Most people consider it horribly ugly. Some people think it will pinch or sting you. Even its name is wrong–it’s not a crab at all. But the Horseshoe Crab deserves a bit more respect than it gets–it is one of the oldest existing lineages of life on Earth.
The Horseshoe Crabs are members of the Limulidae family. Despite the name, they are not crabs–they’re not even Crustaceans. They are Chelicerates, and are closely related to spiders, scorpions and mites. They are the survivors of an ancient group of arthropods that is first found in the fossil record during the Ordovician Period, about 450 million years ago–and the “living fossils” of today still look much the same as they did then. The Limulids survived five mass extinctions, saw the first appearance of animals with bones, watched amphibians invade the land, saw the dinosaurs come and go, and were still here when a primate on the African savanna first stood erect and cracked two stones together.
There are four living species of Limulid. Three of them are found in the Asian Pacific and Indian Ocean coasts, but the fourth, the Atlantic Horseshoe Crab Limulus polyphemus, is found in the US, from the Delaware and Chesapeake Bay down the Atlantic coast around Florida and the Gulf of Mexico.
In appearance, Horsehoe Crabs look . . . well . . prehistoric. In total length, the females can reach as long as two feet from nose to tail tip; the males are much smaller. The Crab’s body is covered by a rounded carapace that looks something like a horse’s hoof. Two compound eyes are visible at the top of the shell–five smaller simple eyes are found along the edge of the carapace, and two more on the underside near the mouth. At the rear end is the long spikey “telson”. Although the spike is scary-looking, it is actually harmless–the Horseshoe uses it only to turn itself back over again if it happens to land on its back. Horseshoe Crabs cannot bite, poke, or pinch.
Pick up a Horseshoe Crab and flip it over, and you will see a whole slew of wiggly things moving around. At the front are the six pairs of legs, with the mouth opening in between them. The first pair of legs has a pair of weak claws known as “chelicerae”, which are used to hold food and carry it to the mouth. The second pair are the pedipalps, which function as feelers–in males the ends of the pedipalps are also used in breeding. The remaining four pairs of legs are used for walking and swimming. And at the very back of the Horseshoe are wide flat structures called “book gills”, used for breathing underwater. The book gills can also hold an internal supply of water, allowing the Horseshoe Crab to crawl onto land and keep breathing for as long as three or four days.
For most of the year, the Horseshoe Crabs wander alone on the ocean floor, feeding on just about anything edible, including small invertebrates and algae. Since they have no jaws, they use their thickly-spined upper legs to mash up food before pushing it into their mouth. In spring and early summer, breeding begins. The males will search for females and then attach themselves to her back end using special hooks on his pedipalps. Often, each female will have a daisy chain of three or four males strung out behind her. At high tide on the night of a new or full moon, the females will emerge from the water onto the beach, dig a small hole, and lay as many as 60,000 small green eggs (each about the size of a BB), then drag the males over them to fertilize them. The eggs are an important source of high-energy food for shorebirds, particularly those who are migrating. The young Crabs will molt several times inside their egg before hatching as miniature copies of their parents, less than an eighth of an inch long. Of the tens of thousands of Horseshoe Crab eggs, only one or two will survive to adulthood. They reach sexual maturity at about age 9, and can live for 20 or 30 years in the wild.
Horseshoe Crab meat is edible, and the eggs in particular are considered a delicacy food item in Asia. In the US, Horseshoe Crab meat is widely harvested and sold as fishing bait, especially for people who trap whelks, conchs, and eels (the females seem to make better bait than the males). In many areas, such as Chesapeake Bay, over-harvesting has led to population declines–which in turn has impacted the number of migratory shorebirds, who depend on the eggs as a food source. As a result, although the Horseshoe Crab is not yet Federally listed as an endangered or threatened species, in 1998 the Atlantic coast states (including Florida) formed a Horseshoe Crab Fishery Management Plan that requires states to identify and protect Horseshoe Crab spawning beaches, and to regulate the number of Crabs taken for commercial use.
The Crabs are also useful to humans for another reason: their blood. Unlike mammals, which use iron-based hemoglobin in their blood, Horseshoe Crabs use copper-based hemocyanin to carry oxygen, which makes their blood blue in color. This blue blood is the only source of Limulus Amebocyte Lysate (LAL), a protein which produces blood clots in the presence of bacterial toxins. Florida’s horseshoe crabs are captured, a quantity of blood extracted from them, and then released unharmed. The LAL is purified from the blood and used to test medical drugs and equipment for the presence of bacterial impurities.