Crab Molting
Crabs (and other crustaceans) cannot grow in a linear fashion like most animals. Because they have a hard outer shell (the exoskeleton) that does not grow, they must shed their shells, a process called molting or ecdysis. Just as we outgrow our clothing, crabs outgrow their shells. Prior to molting, a crab reabsorbs some of the calcium carbonate from the old exoskeleton, then secretes enzymes to separate the old shell from the underlying skin (or epidermis). Then, the epidermis secretes a new, soft, paper-like shell beneath the old one. The crab gradually retracts all of its body parts from the outer shell, a process that can take several weeks, while it begins to secrete a new shell beneath the old one.
Several days to a week before molting the crab stops eating. A day before molting, the crab starts to absorb seawater and begins to swell up like a balloon. This helps to expand the old shell and causes it to come apart at a special seam that runs around the body (the epimeral line). The carapace then opens up like a lid. The crab extracts itself from its old shell by pushing and compressing all of its appendages repeatedly. First it backs out, then pulls out its hind legs, then its front legs, and finally comes completely out of the old shell. This process takes about 15 minutes.
​
When a crab molts, it removes all its legs, its eyestalks, its antennae, all its mouthparts, and its gills. It leaves behind the old shell, the esophagus, its entire stomach lining, and even the last half inch of its intestine. After molting, The new shell is very soft at first, making the crab vulnerable to predators. Within a few days, the shell hardens up, and it becomes very hard after a month.
​
Crabs that have lost legs can regenerate them over time. The leg breaks off at a special joint. Before molting, a new limb bud, with all the remaining leg segments, grows out of the joint (see Dungeness crab page). After molting, the new leg is smaller than the others. The crab needs three molts to grow a leg back to its normal size.
​
Besides allowing the crab to grow, molting helps to get rid of parasites, barnacles, and other animals growing on the shell. It also helps to get rid of shells damaged by bacteria that degrade the chitin in the exoskeleton.
​
The photos at right show:
1. Blue crab Callinectes sapidus undergoing ecdysis;
2. The newly formed shell is about 25% larger than the old one; this difference is called the "Growth Increment";
3. The old shell (exuvium) left behind is empty of tissue. Beachcombers often think these are dead crabs.
4. A red king crab undergoing ecdysis, with the new shell appearing at the rear of the epimeral line.
​
