Horseshoe Crab (Limulus polyphemus)

Despite the name, this isn't really a crab at all — horseshoe crabs are chelicerates, genetically closer to spiders and scorpions than to anything that's earned the word "crab." The lineage predates the dinosaurs by a couple hundred million years, and its blue, copper-based blood is valuable enough to the pharmaceutical industry (it's the basis of the LAL test used to screen for bacterial contamination) that wild horseshoe crabs get harvested, bled, and released just for that. In the tank it's a slow, methodical sand-plower that needs serious room to reach anything close to full size.

Care level: Moderate · Temperament: Peaceful · Max size: ~18" · Min tank: 75 gal, deep sand bed · Reef safe: Yes · Diet: Omnivore, scavenger