Sand Balls. What makes them?

Late in the afternoon as the tide goes out there are patterns that appear on the beach.
They are shown in this BroomeCam photo as the dark areas on the sand. Looking up
close you will see tens of thousands of tiny sand balls. These balls are the work of
Sand Bubbler Crabs. They are almost the same colour as sand, and as round as the sand balls they create. They emerge as soon as the tide recedes. You can almost tell how long the tide has been out by the patterns of their sand balls. The more intricate the pattern of sand balls, the longer the tide has been out. They bolt into their burrows at the slightest sign of danger. To observe them, stay still and avoid casting a shadow over them. Then look out for a moving ball of sand!
Sand bubbler crabs eat the thin coating of detritus on sand grains. They scrape up sand grains with their downward pointing pincers and bring these to their mouthparts that then sift out any tiny food particles. The sifted sand is then discarded in a little ball making these amazing patterns.