These tiny crabs were all over the beaches of Koh Kood. They would scurry into their holes when approached.
According to Wikipedia:
Sand bubbler crabs (or sand-bubblers) are crabs of the genera Scopimera and Dotilla in the family Dotillidae. They are small crabs that live on sandy beaches in the tropical Indo-Pacific. During the low tide, they form inflated[clarification needed] sand pellets that are disintegrated by the incoming high tide.
Filename: Sand Bubbler Crab.jpg
Copyright Sutton Photos Zach Sutton, sutton.photos