Christmas Island Crabs
The amazing march of the red crabs
I am writing this in November, which is around the time of year when something quite amazing happens on a small island 500kms south of Jakarta - Australia's Christmas Island. As to exactly when it occurs is decided by, would you believe, a group of presumably simple creatures known as the red crabs. When the weather, the tides and the phase of the moon are right, millions of them leave the forest and embark on a mass migration across the island heading for the ocean to breed.
These crustaceans have been hiding on the forest floor beneath the debris and in rocky outcrops for three years until they are mature enough to begin their mammoth trek. There are swarms of them covering the ground as they leave their burrows and march en masse along roads, across streams, clambering over rocks, finally ending up on the beaches where they will breed.
The males begin the migration being joined by females along the way and the first rainfall of the wet season is the instigator. This can be as early as October or as late as January the following year, depending on the weather. Other contributing factors are the phase of the moon and the tide. In order for a successful mating and subsequent spawning the crabs must be on the beaches ready for a receding tide during the last quarter of the moon. Bear in mind that theirs is a long trek and how they know exactly when to leave their burrows to arrive at the crucial time for this event is a mystery.
The first rains trigger the migration and once they are falling the red crabs seem to instinctively know that they must set off and how long it will take them from that precise moment to reach the beaches. Should this wet season begin early they take their time, pausing occasionally to eat and drink along the way; however, if the rain is late the speed of the march has to be greatly increased. In some cases when the rain is too late for the crabs to make the beaches for the spawning date they remain in their burrows and set off the following month at the appropriate time.
Once on the beaches many more females congregate, far outnumbering the males; and the first priority for all is to take a dip in the sea, replenishing moisture lost during the long journey. Now rehydrated the males dig burrows in the lower terraces of the island, and because there are so many males engaged in the routine these soon-to-be breeding places are close together resulting in fights for possession breaking out. The females then join the males on the terraces for mating in or near the burrows. After this, their job done, the males have another dip in the sea before starting out to return to their inland homes.
The female crabs stay behind in the burrows. It takes three days for them to produce up to 100,000 eggs which they hold in a brood pouch; and they will wait a further two weeks for them to develop. Before dawn when the tide starts to turn they head for the sea. There are masses of them, up to 100 per square metre, all preparing to release their eggs into the sea. As soon as this has been achieved the females begin the trek back to their original homes, leaving it up to nature and chance to continue the next stage.
The red crab larvae hatch from the eggs the instant they contact water, forming clouds which swirl close to shore, waiting to be taken out to sea by waves and a receding tide. Over the following month they eventually develop into prawn-like creatures called megalopae which congregate in pools near the shoreline for one or two days until they become fully formed baby crabs. Just 5mm across, these tiny crabs begin their own trek, marching inland for about 9 days, heading for the safety of the plateau where they will remain for three years to fully develop.
Unfortunately, the majority of the larvae will not survive, being eaten by fish, manta rays and whale sharks which take advantage of the spawning. Usually, very few of the baby crabs leave the sea; but once or twice a decade a massive amount do make it ensuring the red crab population of Christmas Island is replenished.
The annual march of the red crabs is, without doubt, one of the most amazing events that can be witnessed nowhere else in the world, and is truly one of nature's wonders.
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