Horseshoe Crab Scientific Name: Limulidae Interesting Facts: 1. Horseshoe crabs are more closely related to spiders, ticks and scorpions than they are to true crabs. 2. Horseshoe crabs are ancient. They are the only living members of a group of animals that first appeared about 450 million years ago. 3. Horseshoes crabs usually walk along the bottom of shallow water, but they can also swim awkwardly on their backs by using their gills as paddles.