What is the hard outer covering of a crab called?

The hard, external covering found on crabs is known as an exoskeleton. This rigid outer layer serves as both a protective armor and a structural support system for the crab's soft internal tissues and organs. Unlike vertebrates with internal skeletons, crabs and other arthropods rely on this external shell for their shape, defense against predators, and as a point of attachment for their muscles, which allows them to move their legs and claws. This tough casing is primarily composed of chitin, a strong, fibrous substance.

The presence of this unyielding shell presents a unique challenge for growth. Since the exoskeleton cannot expand, a crab must periodically shed its old covering in a process called molting, or ecdysis. During this vulnerable time, the crab extracts itself from its old shell, often seeking a hidden spot. It then absorbs water to expand its body before a new, soft cuticle hardens over several days. This temporary softness leaves them highly susceptible to predators until their new protective armor fully calcifies.

This remarkable adaptation is a defining characteristic of arthropods, a group that includes not only crustaceans like crabs, but also insects, spiders, and scorpions. The exoskeleton has been a key factor in the evolutionary success of these diverse creatures, allowing them to thrive in a vast array of environments by providing both protection and a framework for complex movement.