It's that time of the year when in October / November the Christmas Island Red Crabs on Christmas Island all march together towards the ocean for the mating season. The Christmas Island Red Crab is by far the most obvious of the 14 species of land crabs found on Christmas Island. It is estimated that there are 40-50 million of these bright red land crabs all over the Island.
Christmas Island is located in the Indian Ocean, 1500 kms West of the Australian mainland and 2600 km from Perth, Although it is an Australian Territory, Christmas Island's nearest neighbour is Indonesia which lies about 350 kms to the North. The Island is around 500 kms from Jakarta.
In 1958, the Island was excised from Singapore and sovereignty was transferred to Australia. As part of the transfer, Australia paid Singapore £2,800,000 as compensation for lost phosphate revenue. Christmas Island became an Australian Territory on 1st October, 1958 - a day still celebrated on the Island as Territory Day.
The Annual Migration To The Ocean
At the beginning of the wet season (usually October / November), most adult Red Crabs suddenly begin a spectacular migration from the forest to the coast, to breed and release eggs into the sea. Breeding is usually synchronized island wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea. The timing of the migration breeding sequence is also linked to the phases of the moon, so that eggs can be released by the female Red Crabs into the sea precisely at the turn of the high tide during the last quarter of the moon. It is thought that this occurs at this time because there is the least difference between high and low tides. The sea level at the base of the cliffs and on the beaches, where the females release their eggs, at this time varies the least for a longer period, and it is therefore safer for the females approaching the water’s edge to release their eggs. Sometimes there are earlier and later migrations of smaller numbers of crabs but all migrations retain this same lunar rhythm.
The main migration commences on the plateau and can last up to 18 days. Masses of crabs gather into broad “streams” as they move toward the coast, climbing down high inland cliff faces, and over or around all obstacles in their way, following routes used year after year for both downward and return migrations. Movement peaks in the early morning and late afternoons when it is cooler and there is more shade. If caught in open areas, in unshaded heat, the crabs soon lose body water and die.
Here is a 5 minute video of the Crabs and the Parks & Wildlife Officer showing you their journey to the ocean.