In the aquarium, these creatures are as peaceful and as docile as they come. They spend most of their time buried in the sand foraging for food. They can often be recognized crawling under the sand's surface by looking for moving sand. The Horseshoe Crab has a hard, spiny shell that is porcelain white in color. Horseshoe Crabs have a number of claw-like legs underneath their hard shell that give them incredible digging ability. They also have a sharp spine that resembles a tail. In nature, this spine is used as a defense mechanism; when threatened the Horseshoe will lift up its spine to impale a predator. The Horseshoe is a very poor swimmer, which can make for great entertainment as they wobble around in the water. These crabs will molt often leaving behind a translucent replica of itself. Crabs belong to the Class Crustacea and Order Decapoda, which is characterized by two pair of antennae, three body parts, and five pairs of legs. The head of a Crab is connected to the thorax and covered by a shell called carapace. They have a smaller abdomen and tail compared to Shrimp and they keep this tucked beneath the carapace. The first pair of their legs are usually developed claws which they use to gather food, use as protection, and to move objects. In order for Crabs to grow they need to shed their exoskeleton, a process called "molting", which allows them to remove their restricting shell and begin a new one. Often times in the home aquarium Crabs will leave this translucent shell in full view so it can serve as a distraction while the Crab finds a hiding place and allows its new shell to harden.