Each year, over 100 million red crabs (Gecarcoidea natalis) embark on a remarkable journey from the forests to the ocean on Christmas Island in the Indian Ocean to reproduce. This migration, which is about one and a half times the population of France, typically begins with the first significant rain of the wet season, usually in October or November, though it can sometimes be delayed. The crabs do not rely on a calendar to begin their journey; instead, they respond to environmental cues, specifically the lunar cycle. The timing of egg-laying follows the moon's phases, with females laying their eggs before dawn during a falling tide and the last quarter of the moon. If the rain is delayed, the crabs may need to hurry to ensure they are not too late for this critical event. Those that arrive too late can return the following month, and egg-laying then spreads over five to six consecutive nights. Each female crab can carry up to 100,000 eggs, which hatch almost immediately upon contact with seawater, forming dense swarms near the shore. The waves and the falling tide then carry the larvae out to sea. However, the survival rate of these larvae is very low, as fish, manta rays, and whale sharks specifically target this annual feast. A month later, crabs that have grown to about 5 mm in size return to the shore, taking only nine days to make the journey back. National park staff prepare extensively for the migration, installing kilometers of temporary barriers along roads to guide the crabs toward underground passages and bridges built specifically for them. When the crab migration becomes too dense, traffic is diverted. Park teams use rakes to clear crabs from the roads and stop traffic while doing so. Some residents choose to work from home or carpool to avoid accidentally crushing crabs. According to a 2025 testimony collected by ABC, a short trip can take 10 to 20 minutes when encountering a stop zone. For visitors, the migration is the island's main attraction. Information panels and local radio provide updates on crab movements, and the park asks visitors to follow its guidelines. Observation points such as Drumsite, Flying Fish Cove, Ethel Beach, and Greta Beach are among the best places to watch the event. The yellow crazy ant (Anoplolepis gracilipes), an invasive species introduced to the island in the early 20th century, kills tens of millions of crabs every year. Without natural predators, the ants have formed massive supercolonies covering hundreds of hectares and containing billions of individuals, with up to 1,000 ants per square meter. The ants use formic acid, which can kill terrestrial crabs. Since the 1990s, tens of millions of crabs have been attributed to the ants. Poison has been used in the past, but it has never fully solved the problem. Aerial spraying is costly and only temporary, as the ants return from inaccessible areas and the process must be repeated after a few years. As a result, the park has turned to biological control, introducing a micro-wasp from Malaysia that attacks the scale insect, a key food source for the ants. In 2023, drones also treated over 56 hectares of forest to create a safer path for the crabs. For 2026, no specific date has been announced, as it will depend on the first rains and the moon's phases. The figure of 100 million crabs remains an estimate, and most of them live in the rainforest. They can also be found in gardens, where they spend the dry season in burrows whose entrances are blocked with leaves.