Read more about Christmas Island's intertidal, subtidal and marine crabs in the book
"Crabs of Christmas Island" by Max Orchard
Intertidal crabs effectively inhabit two environmental regimes. The marine intertidal zone is between the high and low tide levels in a coastal zone within the transition from land to sea, and accordingly, organisms that live within this zone are exposed to marine conditions during high tide and “terrestrial” conditions at low tide.
Organisms living in the intertidal zone experience a suite of physical stresses, including fluctuations in temperature, aerial exposure, salinity, and hydrodynamic forces. Most of the intertidal crab species are active only when covered by water, and are well hidden at low tide. Some species however become active in air when the tide has receded and they will forage across the reef flat during the day and at night.
Below are some of the more easily observed intertidal species.