Horseshoe crabs are considered living fossils because they have changed so little in the 450 million years that they have been around. A horseshoe crab's hard outer shell tends to mask its true identity—the arthropods are related to spiders and scorpions. They live in shallow, sandy habitats that are threatened by coastal development and over-fishing, so conservation efforts in Hong Kong offer hope by sheltering juveniles through their vulnerable moulting process.