Why must crustaceans molt as they grow?

Crustaceans, like other arthropods, possess a rigid exoskeleton that provides essential protection and support for their bodies. Because this outer covering is nonliving and inflexible, it prevents the animal from increasing in size while it is intact.12

To accommodate growth, crustaceans must undergo molting, or ecdysis, which is the periodic shedding of the old exoskeleton. During this process, the animal secretes a new, soft, and flexible cuticle beneath the existing shell. Once the old exoskeleton splits and the crustacean emerges, it rapidly expands its body—often by taking in water—before the new cuticle hardens and calcifies. This cycle allows the animal to achieve steplike increases in size throughout its life.12