This is on a narrow barrier island. It appears to me that the crabs leave their burrows near low tide and march in mass along the beach until they find a breach in the layer of flotsam and grass that borders the beach. There they flood onto the tidal flats to feed. There appears to be a layer of algae attached to the sand about 25 yards out that they migrate towards at the lowest stage of the tide. I wonder if they are vegetarian.