It's an arthropod, a very, very evolutionarily ancient arthropod. They are like cockroach old.

Latin name Limulus polyphemus.

They don't look like crabs, but they do look a bit like horseshoes. They live in the ocean, and come out onto the beach to spawn.

Interestingly, blood from horseshoe crabs is used to detect bacterial components. This assay, called the Limulus Amoebocyte Assay, relies on factors found in horseshoe crabs that respond to LPS (lipopolysaccharide, a component of Gram-negative bacteria.)

I saw a real one up close the other day. There were some people from the Boston Aquarium showing kids stuff at a shopping mall. Horseshoe crabs are deceptively empty. They look like a big bowl, but if you flip it around, you notice that in fact, most of it is just space, and there's very little crab underneath the shell. Kind of weird.

Residents of the eastern seaboard of the United States are familiar with the horseshoe crab: an animal with a big armored head and a long, spiny tail, inhabiting the shallow waters near shore. They are odd-looking things, but are even odder - and even more interesting - than their appearance.


First, in spite of the name, horseshoe crabs are only distantly related to true crabs. They are more closely related to spiders, in fact. (Same subphylum.)

Of more interest yet is the antiquity of the body plan. Fossils 400 million years old have been found which look very like modern horseshoe crabs. Though each animal itself lives to be only about ten years old, the type is so ancient, and so unchanged, as to deserve the common name "living fossil." Horseshoe crabs in their modern form were around long before anything even vaguely resembling modern mammals evolved; while all the furious activity of development of other forms proceeded, these animals stuck to a simple, effective structure and preserved it virtually unchanged over almost unimaginable lengths of time.

Life style

Horseshoe crabs lived in ancient times as they live now, mostly in the brackish water of bays and estuaries, spending most of their lives on the shallow subtidal and intertidal sand and mud flats of the seashore. Asian species live among mangrove roots in similar conditions. They are tough, able to withstand the wide swings in salinity and temperature common in their environment. They are also quite tolerant of the pollution now common in that same environment. Survivors.

General anatomy

Large for invertebrates, adult horseshoe crabs measure about 8 to 9 inches (20 cm) across. Horseshoe crabs resemble large horseshoe shaped helmets with a long spike-like tail or telson. (Most of the body parts are under the huge, armored carapace.) Two large compound eyes are visible on the top of the animal. These compound eyes are made up of units of 8 to 14 light-sensitive cells. Of course they can't tell us what they see with these primitive compound eyes, but it is not probable that they are able to form distinct images.

The carapace is the bulk of the animal's size. It forms a crescent around the next section, the abdomen. The abdomen possesses 6 pair of spines along the margin which further protect the underside.

The body ends in a long spine called a "telson" which can be rotated by the crab and is used to right itself when it is flipped over.

Under the crab carapace are five pairs of walking legs and the chelicera, jointed feeding appendages.

Horseshoe crabs are scavengers and feed on mollusks, worms and other benthic organisms. They grab their food with the chelicera. Unlike some American politicians who are said to be unable to carry on both activities at once, horseshoe crabs must walk while chewing, since they use spiny leg segments to grind their food on its way to their mouths. They have no jaws.


To reproduce, the smaller male clings to the larger female and is towed along, sometimes for days, until she is ready to spawn. When she is ready, she digs a depression in the sand near mid or high tide mark. As she deposits her eggs, the male sheds his sperm over them. The eggs hatch into free-swimming young called "trilobite" larvae because they look strikingly like trilobites, the now-extinct ancient life form. As the larvae mature they develop the long telson and other adult features, and end up on the bottom like their parents.

Blue blood

Like many crustaceans, horseshoe crabs have blue blood rather than red blood like ours. Our blood is red because of the oxygen-carrying molecule hemoglobin, which has an iron molecule in the center. (Rust gets its red color similarly, being also oxidized iron.) The oxygen-carrying molecule in horseshoe crab blood, hemocyanin, contains a copper molecule instead of an iron molecule, and accordingly is blue (oxidized copper).

