Each year as soon as the wet season rains start, Christmas Island's red crabs will race to meet their partner before the new moon rises, Australian scientists said on Wednesday.

The Territory of Christmas Island is a territory of Australia in the Indian Ocean. The island has a tropical monsoonal climate, with highly variable annual rainfall.

When the rain starts to fall in October and November, the island became a seething mass of crustaceans, as countless red crabs travel across roads and vertical cliffs to reach the sea and breed.

"Tens of millions of the bright red crabs found in shady sites all over the island," ABC News reported on Wednesday.

The red crab named Gecarcoidea natalis is one of 23 land and freshwater crab species inhabiting Christmas Island. The species are highly sensitive to moisture loss.

"Red crabs have no natural predators and drying out is their main natural threat," red crab expert and Christmas Island National Park ranger Max Orchard told ABC News on Wednesday. " Conserving moisture drives everything they do, from when they eat, to when they migrate."

As other land crabs mate inland with only the females making the long march to the coast to deposit their fertilised eggs into the sea, red crabs Gecarcoidea natalis are the only land crab species where both males and females migrate to the seaside in order to breed.