Infra-order Brachyura, Family Gecarcinidea
Purple crab specimens collected on Christmas Island have genetically split into two groups. One conforms, (in terms of morphology and genetics), to those of Gecarcoidea humei from the Straits of Malacca, Aceh, Nicobar Islands and the Andaman coast of South Thailand.
The other appears to be affiliated with Gecarcoidea lalandii specimens collected from the Pacific side of its distribution.
It is clear that there are two species of purple crab on the island, although it is puzzling why the island has the two species. Research is continuing into this issue.
Purple crab | Gecarcoidea humei Wood-Mason, 1873
A large crab that grows to 150 mm carapace width. Top of carapace generally an overall light purplish-grey or dark purplish-brown grading to creamish brown under body. Slender legs of similar colour; claws white-brown with purplish grey tinge. Scars and creases on carapace yellowish white. Has small stark white spots on the carapace to the outside of the eye cavities.
Distribution is Straits of Malacca, Aceh, Nicobar Islands and the Andaman coast of South Thailand, Christmas Island.
Found in moist areas along the coasts of islands, typically within the forest. Found all over Christmas Island but very uncommon.
Purple crab | Gecarcoidea lalandii H. Milne Edwards, 1837
A large crab. Grows to 160 mm carapace width. Dark purple colouring on upper body grading to cream
underneath. Has small stark white spots on the carapace to the outside of the eye stalks. The large rounded claws are generally similar but lighter in colour than the carapace with white fingers.
Gecarcoidea lalandii is the rarer of the two purple crab species on the island although it has been found at
widespread locations. Found in moist areas along the coast and within forest. Have been found all over the island however are very uncommon.
Read more about Christmas Island's purple crabs in the book
"Crabs of Christmas Island" by Max Orchard