Sand fleas, the size of your thumb, live in the wake zone where ocean waves break along the sandy beach. These creatures are not really fleas; they are a species of crab called mole crab or more appropriately named the Atlantic sand crab. Mole crabs live buried in the sand facing the direction of the waves where they filter plankton (micro-organisms) from the sea water.
Shorebirds, seabirds and fish eat these small crabs. Sand fleas are often harvested by anglers for bait to catch ocean faring fish. Some people eat them too. Yummy!