The American horseshoe crab is not a true crab and is not even a crustacean at all. In fact, this species is more closely related to spiders and scorpions than to crabs, shrimps, and lobsters. As a broader group, horseshoe crabs have been around for at least 450 million years, making them one of the oldest lineages of animals alive today. Horseshoe crabs were roaming the seafloor more than 200 million years before dinosaurs existed!
American horseshoe crabs live on sandy bottoms or other soft sediments and roam the seabed in search of benthic, immobile invertebrates or dead and decaying organic matter, which they grind up before passing to their mouths. Using this method, they eat most things that they can find. The American horseshoe crab is covered with a strong exoskeleton that provides it some protection from potential predators. Like in the distantly crustaceans, the American horseshoe crab’s shell really is a skeleton on the outside of its body. The exoskeleton does not expand, and therefore individuals must molt (=shed) it regularly in order to grow bigger. Before molting, an individual begins building a new, larger skeleton inside the existing one. As it gets too big to be contained, it splits open the outer shell, and the new exoskeleton hardens. During this process, the new exoskeleton can be soft for several hours, and the American horseshoe crab is vulnerable to predation.
American horseshoe crabs spawn their eggs on beaches, and thousands of individuals migrate to form massive spawning aggregations at a relatively small number of places throughout its range. The largest such aggregation forms every year in the Deleware Bay, along the mid-Atlantic coast of the United States. Males climb onto the backs of the significantly larger females and ride them, with the high tide, to their preferred nesting sites. The female digs a burrow and spawns her eggs, and one or more males immediately fertilize them, after which they are covered with sand. After approximately 20 days, the eggs hatch during another high tide, and larval horseshoe crabs float out to sea. Occasionally during mating, large numbers of individuals become stranded on the beach and die in the sun.
Two more interesting facts about the American horseshoe crab involve its vision and blood. Vision involves a complicated arrangement of nine eyes, on both the top and underside of the body, some that simply detect light and others that can develop images. The American horseshoe crab’s vision system has been the subject of a huge amount of physiological research.
Unlike in mammals and most animals, the American horseshoe crab’s blood is not iron-based and is instead full of copper compounds. Therefore, rather than being red, it is bright blue! The blood is also a valuable natural product in several medical tests and forms the basis for a large fishery for American horseshoe crabs. Amazingly, individuals do not die after being almost completely drained of their blood, so fishers capture them alive, bleed them in special facilities, and then return them to the ocean to continue living.
Population trends of the American horseshoe crab are not clearly known, but the species is generally considered near threatened with extinction. While the fishery for the medical industry claims to return individuals to the ocean alive, their practices may interrupt mating cycles and other natural behavior. Furthermore, for many years there has been a fishery for American horseshoe crabs to be used as bait in lobster traps. The combination of these activities with coastal development and beach tourism may threaten American horseshoe crab populations.