Horseshoe Crabs

Often referred to as a living fossil, the horseshoe crab has changed very little in over 400 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 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 four 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 subphylum Chelicerata includes spiders and their relatives. This subphylum can be broken down into three classes: (1) class Arachnida (Otherwise known as arachnids, this class includes true spiders and scorpions); (2) class Pantopoda (also known as sea spiders); and (3) class Merostomata (referred to as Merostomates). Within the Merostomata class, 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.

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 their blood when diagnosing human bacterial diseases and checking for toxins in intravenous solutions.