Often referred to as a living fossil, the horseshoe crab has changed very little in over 350 million years. Related to spiders, this animal is easily identified by the large greenish brown, helmet-like dorsal plate, called either the cephalothorax or prosoma. A separate plate covers its abdomen. A long tail spine, referred to as the caudal spine or telson, extends from its abdomen. Measured from the front of its dorsal plate to the tip of its tail spine, the horseshoe crab can reach a length of 24 in (60 cm). Its mouth and six body segments lie underneath its dorsal plate; a pair of limbs is attached to each segment. Today’s horseshoe crab populations are rather sporadically distributed. One species—Limulus polyphemus —lives off the coast of the eastern United States, and three species live in the marine waters of Southeast Asia.
The phylum Arthropoda is the largest phylum in the animal kingdom, containing more than one million species. Within this phylum, the class Chelicerata includes
spiders and their relatives. This class can be broken down into three subclasses: 1) Arachnida, otherwise known as arachnids, this class includes true spiders and scorpions; 2) Pantopoda, also known as sea spiders: and 3) Merostomata, referred to as merostomates. Within the Merostomata subclass, there are two orders. One extinct order, the order Eurypterida, contained sea scorpions; the other order, Xiphosura, includes only horseshoe crabs. There is one family, Limulidae, and three genera within this family—Limulus, Tachypleus, and Carcinoscorpius. In total, there are four species.
Although fossils confirm that chelicerates developed in the sea, there is some debate over their evolutionary history. Some research suggests that animals in this class are descendants of trilobites, the earliest known arthropods which lived 570 million years ago during the Cambrian period; other research suggests that segmented worms are their true ancestors. Whatever the case, the history of the horseshoe crab can definitely be traced back to the Ordovician period, about 500-440 million years ago.
Ancestors and relatives of the horseshoe crab include very diverse animals. For instance, horseshoe crabs are related to mites that never surpass 0.04 in (1 mm) long and to the biggest segmented animal that ever lived—the giant sea scorpion (Pterygotus rhenanus )—which grew to over 6 ft (180 cm) long. Members of this subphylum have adapted to nearly every habitat on land and sea, and most have retained their primitive behaviors.
The horseshoe crab’s body is composed of two parts: the cephalothorax and the abdomen. The cephalothorax is basically the crab’s head and thorax fused together. Under the cephalothorax, there are six body segments, each equipped with a pair of limbs. Under the abdominal shell is located the circulatory, respiratory, reproductive, and nervous systems. Further, the abdomen houses part of the crab’s digestive system and an abundant number of glands.
Like all members of the class Chelicerata—but unlike other anthropods—the horseshoe crab does not have antennae. Instead, it uses its first pair of appendages (called cheliceras), located in front and to the sides of its mouth, to feed itself. The cheliceras, and all of their appendages except for their walking legs, are equipped with pinchers (called chelas) with which the animal grabs food from the sea floor. The second pair of legs (called the pedipalp) evidently used to be used for walking, but, over time, evolved more specialized functions. Currently, the second pair of legs are used in different ways, depending on the species; basically, these legs can be used for gripping, chewing, or sensing.
While the horseshoe crab does not have a conventional jaw, its four pairs of walking legs have special equipment attached to them. Known as gna-thobases, these are primitive devices that the crab uses to manipulate and shred food before passing it to its mouth. The last pair of walking legs can be used to break shells and to crush tough food. Because the crab often swallows sand and shell fragments, its gizzard is quite powerful and can grind up almost anything it consumes.
The horseshoe crab has two sets of eyes. The first pair are large and compound, meaning that they are composed of numerous simple eyes clustered tightly
Caudal spine —Also called the telson, this appendage extends from the crab’s abdomen and resembles a tail; it is often as long as the crab’s body. It is used by the crab to right itself if it falls on its back; the crab flaps it against the abdomen when it swims.
Cephalothorax —The head and thorax (upper part of the body) combined.
Chelas —Pincers on the last pair of walking legs with which the crab grabs food from the sea floor.
Chelicerae —Feeding appendages.
Compound eyes —Two large eyes appearing widely separated on the anterior of the dorsal plate. They are actually composed of numerous simple eyes clustered together.
Gnathobases —Attached to the legs, these spiny devices function like jaws, shredding and manipulating food before passing it to the mouth.
Pedipalp —Their second pair of legs, highly specialized, depending on the species.
Prosoma —See cephalothorax.
Salinity —The amountdissolved salts in water.
Simple eyes —Located fairly close to each other at the anterior of the crab’s back. Easy to overlook.
Thorax —The area just below the head and neck; the chest.
together. These large eyes are located far apart on the front side of the dorsal plate. Much less noticeable, the two small, simple eyes are located fairly close to each other at the anterior of the crab’s back. Little is known about the animal’s other senses.
The horseshoe crab lives in shallow, coastal waters, usually partially covered by mud or sand. It covers itself like this by driving the front of its round dorsal plate forward and downward into the earth. This crab is a sturdy creature, tolerating wide swings in salinity and temperature. As a scavenger, it spends much of its life feeding on all types of marine animals, including small fish, crustaceans, and worms. Interestingly, it swims through the water with its dorsal plate facing the bottom (on its back) by flapping its tail spine into its abdomen.
Horseshoe crabs mature sexually when they are between nine and 12 years old. Typically, when they breed, horseshoe crabs congregate in large numbers in shallow coastal waters. At such times, the male climbs onto the female’s back, and holds the sides of her dorsal plate. (The male is significantly smaller than the female.) She carries him around, sometimes for days, until spawning takes place. When ready to lay her eggs, she digs holes about 6 in (15 cm) deep in a tidal area and lays up to 1,000 eggs in each hole. While she lays these eggs, the male fertilizes them. In approximately six weeks, the eggs hatch into free-swimming larvae that look a lot like their parents, but their tail spines are missing. Because of the inflexibility of their dorsal plates, it is difficult for these animals to grow within their shells; thus they molt several times before their growth stops at sexual maturity.
When a horseshoe crab is wounded, its blood cells release a special protein to clot the bleeding. The same thing happens when certain toxins are introduced to stop invading bacteria. (Horseshoe crabs are a favorite host of flatworms.) Thus, hospitals sometimes use extracts of horseshoe crab blood when diagnosing human bacterial diseases and checking for toxins in intravenous solutions.
Bonaventura, Joseph, Celia Bonaventura, and Shirley Tesh, eds. Physiology and Biology of Horseshoe Crabs: Studies on Normal and Environmentally Stressed Animals. New York: Alan R. Liss, 1982.
Shuster, Carl N., H. Jane Brockmann, and Robert B. Barlow, eds. The American Horseshoe Crab. Cambridge, MA: Harvard University Press, 2004.
Tancredi, John T., ed. Limulus in the Limelight: A Species 350 Million Years in the Making and in Peril? New York: Kluwer/Plenum, 2001.
"Horseshoe Crabs." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 18, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/horseshoe-crabs
"Horseshoe Crabs." The Gale Encyclopedia of Science. . Retrieved September 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/horseshoe-crabs