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Planet Earth II

March of the red crabs

By Renate Ruge

At the end of each year on Christmas Island, millions of red crabs embark on an epic journey. Their goal? To spawn at the turn of the high tide during the last quarter of the moon.

As the first cooling drops of rain from the monsoon fall, 40 to 50 million adult red crabs (Gecarcoidea natalis) leave their burrows on Christmas Island, north-west of Australia. The surge of movement spreads around the island and the annual three-week migration to the ocean is underway.

The Christmas Island crab’s journey is a treacherous one as it risks dehydration under the hot tropical sun, and has to get past traffic on busy roads to reach the sea – though bridges are being built to aid safe crossing. Males go first and females follow, but another threat lies in wait en route. Super colonies of yellow crazy ants (Anoplolepis gracilipes) attack their prey by dousing them with acid, targeting their eyes and mouth. Blinded crabs are left helpless and die. The ones who survive climb down to the beach, risking more attacks by sea birds or Moray eels, before they can rehydrate in the sea. A few days later, the red creatures sidle along the beach, disappearing to create breeding burrows on terraces. Only then do they mate. Finally, the male crabs retrace their path back home and the females follow after spawning (releasing their eggs into the sea).

It is an incredible journey and the crabs’ battle for survival shows the unintended effect we humans have on our world.

The crazy ants are thought to be native to Africa and arrived in Australia via cargo boats. Studies show they’ve killed or displaced 15 to 20 million crabs. This in turn has changed the structure of the forest, as the crabs performed vital functions for its ecology: digging and fertilising the soil and keeping weeds in check. So now, Parks Australia is working to limit ant numbers. Humans created the problem by bringing the ants and now humans are dealing with the effects.

Photograph © Elizabeth White BBC NHU 2016

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