The Horseshoe Crab is a fascinating organism and is one of the world?s oldest. It is a ?living fossil?. Though it is called crab, it is not really a crab; instead it is closely related to spiders and scorpions. The Horseshoe Crab has three main parts of the body: the head region, known as the ?prosoma?, the abdominal region or ?opisthosoma? and the spine-like tail or ?telson?. It has five pairs of jointed legs and a pair of pincers, and it molts its skin many times as it grows. The Horseshoe Crab is a hard-shelled invertebrate that lives in warm, shallow coastal waters on the sea floor. It has light blue, copper-based blood and breathes using gills. The Horseshoe Crab also has a long tail which acts as steering and it helps in righting itself when it is flipped upside down. Both the sexes look similar, but female horseshoe crabs are much larger than males. The Horseshoe Crab has a compound eye on each side of the prosoma, five eyes on the top of the carapace, and two eyes near to the mouth, making a total of nine eyes. It is essentially a nocturnal animal and it digs for food under the sand bed. Therefore, it is suggested to keep a thick sand bed in your aquarium. The Horseshoe Crab can feed on worms, algae and mollusks in the sediment. They are very effective scavengers at an early age, and are great for aerating and maintaining sand beds through sifting the sand and gravel in search of its food. The Horseshoe Crab is reef safe but on attaining bigger size it might harm other tank mates.