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Molting is the process where blue crabs shed their rigid outer shell (exoskeleton) to allow their bodies to increase in size.  This must be done several times during their lifetime.




Blue Crab


Fiddler Crabs



 Blue Crab                    Molting

As a blue crab mature, its body size increases. The problem is that its shell is composed of chitin, a hard material that does not grow, so the crab sheds it shell and and a new one is formed on the larger body.


Crabs molt more often when they are small an less often as they grow.

The result of each molt is typically a 25%-40% increase in carapace width.


The pictures to the right show the stages of the molting process.


 <double click on any image to enlarge>


When a blue crab is ready to molt it is known as a "peeler" crab.

When it starts to molt, the crab cracks the back of the shell between the carapace and the abdomen.

The crab then slowly backs out. It is then in a soft-shell condition. This is also called "busting".

The time to complete this phase varies, but can take about 2 to 3 hours to complete the molt.

After shedding its old shell, the crab expands by pumping water into its body and the new shell begins to form.

The soft-shell crab is very vulnerable to predators (including other crabs) and hide in rocks, aquatic vegetation (SAV) or bury in mud or sand.

Within a few hours after molting, the new shell begins to harden.  After about 12 hours the shell is slightly stiff and the crab is referred to as a papershell.

After another 12 hours, the shell becomes harder, yet still pliable and is referred to as a buckram.

The total process of molting takes about three to four days to complete resulting in a (bigger) hard shell crab.







Without its old hard shell (which, is called a "slough" when it is discarded), the crab is now temporarily a soft-shelled crab.

The new exoskeleton is there, but it is still very soft.





Another image of a blue crab molting















photo by Alicia Young-Williams

photo by Alicia Young-Williams

photo by Alicia Young-Williams

photo by Alicia Young-Williams

photo by Alicia Young-Williams

photo by Alicia Young-Williams

photo by Alicia Young-Williams




Smithsonian Environmental Research Center  and Alicia Young-Williams