In the center you can see one of our tiny zen artists:   a Thai Bubble Crab.    They are about 2-3mm in diameter. They make the balls as they feed off of the algea attached to individual grains of sand.   When the ball (or sand bubble) gets too big to sit on, they roll it to one side and start over again... creating the most fantastic sandpaintings as they lunch away the day.   

They're a bit shy so I had to make several efforts to get a portrait.

See http://en.wikipedia.org/wiki/Sand_bubbler_crab.