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Public Radio's Environmental News Magazine (follow us on Google News)

Denizen of the Sea

Air Date: Week of

The humble horseshoe crab has been around for 350 millions years. Over time, humans have found multiple uses for this denizen of the sea, and its most lucrative attribute turns out to be its blood. Host Steve Curwood visits with scientist Bill Sargent on the shores of Cape Cod, where these prehistoric creatures abound.


CURWOOD: It’s Living on Earth. I’m Steve Curwood. With its spiked tail that resembles a weapon, and its dark, mud-colored shell shaped like prehistoric armor, the horseshoe crab looks like its been around for 350 million years. And it has. Various creatures have found it handy. For perhaps thousands of years the red knot, a modest sized shore bird, has found horseshoe crab eggs to be key to its survival during its lengthy migration from the Antarctic to the Arctic each spring.

Now humans have found that blood from the horseshoe crab makes a handy and almost instantaneous test for diseases that doctors used to have to diagnose by injecting a rabbit and waiting a day or more. With birds and humans alike on the hunt for horseshoe crabs, the population has been under some pressure. I spent an afternoon with a man who’s made the horseshoe crab part of his life’s work, and who argues that the critters are much more valuable than people who cut them up for fish bait might think. Bill Sargent wrote a book called “Crab Wars: Horseshoe Crabs, Bioterrorism, and Human Health,” and took me wading in the waters of Pleasant Bay on Cape Cod.


SARGENT: Okay, we can enter the water here, and we’re going out on a very shallow sand bar.


SARGENT: And you should be able to… Here’s one right here. You can see… Here’s a nice fellow right here. And you can tell right away that this is a female. The way you tell that is that the males have this forward claw that becomes a little bit like a fist.

CURWOOD: Now, give me the big biological picture of horseshoe crabs: what are they, where they come from, and where they fit in the ecology.

SARGENT: Basically, they are one of the oldest animals that we have. They’re arthropods.

CURWOOD: Arthropods? That means they’re spiders?

SARGENT: Yes, their closest living relative is a spider. They’re sort of an integral part of the ecology of the bay. Actually, a lot of people think by turning over the sediment, it’s almost like a plow. So they’re oxygenating the sediments, and you’ll find more shellfish because of that.

CURWOOD: What are the biggest threats to these crabs? They’ve been here for 350 million years but, as I understand it, there’s a lot of pressure on them now.

SARGENT: Basically, you’re looking at ‘em. We’re the biggest threat, humans. Basically, these animals have been used as research animals for hundreds of years—pretty much every aspect of the crab, the behavior of the crab, vision of the crab, and the horseshoe crab blood. What’s happened is that the horseshoe crab test is probably the most common medical procedure used in medicine today that is all based on a single species of wild animal. So if we get a wound, we have a whole series of antibodies that go to the area and fight the infection.

What horseshoe crabs have are very large, what are called, amoebocyte cells, and the amoebocyte cell simply goes to the area and coagulates and keeps the infection out. So, what scientists have been able to do is they catch the crabs, they extract its blue, copper-based blood, separate out these amoebocyte cells and make what’s called lymulus amoebocyte lysate, which is used as a test for gram-negative bacteria. And whenever you got into the hospital, anything that’s going to come into contact with the human blood system has to be checked for gram-negative bacteria.

CURWOOD: Remind me, what are some of the typical diseases of gram-negative infections.

SARGENT: Well, when you go into septic shock that’s one of the things. A lot of e-coli are gram-negative bacteria.

CURWOOD: I’ve got to ask you this: how do you get the blood out of a horseshoe crab without killing it?

SARGENT: Basically, what you do, there’s this little hinge right here, and you put these crabs in a rack. And then they put a needle right through that hinge and then there’s a free flow of blood; and when the blood stops flowing, then they stop the bleeding and then release it. And if you do that right, you get less than ten percent mortality which is not perfect but it’s okay.

Each one of these crabs that we’re looking at is worth about as much as a laptop computer. Because they get $250 worth of lysate each time they bleed them, and they can bleed them every summer for all their lives. So they have about ten years that they can bleed them.

CURWOOD: I imagine when people discovered that you could use it for this medical test that it was quite popular and they probably started to disappear.

SARGENT: Yeah, we started noticing that there were very, very few immature crabs, almost none. The whole population seems to have crashed. And then we realized what was happening is the crabs were being caught in the shallow waters when they were laying their eggs. They have about a four-day window of opportunity to lay their eggs, right around the full-moon and the new-moon high tides. They were being caught and then bled in Falmouth and then returned back to the deeper waters of the bay, and it would take them about 30 days to get from the deep water to the shallow water. So, essentially, what you’d done is taken them out of the breeding population. Then what happened about three years ago, the Cape Cod National Seashore established a reserve for horseshoe crabs on the eastern part of the bay. And it used to be that you would walk a hundred meters along this beach and not see any immature crabs, and now you’re seeing lots of them.

CURWOOD: Okay, so now we’re going to take this fellow back to the water.


CURWOOD: Let’s see how he responds

SARGENT: Usually, they have two responses when you put them back. Either you put them in and they turn right over and they’ll disappear quickly, or sometimes they sit on the bottom for a while, so we’ll see what this one does.

[Sound of splashing water]

CURWOOD: He’s sitting.

SARGENT: He’s sitting. Oh, now he’s flipping over.

CURWOOD: There he goes.

SARGENT: There he goes. He made it. So he’s fine, and now he’ll just shoot right off into the deeper water.

CURWOOD: Bill Sargent’s book is called “Crab Wars: Horseshoe Crabs, Bioterrorism and Human Health.” Thanks for taking this time with me today, Bill.

SARGENT: Thank you.

[MUSIC: Ry Cooder/U.M Bhatt “Isa Lei” A MEETING BY THE RIVER (Water Lily Acoustics – 1993)]



“Crab Wars: A Tale of Horseshoe Crabs, Bioterrorism, and Human Health” by William Sargent

The Horseshoe Crab: Natural History, Anatomy, Conservation, and Research

Horseshoe Crabs “A Living Fossil” – Maryland Department of Natural Resources

NOAA Spotlight Feature Article


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