Mother and son at Cox’s Bazar, Bangladesh!
Sand bubbler crabs live in burrows in the sand, where they remain during high tide. When the tide is out, they emerge on to the surface of the sand, and scour the sand for food, forming it into inflated pellets, which cover the sand. The crabs work radially from the entrance to their burrow, which they re-enter as the tide rises and destroys the pellets. The material consumed by sand bubbler crabs has a very low concentration of organic matter, which is concentrated by egestion of indigestible material.
The Beach at Cox’s Bazaar had lots of these pieces of art!