Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species of horseshoe crabs. Horseshoe crabs fossils have been first seen in the late Devonian. Horseshoe crabs are found in shallow water on soft sandy or muddy bottoms. There has been a decline in number of individuals, as a consequence of coastal habitat destruction and pollution.
The entire body of horseshoe crab is protected by a hard, dark brown carapace. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. While they can swim upside down, they usually are found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish. Females are larger than males, C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60cm long (including tail).