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Some crabs swim. Most crabs, like stone crabs and spider crabs, walk or run across the bottom. However, crabs in the family Portunidae have specially modified back legs called swimmerettes. These paddle-shaped legs rotate at 20 to 40 revolutions per minute, allowing the crab to quickly swim through the water. One of the most well known crabs in the Portunidae family is the blue crab. In fact, the scientific name of the blue crab is Callinectes sapidus, which translates to "beautiful, savory swimmer."

Just like fish, blue crabs breathe using gills. However, unlike fish, blue crabs can survive out of water for long periods of time-even over 24 hours-as long as their gills are kept moist. When out of water, crabs will seek out dark, cool, moist places to help prevent their gills from drying out and to hide from predators. Crabs also have special articulating plates around their gills. They use these articulating plates to seal off their gills and help keep them moist.

That depends on how well they avoid predators. Typically, the life span for a female blue crab is 1-2 years and a male is 1-3 years; however, in some tagging studies, crabs aged 5 to 8 years old were caught.

Soft-shelled crabs and hard-shelled crabs are of the same species. Blue crabs have a hard shell or exoskeleton. In order for the crab to grow, it must periodically shed its shell in a process called molting. Typically, a crab will seek shelter during this process because it is highly vulnerable to predators. The crab absorbs water, which causes the tissues to swell and split the shell in the back between the lateral spines. The crab then backs out of its old shell and discards it. It continues to pump water into its tissues causing a new shell that grows approximately 33% larger than its original size. The shell hardens again within 2 to 4 days. However, the shell will only harden in water; if the crab is removed from the water, the process is halted. To sell soft-shelled blue crabs, harvesters catch blue crabs just before the crabs are going to molt. The harvester can tell if a crab is in that "molt stage" by the color of the crab's exoskeleton in specific areas. Harvesters place the crabs in large shallow pans, watch them carefully, and take them out of the water soon after they molt.