Nature lovers each year flock to Christmas Island, Australia, around late November to witness a scarlet sea of tens of millions of red crabs migrating down from the forest to mate and spawn on the shoreline.
Male red crabs set out on the journey first, triggered by the first rainfall of the wet season; they are soon followed by larger numbers of female red crabs.
“World-renowned naturalist Sir David Attenborough described the red crab migration as ‘like a great scarlet curtain moving down the cliffs and rocks towards the sea’ and considered filming the spectacle as one of his 10 greatest TV moments,” said Bianca Priest, Christmas Island National Park acting manager.
“Over the years, visitors have traveled from every corner of the world to witness this wildlife phenomenon.”
Visitors hoping to take in the remarkable wave of red crabs may find some of the roadways blocked on their migration path for all or part of the day when the crimson crustaceans make their way to and from the ocean.
Island officials have set up barriers to funnel the red crabs toward especially-designed crab-crossing overpasses for the critters to safely traverse otherwise-perilous roadways.
These crab crossings happen to be an excellent spot for visitors to watch the river of crabs as they make their yearly mass migration.
Incredibly, their journey is perfectly timed with the tides so that, upon setting out at first rainfall, they arrive exactly in time to spawn at dawn as the high tide recedes in the last quarter of the moon.
Should that rain arrive too early, the crabs will adopt a slower pace, taking time to eat and drink along the way. Should the rain come closer to that lunar date, they move rapidly for the sea.
Should that rain come too late, some crabs will remain in their burrows until the next month.
The larger male red crabs usually reach the seashore first but are soon outnumbered by females arriving. After the arduous journey, they take a dip in the sea to replenish their moisture.
The male red crabs will then make burrows, which are packed closely next to each other, and will sometimes fight other males for possession of them. After this, they are joined by the females for mating, before then taking another dip and setting out for the forest again.
The female red crabs will remain inside the moist burrows to produce their eggs until the high tide begins to recede on the specified lunar date. Laden with as many as 100,000 eggs, they return to the sea and release their eggs—which turn the shoreline into a turbid, black soup.
The crab eggs hatch immediately upon entering the ocean. The emerging larvae over the next month float and undergo several larval stages—eventually developing into prawn-like animals called megalopae, which gather in pools for one or two days before becoming fully formed baby crabs.
Some years, few or no baby crabs will emerge from the sea—instead being eaten by fish, manta rays, and enormous whale sharks visiting the island to take advantage of this annual feast, according to Parks Australia.
But once or twice every decade, a huge number will survive to replenish their populations, which set out on the long journey inland, taking around nine days to reach the safety of the plateau where they will stay hidden in rocky outcrops for the first three years of their life.
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