Horseshoe crabs are marine and brackish water arthropods of the family Limulidae, suborder Xiphosura or Horseshoe crabs. They live primarily in and around shallow coastal waters on soft sandy or muddy bottoms. Horseshoe crabs are so-named for their horseshoe-shaped carapaces. There are four extant species: Carcinoscorpius rotundicauda, found in Southeast Asia; Tachypleus gigas, found throughout East Asia; Tachypleus tridentatus, distributed in South and Southeast Asia; and Limulus polyphemus, which inhabits the Eastern coast of North America. The extinct Mesozoic era genus Mesolimulus was very similar to modern horseshoe crabs, although it differed from them in having a long tail spike.
Horseshoe crabs are hard-shelled animals with a segmented body protected by a tough exoskeleton made of chitin. They have an unsegmented abdomen that tucks underneath their cephalothorax (headshield). Their mouthparts are located on the underside of their headshield and include chelicerae (pincers), mandibles (jaws), and maxillipeds (mouthparts used for feeding). They have five pairs of legs: the first pair is modified into large pincers used for defense and mating; the second pair is smaller than the first and functions as walking legs; the third and fourth pairs are also used for walking but are much smaller than the second pair; and the fifth pair is very small and may be absent in some species. Males have two testes located behind their hearts while females have ovaries located near their stomachs. Both sexes also have a single genital opening located on their ventral side between their third and fourth pairs of legs.
Horseshoe crab blood contains hemocyanin, which gives it a bluish coloration. This respiratory pigment binds oxygen reversibly through copper atoms rather than iron atoms like hemoglobin does in vertebrates, making it less efficient at delivering oxygen to tissues but more resistant to changes in pH levels. When threatened, horseshoe crabs can release large quantities of coagulogen from specialized cells called Leydig’s organs near their tails. This clotting agent quickly stops bleeding by forming clots around wounds when it comes into contact with air or water. It has been used medically since ancient times to treat various ailments such as nosebleeds, hemorrhages, burns, cuts, etc.,