Little nipper - Geograpsus greyi
Infra-order Brachyura, Family Grapsidae
Grapsid crabs are a cosmopolitan family of crabs including pelagic and littoral and shore crabs as well as a few that have adapted to a strictly terrestrial or to a freshwater mode of life.
The terrestrial nippers of the Geograpsus genera are principally air breathers with reduced gills and enlarged branchial chambers, the interior lining of which functions as a respiratory structure similar to a lung. The crabs must keep this structure moist, but they do not keep it full of water. All nipper crabs are distinguished by having a flattened carapace and a serrated forward edge on the merus of the claws.
Grapsus and Pachygrapsus are the most common genera of the family that live in the splash zone. These crabs are visible during the day but are principally active at night scavenging and grazing on algae off the rocks. They are extremely fast and agile crabs, and also have a flattened carapace, which likely assists them from being swept away underneath crashing waves.
Read more about Christmas Island's grapsid crabs in the book
"Crabs of Christmas Island" by Max Orchard