The robber crab Birgus latro is the largest land-living arthropod in the world. Measuring up to 1.5 metres and weighing up to 4 kg. They can live for more than 30 years. Photo by K Singer
Christmas Island is an isolated, oceanic island that has never been near a large land mass. This isolation has created a unique set of ecological relationships characterised, among other aspects, by the evolution of new species and sub-species restricted to Christmas Island, and profound changes in the biology of immigrant species establishing their niche on the island. The island is also a focal point for seabirds of various species.
The land crabs of Christmas Island are remarkable for their abundance and the role they play in the ecology of the island's rainforest. Through their scavenging activities they recycle nutrients locked in fallen leaves, their burrowing tills the soil and their browsing on tree fruits and shoots probably is an important factor in determining forest composition. It has been suggested that plant life on the island reached a stable climax stage long ago due to the presence of land crabs.
21 land or freshwater crabs have been identified on the island. Of these, the red crab, blue crab and Jackson's crab are thought to be endemic. The three largest and most common crabs are the red crab (Gecarcoidea natalis), the robber crab (Birgus latro) and the blue crab (Discoplax hirtipes); they live mainly on fallen leaves and fruits of the rainforest.
The land crabs have evolved from marine ancestors as shown by their return to the sea for spawning.
The sight of the massive population of red crabs on their annual breeding migration or a crowd of huge robber crabs around a fruiting Arenga palm must rank with the outstanding natural spectacles of the world.