Blue crabs are active predators and effective scavengers. They occur throughout the estuary. They can swim by using their last pair of legs, which have been modified into paddles instead of walking legs. Blue crabs can mate only when the exoskeleton of the female is soft. Just before mating, and also whenever the females or males grow, they shed the exoskeleton in a molt. Exoskeletons are very useful as protection against drying out and predation, but must be molted in order to grow. When you eat a soft-shell crab, you are eating a crab that has just molted.