Soft-shell crab is a prized dish in many ocean-side resorts. why are the crabs' shells soft? the crab has just molted and the new exoskeleton has not yet hardened. these species have evolved a light and soft exoskeleton so that they can float in the water column. these species have evolved a light and soft exoskeleton so that gases can diffuse directly between body of the crab and the surrounding water. these species of crabs live under rocks and therefore have lost the hard exoskeleton over evolutionary time.