Red crabs

Tens of millions of red crabs live on the island. They are the island's keystone species, recycling nutrients and shaping and maintaining the structure of the rainforests.

 

The migration

At the beginning of the wet season (around October to December) every year adult red crabs begin a spectacular migration from the forest to the coast, to breed and spawn.

While the rains provide the moist preconditions for the march to begin, the timing of the migration breeding sequence is also linked to the phases of the moon. Eggs are released by the female red crabs into the sea precisely at the turn of the high tide during the last lunar quarter.

The migration sequence

Males lead the first wave of the downward migration and are joined by females as they progress. When they reach the shore the males dig burrows - they are joined by the females and mating occurs. Males take another dip in the ocean before returning inland.

The females produce eggs within three days of mating and remain in the moist burrows for 12-13 days while they develop.

Baby crabs

The eggs released by the females hatch immediately on contact with the sea water and clouds of young larvae swirl near the shore before being washed out to sea by waves and tides.

After about a month in the ocean, and after growing through several larval stages, the larvae have developed into prawn-like animals called megalopae. The megalopae gather in pools close to the shore for one to two days before changing into young crabs and leaving the water.

Although only five millimeters across, the baby crabs begin their march inland, taking about nine days to reach the plateau. Here they are rarely seen, disappearing into rocky outcrops and fallen tree branches and debris on the forest floor for the first three years of their life.