Soldier crabs (FAMILY MICTYRIDAE) are a beautiful small and unique group of crabs that live on tidal mudflats in sandy estuaries and mangroves along the coast of most of Australia and in the islands of the western Pacific Ocean and South-East Asia.
They have a rounded body shape, and they walk forward instead of sideways like other crab specie. They emerge from their holes in the sand at low tide and move in large groups of dozens or even thousands of individuals leaving the sand patterned with sand nodules. .
They move together in the same direction creating the illusion of moving sand, I have seen, this moving pattern in the sand, made by the soldier crabs, it is fascinating, to watch.
As they move they place sand into their mouths with their pincers eating any organic matter contained amongst the grains leaving behind small, round pellets of sand now devoid of organic material.
The soldier crabs are most active on the surface at low tide and at high tide; they protect themselves from fish by burrowing into the sand with a corkscrew motion, seal themselves in an air chamber with a plug of sand.