horseshoe crabs are a major part of the Delaware tidal flat ecosystem. The eggs they lay each year singlehandedly support huge populations of migratory birds that make one stop and one stop only – a stop in Delaware that is perfectly timed with the arrival of the horseshoe crab eggs. Some of these birds are endangered, and keeping the crab populations stable will have a trickle-up effect for all of these bird populations.

Some LAL manufacturers gather crabs during the breeding season, when they’re close to shore and easy to find. Chris Chabot, a neurobiology professor at PSU studies the effects of bleeding on crabs, says “If the biomedical industry could delay the blood harvest, it would probably help these animals.” He says if they were collected after the breeding season, they’d have plenty of time to recover and fatten up to survive the winter and breed again the in spring. The egg numbers wouldn’t be impacted, and the crabs wouldn’t be disrupted during their most vulnerable time of the year. Other scientists say giving the crabs more time to recover before they’re released, making sure to tag the crabs so they’re only bled once a year, or taking great care to release a crab where it was found could make a huge positive impact on the crabs’ way of life. Making these small changes would allow a multibillion dollar industry to continue keeping our pharmaceuticals safe from toxins, while reducing our impact on the naturally occurring ecosystem – doesn’t that sound like the best of both worlds?


Thanks to Christy Mannering and Flickr’s creative commons for the lovely picture of a horseshoe crab.
Thanks also to Slaughter Beach, DE for the wonderful vacation and the inspiration.
And, as always, a special thank you to the masses of Wikipedia, for most of the background knowledge needed for this post.

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