The shell s abdominal covering. The shape of the apron is one way to determine the sex of a crab. The male has a t-shaped apron.
Immature females have a triangular apron sealed to the body.
Mature females have a broadly rounded apron, which is not sealed to bottom portion of the shell. Crabs in the stage following papershell, when the shell is harder but still pliable. A mating pair of crabs, when the male carries the female beneath him. During this period, which can last two days or more, the male protects the female during her vulnerable soft-shell stage. She does not leave him until her shell has hardened and she can protect herself.