The exact species used as soft-shells varies from region to region. In the United States, the blue crab (Callinectes sapidus) is used typically, although the use of the mangrove crab in Asia has provided another source for this seasonal food.
As these crabs grow larger, their shells cannot expand, so they molt the exteriors and have a soft covering for a matter of days when they are vulnerable and considered usable. Fishermen often put crabs beginning to molt aside, until the molting process is complete in order to send them to market as soft-shells. Crabs should be kept alive until immediately before cooking by the customer or restaurant so that they are fresh. Usually crabs must be eaten within four days of molting to be useful as soft-shell crabs. They begin to rebuild their shells after that, and when eaten, have a thin shell developing. These are often referred to as "papershells" or "tinbacks" and are more crunchy when eaten, making them less desirable to many people.
With the blue crab in cold waters this molting is highly seasonal and usually lasts from early May to July. Demand for this delicacy has increased with the use in Japanese and other cuisines, so that the mangrove crab has been used as an alternative source from Asia. Because mangrove crabs grow in tropical muddy flats all year round, such swamps provide a continual source of soft-shell crabs. In warmer waters such as the Gulf of Mexico, soft-shell crabs are available for longer periods. The crabs continue to molt throughout the year, but in smaller numbers, sometimes making it unprofitable to fishermen to maintain traps through those periods.