Each female red crab can carry 100,000 eggs to the sea; Christmas Island closes roads and deploys bridges to protect the moon-timed journey

Every year, millions of red crabs leave the rainforest floor of Christmas Island and begin one of nature’s most extraordinary journeys. They do not simply head toward the ocean whenever conditions seem right. Rainfall starts the migration, while the moon and tides help determine when the females must reach the shoreline to release their eggs, as per a report by Christmas Island.

For the crabs, timing matters enormously. A female can carry up to 100,000 eggs, and the journey from the forest to the sea has to happen at precisely the right moment.

That is why roads can close during the migration, while crab bridges and underpasses help the moving masses cross human-made obstacles on their way to the coast.

Why do the crabs begin moving?

More than 120 million red crabs can live across Christmas Island’s rainforest floor, where they spend most of the year in individual dirt burrows or deep rock crevices. The island has a tropical climate, with a wet season from December through April and a dry season from May through November. As the return of wet conditions approaches, usually in October or November, the crabs begin leaving their forest homes.

The first suitable rainfall triggers the migration. Males generally begin the journey first, moving toward the lower terraces and eventually digging burrows. Females follow, and mating takes place roughly two weeks before spawning.

The timing can change depending on when the rain arrives. If rainfall comes close to the ideal spawning period, the crabs may move quickly. When rain arrives earlier, they can take more time, stopping to feed and drink. If it comes too late, some crabs wait for the following month.

How does the moon control spawning?

After mating, the females remain in moist burrows while their eggs develop. They produce the eggs within three days of mating and remain in the burrows for roughly another two weeks. Each female can carry as many as 100,000 eggs in her brood pouch. Then comes the crucial part of the journey. When the moon reaches its last-quarter phase, the females leave their burrows and gather near the shoreline. In some places, as many as 100 crabs can occupy a single square metre of beach or rock.

The females release their eggs before dawn as the high tide begins to recede.

The process may continue for five or six consecutive nights.

The timing is so closely connected to the lunar cycle that the migration can appear almost choreographed. The crabs need both suitable rainfall for the journey and the right tidal conditions for spawning.

For 2026, the possible spawning dates supplied for Christmas Island are November 4–6, December 5–6 and January 2–3, 2027. Spawning could occur a day either side of those dates, with migration typically beginning around four to five weeks beforehand if suitable rain arrives.

Why are roads closed during the migration?

The scale of the migration creates an unusual problem. Millions of crabs move across roads and other parts of the island while following their route toward the coast.

Christmas Island responds by closing some sections of road during peak migration, sometimes for part or all of the day and often at short notice.

Rangers have also installed crab bridges over roads, allowing the animals to continue moving without having to cross directly beneath vehicles. Underpasses have also been constructed across the island to provide additional routes.

For visitors, these structures have become part of the spectacle. The crab migration is Christmas Island’s biggest tourist attraction, and people can watch the moving streams of red crabs at places including Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach.

The sight can be extraordinary, with crabs moving across the landscape in huge numbers as they travel between the rainforest and the ocean.

What happens after the eggs reach the sea?

The eggs hatch as soon as they come into contact with seawater. The larvae initially gather close to shore before waves and the receding tide carry them out to sea. Over the following month, they pass through several larval stages before becoming prawn-like megalopae.

Eventually, the megalopae gather in pools near the shoreline for a day or two and develop into tiny crabs. At only about 5 millimetres across, the young crabs then begin another journey, this time inland. It takes around nine days for them to reach the plateau, where they remain hidden among rocky outcrops and forest debris during their first three years.

The journey is extremely difficult. Most larvae never return to land because they are eaten by fish, manta rays and whale sharks that gather in the area for the annual food supply.

In most years, few or no baby crabs emerge from the sea. Once or twice a decade, however, huge numbers survive, helping sustain the island’s enormous red crab population.

The cycle then begins again. For the adult crabs, the journey is a race against weather, tides and time. For the females, it culminates in a brief moment at the water’s edge, where thousands of eggs are released into the sea.

And across Christmas Island, the roads, bridges and underpasses become part of the effort to let that ancient migration continue.

FAQs

How many eggs can a female red crab carry?
Up to 100,000 eggs.

Why do Christmas Island roads close?
To help protect migrating red crabs from vehicles.