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When walking on the beach you will invariably encounter crabs of various types. But did you know that almost all of these crabs do not live on the beach but are washed up from deeper water?
The only crab that actually lives most of its life on the beach out of water is the amphibious ghost crab, which lives in burrows and comes out to forage on the beach. It can run quite fast across the dunes but must release its eggs in the water to hatch and pass through a planktonic phase.
Some other crabs that spend their entire lives underwater but are occasionally washed up on the beach are calico, spider, flame box and speckled. The first three are bottom walkers that may bury themselves in the sand. The speckled is quite different in that it is a swimming crab (as is the blue crab) with flippers on the rear-most leg that allow it to move quickly through the water.
The beauty of the beach is deceptive in that it is actually a harsh environment to which few species can adjust. Salinity, moisture, tide height, wave action, temperature and the sandy substrate are quite variable which poses problems for adaptation. Thus biodiversity increases many-fold as you leave the beach and go offshore. So next time when you are walking the beach, see if you can separate the true inhabitants of the beach and dunes from those which are just washed up from their homes below the water line.
Bill Dunson, Englewood, FL, and Galax, VA