Crabs have short tails. A crab's tail and reduced abdomen is entirely hidden under the thorax. It is folded under its body, and may not be visible at all unless the crab is turned over. Usually they have a very hard exoskeleton. This means they are well protected against predators. Crabs are armed with a single pair of claws. Crabs can be found in all oceans. Some crabs also live in fresh water, or live completely on land.