Scopimera sp. -or- Dotilla sp.
Sand bubbler crab of the genus Scopimera or Dotilla (family Dotillidae). These small crabs live on sandy beaches in the tropical Indo-Pacific, and are abundant on the beaches of this part of Papua. They form inflated sand pellets as they sift sand during feeding. These are pushed out of their burrow and destroyed during tidal inundation. At high tide the reside in burrows that contain a trapped pocket of air which they are able to breathe until low tide.
Intertidal ocean beach.
The discarded "sand pebbles", an artifact of the feeding process, form attractive patterns on the beach at low tide. These crabs are very nervous and shy and readily return to their burrows on the slightest approach.