Crabs, like all crustaceans, possess a rigid external covering known as an exoskeleton. This hard shell provides protection and support, yet it does not grow alongside the crab’s body. To accommodate their increasing size, crabs undergo a biological process of shedding this outer layer. This process is called molting, or ecdysis.
Why Crabs Molt
Crabs primarily molt to grow. Their rigid exoskeleton cannot expand, so they must periodically shed it to increase in size, allowing their soft tissues to grow and fill a new, larger shell. Molting also serves other biological purposes. Crabs can repair damaged shells or regenerate lost limbs, like legs or claws, during this process. It also helps shed accumulated parasites, barnacles, or other attached organisms.
The Molting Process
The molting cycle is initiated by hormonal changes within the crab. In the pre-molt phase, the crab reabsorbs calcium and minerals from its old shell, recycling these resources for the new exoskeleton. Simultaneously, a new, soft, and flexible shell begins to form underneath the existing one. The crab may also store water and nutrients, and muscles, like those in the claws, can shrink to facilitate extraction from the old shell.
When ready, the crab pumps water into its body, building hydrostatic pressure that causes the old exoskeleton to crack, often along a predetermined line at the back between the carapace and abdomen. The crab then extracts itself from its old shell, pulling out all its appendages, eyestalks, mouthparts, and even the lining of its digestive tract. This emergence can take 10 minutes to several hours, leaving behind an empty shell.
Life as a Soft-Shell Crab
Immediately after molting, the newly emerged crab is soft and vulnerable, often called a “soft-shell crab.” To expand its new shell, the crab rapidly absorbs water, inflating its body and stretching the new, soft casing.
During this period, the crab seeks a safe, secluded hiding place, such as burying itself in sand or mud, or finding shelter among rocks or vegetation. The new shell then hardens through calcification and sclerotization, processes where calcium carbonate is deposited and proteins form rigid structures. This hardening process can take a few days to several weeks, depending on the crab’s size and species. Many crabs also consume their discarded exoskeleton, recycling valuable calcium and other minerals to strengthen their new shell.