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Crab
Florida keys
I have moved this to arthropods for you.
Agree Nance its type of Fiddler Crab...
http://www.nps.gov/pais/naturescience/fi...
My guess is that this is a fiddler crab: http://myfwc.com/wildlifehabitats/profil...
They are seen all over coastal Florida. The species above are from Maine and Alaska.