Horseshoe crab. Close-up of the underside of a xiphosuran (horseshoe crab), showing its pincers. Despite their name, horseshoe crabs are not closely related to true crabs, instead being closer to arachnids. It is a bottom-dwelling marine animal that inhabits the cool waters of the western Atlantic Ocean and has remained essentially unchanged for the last 500 million years. As such it is considered to be a 'living fossil'. It is known for its mass spawning, in which thousands of horseshoe crabs mount the beaches of north-eastern North America to spawn and lay their eggs.