The Christmas Island red crab spawning migration is synchronized with which lunar phase?

The Christmas Island red crab’s spawning migration is synchronized with the last quarter moon. This timing helps female crabs release their eggs into the sea at the turn of a high tide, giving the larvae the best chance of being carried into offshore waters.

Millions of red crabs leave the island’s forest and travel toward the coast during the wet season. The migration is triggered by rainfall and follows a carefully timed schedule, with males generally reaching the shore first and females following behind. Roads and other obstacles can become covered by moving crabs.

After mating, females carry fertilized eggs in a brood pouch beneath the abdomen. At the appropriate lunar phase, they release the eggs into the ocean. The larvae remain at sea for roughly three to four weeks before returning toward land as young crabs. The full moon is a common mix-up, but the egg release is linked specifically to the last quarter moon.