Coquina are tiny little clams which are seen at the water's edge, or the "swash zone".  When the waves come up, and the sand is loosened, they wriggle down into the wet sand; after a bit they squirm back up again and the next wave carries them to a different place, where they wriggle down again. They are all different colors. Here is a movie:

The coquina feed on small particles in the sand and water, and the fish and shorebirds, as well as the olive snails, feed on the coquina.