Infra-order Brachyura, Family Grapsidae
Grapsus crab | Grapsus tenuicrustatus Herbst, 1783
The greenish “rock-hopping” crabs present in high numbers around the island coastline. Has a flat, thin disc
like carapace up to 65–70 mm wide. Body coloration varies widely from dark blue-green to light streaked brownish. Legs usually reddish brown marbled with green. Claws and face reddish. Also known as Spider crab, Sally-lightfoot crab or Rock crab in other areas of their range.
Read more about Christmas Island's grapsus crabs in the book
"Crabs of Christmas Island" by Max Orchard