The Atlantic Sand Crab (Emerita talpoida) is also known as the mole crab and the sand flea (confusingly, since there are, in fact, amphipod sand fleas).
These streamlined animals are, at any rate, crustaceans. As Sarah Oktay explains from the place I first came across them, they are surf-zone specialists, and pretty important in that harsh habitat. 
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This work by Matthew Wills is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.
Backyard and Beyond 
Very cool. I’ve often thought of trying to catch one of these, but never have.
This one was deceased. I’ve never come across one so large and intact before.