Tachypleus limulus or Horseshoe crab is usually hidden in the sand of the aquarium. You may distinguish it from its tail, which protrudes slightly from the sand. It is a very ancient, evolutionary, organism, that is, we know that its ancestors existed before humans, plants, dinosaurs, when life first appeared on our planet. It is noteworthy that its blood is blue and is used in various fields of medicine, such as vaccines, medical implants and more.