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,