This culinary term refers to any crab that has just molted its old exoskeleton and is still soft and flexible on the outside. The species most commonly sold as soft-shell crab in the U.S. is the blue crab, Callinectes sapidus, which has the convenient habit of mass molting between April and September on the East Coast.
At any time in its life, a crab has two exoskeletons—the one we see on the outside and another one just underneath it, which is always in the process of growing. A few days before the old exoskeleton is shed, the crab stops eating. Hormones, directed by the eyes of the crab, initiate the separation of the outer from the inner exoskeleton and from the fleshy part of the crab. Within a few days, the back of the crab’s carapace pops up and the crab pulls itself out, completing the molt. The new exoskeleton hardens over the next several hours to several days; during this time, the crab sucks up water to expand the new pale-colored exoskeleton.