After the tide goes out the crabs come out to feed. In the top left corner of the photo above you might be able to see some darker dots. They're the perfect round balls moulded by the little Soldier Crabs Mictyris platycheles. Each crab picks up a lump of sand and sifts through it to get the bacteria and algae that film the sand grains, and then rolls the sand grains into a ball before putting it back on the sandflat. The accidental patterns made by the crabs as they work their way across the surface are beautifully artistic.
The crabs themselves are hard to see because as you walk up to them they spiral very quickly below the surface - good fun for children (and adults).