Blue Crab Molting

University of Delaware Sea Grant Marine Advisory Service

The young crab is only 25 mm (1 inch) wide but it grows quickly during the summer, molting every 3 to 5 days and increasing up to 1/3 its size with each molt.

Molting usually does not occur from November until the first 2 weeks in April.

Females molt 18 to 20 times to reach their final molt, while males molt 21 to 23 times and reach a larger size.

When molting, the shell cracks along the back between the carapace and the abdomen. The crab or buster slowly backs out in a soft shell condition.

Times vary, but a 4- to 5-inch crab takes up to 2 or 3 hours to molt.

Over the next 9 to 12 hours, the shell has a leathery feel and is called a Paper Shell. The crab then becomes stiff and brittle during the next 12 to 24 hours.

The shell becomes hard after 72 hours but the crab contains little meat. As days go by, the meat content increases and a new soft shell forms under the hard outer shell.

The White Sign, a faint line or hair sign, then appears just inside the outer brown edge of the hard shell, meaning another 5 to 10 days before the next molt.

The White Sign crab becomes the Pink Sign crab 2 to 5 days before molting and the Peeler Crab or Red Sign crab 1 to 3 days before molting.

In the fall, when the water cools, crabs migrate to deeper bay channels and bury themselves in the mud at 45-degree angles with only their eyes and antennae exposed. They remain this way for the winter.

Source: University of Delaware Sea Grant Marine Advisory Service