Hermit crabs are decapod crustaceans of the superfamily Paguroidea. There are about five hundred known species of hermit crabs in the world, most of which are aquatic and live in saltwater at depths ranging from shallow coral reefs and shorelines to deep sea bottoms.
Hermit crabs are not "true crabs." They have a smaller, soft abdomen which they protect by living in an empty snail shell. Like all other crustacean Hermit Crabs have an exoskeleton. This hard exoskeleton only covers the front portion of the body. The tail is very soft and must be protected inside of the shell. When their home gets too small they will go searching for a new one. In that case you should provide a variety of empty shells that are slightly larger than the one your crab is currently in. Get shells of slightly different shapes and styles and opening size available in their aquarium.
Hermit Crabs have the ability to grow back any missing part through a process called molting. If a Hermit Crab looses a leg, claw or even an eye, it can grow it back when it molts again. Since they lose 90% of their calcium when they shed their exoskeleton, they will eat sometimes their old exoskeleton to regain it.