Sand crabs are oval-shaped crustaceans found on the Atlantic and Pacific coasts of North and South America. Emerita are popularly known as mole crabs, sand fleas, sand crabs, sea pigs or beach hoppers.

Sand crabs live under the sand in shallow water near the shore. Their lifespan is two to three years. They are the color of the rippled sand at the water's edge and live mostly buried in the sand in the intertidal zone. Their antennae reach into the water, forming a V-shaped obstacle as the waves recede. These antennae filter plankton and organic debris from the water. Sand crabs also eat the tentacles of Portuguese man o' war, which are collected by winding the tentacle around the sand crab's leg. Their camouflage protects them from their predators—chiefly fish and birds.

More: http://en.wikipedia.org/wiki/Emerita_(genus)

This is actually the butt, but it looks like a face to me :P