The Christmas Island Red Crab is a species of land crab that is endemic to Christmas Island and Cocos (Keeling) Islands in the Indian Ocean. Although restricted to a relatively small area, an estimated 43.7 million adult red crabs once lived on Christmas Island alone, but the accidental introduction of the yellow crazy ant is believed to have killed about 10 – 15 million of these in recent years. Christmas Island red crabs make an annual mass migration to the sea to lay their eggs in the ocean. Although its population is under great assault by the ants, as of 2020 the red crab had not been assessed by the International Union for Conservation of Nature (IUCN)
For most of the year, red crabs can be found within Christmas Islands’ forests. Each year, however, they migrate to the coast to breed. The beginning of the wet season (usually October, November) allows the crabs to increase their activity and stimulates their annual migration. The timing of their migration is also linked to the phases of the moon.
This normally requires at least a week, with the male crabs usually arriving before the females. Once on the shore, the male crabs excavate burrows, which they must defend from other males. Mating occurs in or near the burrows. Soon after mating the males return to the forest while the females remain in the burrow for another two weeks. During this period they lay their eggs and incubate them in their abdominal brood pouch to facilitate their development.
At the end of the incubation period the females leave their burrows and release their eggs into the ocean, precisely at the turn of the high tide during the last quarter of the moon. The females then return to the forest while the crab larvae spend another 3 – 4 weeks at sea before returning to land as juvenile crabs.
The crabs’ breeding timetable is fixed around the phases of the moon. Spawning (the dropping of their eggs into the sea) must occur before sunrise on spring tides during the last quarter of the moon, regardless of any other factor. The timing of spawning is the only certain and predictable part of the whole migration. All other stages of the migration will vary with the prevailing weather.
The red crab breeding migration comprises a series of separate actions on the crabs’ part that follow on from one to the other in a programmed sequence. These separate actions in combination make up the breeding migration and one action will not occur unless the preceding action is accomplished. If there isn’t enough time for them to be able to do all of these things before the next spawning opportunity, they will delay the start of their migration and attempt to meet the following month’s spawning date.
All phases of the crabs’ breeding migration involve colossal numbers of crabs and usually occur all over the island. If the rains continue, there is usually a second, and sometimes even a third, smaller, downward migration by crabs that did not join in the first migration. When this happens it is possible to see crabs on return journeys mingling with the crabs on their downward migration. It can become confusing for all concerned.