Thousands of these sand crustaceans were feeding just below the sand, right at the ocean's edge. As the ocean wave recedes, you can detect their bodies as they create v's in the sand. Their heads face toward the ocean (they bury themselves backward--quickly) and front appendages angle outward, creating a temporary "v" in the sand as the water runs off. I suspect they were so abundant, in part, because the masses of people at Moonlight Beach keep the shorebirds at bay.