Infra-order Brachyura, Family Grapsidae, Subfamily Sesarminae
Mottled crab | Metasesarma obesum (Dana, 1851)
A tiny crab with a squarish carapace up to about 20 mm wide. Color is widely variable.
Usually found under rocks, leaf litter and in rock crevices at the top of island beaches in the intertidal zone to 5 metres above sealevel.
Read more about Christmas Island's mottled crab in the book
"Crabs of Christmas Island" by Max Orchard