It is not an individual species, as some believe. In order to grow, a crab (or lobster) must periodically shed its shell and grow a new, larger one. Immediately after undergoing this process (called "molting"), the crab has a soft exterior. It takes days, or longer, for the crab's exposed surface area to ossify. Until it does, the crab is highly vulnerable to predators (including two-legged diners) and is known as a soft-shell crab. The nonpareil soft-shell crab, from a seafood buff's perspective, is a young blue crab that has molted within the hour.
** Asian Recipes