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Exoskeleton
As invertebrates, crabs do not have a backbone. Instead, they have a hard outer-covering called an exoskeleton that protects their internal organs. The shell of a crab, called a carapace, is made from chitin. When a predator attacks, the crab can tuck its head and legs against its body under the shell. Other animals trying to bite a crab have difficulty penetrating the carapace. Most will give up on the crab and seek other prey.
Chelipeds
The chelipeds, or "claws" of the crab are extremely important in various aspects of the crab's survival. The claws are evolutionarily modified legs that allow the crab to grasp objects. They can grab or carry food, as well as dig into the ground, which is necessary for crab species that like to bury themselves. The crab also uses the claws to warn off attackers or rival crabs when there is a food source or breeding rights at stake.
Advanced Senses
Crabs have an advanced neurological system that includes highly developed senses of smell and sight. Its compound eyes include several thousand optical units it uses to spot food, danger, or potential mates from long distances. The antennae have specialized receptors that can sense chemical stimuli, which helps them search for food. Their mouth parts and legs also have similar receptors. When a crab comes in contact with a food source it quickly passes it into its mouth.
Defense Coalitions
Some species of crabs have developed advanced social relationships. One example is the fiddler crab found in Australia. When a male fiddler crab invades a female crab's territory, a neighboring male crab will often defend her because he knows he is likely to mate with her. This form of "defense coalition" allows females to exchange their mating rights for protection.