The horseshoe crab circulatory system also differs from ours in interesting, and, as it turns out, useful ways. Our circulatory system is closed: all the blood is contained in capillaries, arteries, veins and the heart. Not so in the horseshoe crab: Large sinuses exist that allow blood direct contact with tissues.

However, this raises an interesting problem for the crab. Suppose the shell is cracked or some other injury occurs which allows sea water to flow into such an open sinus? Sea water is full of bacteria. Bacteria which invade a human body have to negotiate the complicated circulatory system, evading the body's defenses all the way. Wouldn't the open spaces in the horseshoe crab system allow those bacteria immediate free access to the crab's blood, and, hence, its entire circulatory system? Furthermore, these crabs, being cold-blooded, cannot use the human body's prime defense against infection, fever.

You know they solved it, or they wouldn't be here.

The horseshoe crab's chief defense is the same single type of blood cell which carries oxygen, the amoebocyte. These cells appear oval when seen inside a living crab, but are packed with small granules containing a clotting factor called coagulogen. When an invading bacteria, or even small amounts of fragments from the cell wall of a bacteria, are detected, the cells immediately release this coagulogen in great quantities. The thought is that by clotting the immediate surroundings very quickly, the invading bacteria can become enmeshed and therefore stopped. Larger clots may not only stop enmeshed bacteria but serve as a barrier to the outside environment in the case of a severed limb or large incision.

Important medical use of horseshoe crab blood

Human beings react badly to bacteria too, and our immune systems also react, as does the crab system, to fragments of the cell wall ("endotoxin") of gram-negative bacteria, the thin-walled kind common in the water in which horseshoe crabs live. A person can develop fever and other complications merely from an exposure to the endotoxin from bacteria. Anything that goes into the body during surgery, by injection, or for therapy, has to be free not only of living bacteria (which can easily be killed by heat), but must also be free of bacterial endotoxin.

The industry of ensuring that injectable drugs, irrigation fluids, surgical tubing, and the like are free of bacterial endotoxins is a big business. In the past, drug and surgical supply companies maintained large rabbit colonies. Rabbits, like us, are sensitive to endotoxin, and if a suspect sample of saline injected into a rabbit caused a fever then it was contaminated. No fever, no contamination. This method was expensive and slow, and, some felt, inhumane.

But it has been discovered that the clotting agent in horseshoe crab blood can be easily isolated, and used as an instant, inexpensive test for endotoxin: add the clotting agent to a sample of the material to be tested, and if clotting develops, the sample is contaminated; if not, not.

Bizarre as it sounds, hundreds of horseshoe crabs are captured and bled every year to get the blood from which the clotting agent can be refined. The blood is taken from a large dorsal blood sinus, the pericardium. The crabs are returned to the water within 24 hours and completely recover. Tests have shown that the same crab may be bled year after year (about 30% of its blood taken each time) without ill effects, just as human blood donors give blood repeatedly. (Of course the crabs aren't exactly "volunteers.")

Current status, and prospects

The Carboniferous Period, between about 360 to 300 million years ago, appears to have been the heyday of horseshoe crabs. From the hundreds of species extant then, only four are now alive:

Tachypleus tridentatus
Tachypleus gigas
Carcinoscorpius rotundicaudata
Limulus polyphemus

The first three are found in the waters off Asia; Limulus polyphemus is our American species, common up and down the Atlantic seaboard.

They are hardy beasts. Essentially all armored head, they are difficult for predators to assail, and are very tolerant of a harsh environment of fluctuating oxygen, temperature, salinity and food supplies. They withstand pollution well too.

As they came before us, so also they may well outlast us.

(KINGDOM, Animalia; PHYLUM, Arthropoda; SUBPHYLUM, Chelicerata; CLASS, Merostomata; SUBCLASS, Xiphosura; ORDER, Xiphosurida; SUBORDER, Limulina)

Ward, Peter Douglas, On Methuselah's Trail, W.H. Freeman and Company, New York 1992
Pearse/Buchsbaum, Living Invertebrates, Blackwell Scientific Publications, Boston 1987
Marine Biological Laboratory, Woods Hole, Massachusetts,

Spiders, really. And like spiders, feared.

Kids, really. And like kids, fearless. I was 8, old enough to know better.

