Christmas Island is home to the most remarkable abundance and diversity of land crabs in the world. Their scavenging activities recycle nutrients locked in fallen leaves, the burrowing tills the soil and their grazing on fruit trees and seedlings is an important factor in determining forest composition. The land crabs have evolved from marine ancestors as shown by their return to the sea for spawning. Apart from the 20 land crabs shown here, over 160 other species have been recorded from the reefs and shallows of Christmas Island.
Crabs are classified in the vast group of animals called arthropods which have a hard external skeleton. Insects are arthropods, as are spiders, scorpions, millipedes and centipedes. Crabs belong in the arthropod division called crustacea. Crustacea are distinguished from other arthropods by the simple feature of having two pairs of antennae.
Robber and hermit crabs - Infra-order Anomura, Family Coenobitidae
Robber crab - Birgus latro
Robber crabs are a close relative of hermit crabs, but do not (at least as adults) carry shells to protect their abdomen. They are found in most parts of the island. From the shore terrace to the highest plateau areas, generally sheltering during the day and venturing out at night or on overcast days. They forage on any vegetable material or carrion on the forest floor. The fruits of some trees and the pith of fallen Arenga palms are favourite foods. Though found through the tropical islands of the Pacific and Indian Oceans, Christmas Island now supports the world's largest population.
Hermit crabs - Coenobita sp.
There are three species of the genus Coenobita recorded on Christmas Island. This number may be revised with further research. As many as eight species are found in the Indo-West Pacific. When spawning, hermit crabs carry their eggs inside the shell attached to hairs on the pleopods on the left side of their body.
Red hermit crab - Coenobita perlatus
Carapace length > 34mm. Distinguished by flattened triangular eyestalks and red colour of the carapace or red about the joints of the legs on a white to pink background. Recorded on most beaches, they prefer rubble beaches. Hermit crabs are shy and stay close to cliffs.
Purple hermit crab - Coenobita brevimanus
The largest of the island's hermit crabs (upto 45mm carapace length). A distinctive purplish brown crab with cylindrical eyestalks. Found mainly on the beaches and shore terraces.
Ghost crabs | infra-order Brachyura, family Ocypodidae
Horn-eyed ghost crab - Ocypode ceratophthalmus
Carapace > 45mm. There are two colour forms on Christmas Island - one is deep olive green (common on Dolly Beach and the colour form common on east Indian Ocean islands) and the other is greyish yellow to cream. It may be that these are two distinct species. The 'horned eyes' are shorter in females and half grown crabs and undeveloped in juveniles.