The lifeguard, bronzed and confident, draped in red, asked us to slaughter the horseshoe crabs. "Hold them by the tail so they can't sting you, then smash them on the wall." The wall--a creosote bulkhead jutting into the beach, already slimy with the blue blood and cracked shells of these fearsome creatures.

It was mating season. Not yet old enough to understand a desire that drove these beasts onto my beach, Ideal Beach, I smashed one creature on top of the others, picking the same one up over and over until the shells splintered and the sky blue blood soaked the shells underneath. Sea gulls pecked into the soft flesh of our dying prey as the legs were still scratching at the sky.

When we were done, tired, stinking, and proud of our aching muscles, we jumped back into the water, washed ourselves, then went home.



Hemocyanin: blue blood. We wore it that day like war paint--stinking of death, sweating under the early July sun. 20 mg of high purity hemocyanin will fetch $175.00 20 milligrams. Less than the weight of a fat housefly. Less than the weight of our soul.

Our blood runs red from the iron in hemoglobin; a horseshoe crab's blood runs blue from copper. Hemoglobin is cheaper.

Every spring the she crabs come to the beach, several smaller males trailing behind. The bayshore teems with horseshoe crab eggs, tiny bean curds at the lip of the beach. Millions upon millions, so many that none mean anything at all to me. Birds crowd along the beach, feasting.

I used to think the birds were what mattered.


Female crab can lay 80,000 eggs in a season. She creeps up onto the beach, riding the spring tide on the full moon. She crabs reach sexual maturity in 9 to 12 years, not much younger than when humans do. May is crazy with life everywhere. Within 2 to 4 weeks, they are ready to hatch. I never really looked at the eggs closely, until last year.

It took me a long moment to believe what I was seeing. Horseshoe crab eggs do not remain opaque. Soon before they egg splits, the tiny pale crab spins wildly. Half the egg is clear, the other half white from the curled embryo. If you look carefully, you can tell that this tiny critter is a curled up horseshoe crab.

As it spins, it looks like a tiny blinking eye.

If you watch one long enough, you will catch its birth. It will already have survived longer than most of its siblings. And it will not likely survive the next high tide.

Still, catching the exuberance of a newly hatched critter the size of an ice cream sprinkle on a warm, June afternoon changed me. Strangers saw a wild-haired middle-aged man squatting by the water's edge, staring intently at nothing, gesticulating a bit too much for others to come share his excitement.

I would not have let my kids close to me, either, had our roles been switched. Fortunately for my kids, I am not a stranger.


The smell of dead or dying crustaceans can overwhelm a kitchen. On an open beach, however, mixed in with the salty life-teeming spray, a balance is reached. At high tide, the smell is almost too clean; at low tide, whiffs of the decaying mud are sharp, but not repulsive. The tide washes over us twice a day, the rhythm of mortality.


I occasionally find horseshoe crabs stranded on the beach. I will gently pick them up, and return them to the water. I may have returned thousands by now. I cannot make up for the hundred I slaughtered. That is not why I do it.

Every summer I show children how to pick up a horseshoe crab. Cradle the carapace with your hand. Do not carry them by their tails. I touch the point of the tail, show a child there is no stinger. They are gentle creatures. Omnivorous, true--clams, worms, and algae, so perhaps not so gentle, but certainly not harmful to humans.

Young loggerhead turtles snack on horseshoe crabs. Humans use their blood in medical research. Otherwise, they have few "enemies." Not sure being higher up in the food chain makes one an enemy. Someday I will feed the worms, unless some stranger stuffs my veins with formaldehyde and buries me too deep to be useful.

Horseshoe crabs live an average of 19 years, or so the scientists will tell you. I doubt the average longevity matters to a horseshoe crab--the mad, exuberant spinning of horseshoe crab embryos one June afternoon reminded me what matters.

Ask me someday....ask me in June. I will show you. Words will not do. I bet you smile like an idiot, too....

Sources: Personal observations; "The Horseshoe Crab,(Limulus polyphemus)," Maryland Sea Grant Schools Online Network A.G. Scientific, Inc., Product Catalog,


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