Every year, millions of these large crabs emerge from the forest and make their way to the ocean to breed, swarming across roads, streams, rocks and beaches.
It’s a truly spectacular sight that world-renowned naturalist Sir David Attenborough described as one of his greatest TV moments.
When does the red crab migration occur?
When wet season returns in October or November they begin a legendary mass migration to their seaside breeding grounds, moving in colorful waves that wash over all obstacles including roads (necessitating crab tunnels and road closings) and even seaside cliffs.
Red crabs all over the island leave their homes at the same time and start marching towards the ocean to mate and spawn. Male crabs lead the migration and are joined by females along the way.
The exact timing and speed of the migration is determined by the phase of the moon. Red crabs always spawn before dawn on a receding high-tide during the last quarter of the moon. Incredibly, they know exactly when to leave their burrows to make this lunar date.
However, because crabs wait until the first rainfall to start their trek, they sometimes have to hurry. If the rains arrive close to the optimal spawning date, they will move rapidly. But if the rain comes early they may take their time, stopping to eat and drink on their way to the coast.
If it begins raining too late to make the spawning date, some crabs will stay in their burrows and migrate the following month instead.
The larger male crabs usually arrive at the sea first but are soon outnumbered by females. After their arduous journey from the plateau, the crabs take a dip in the sea to replenish moisture.
Next, the male crabs retreat to the lower terraces of the island to dig burrows. The huge number of crabs means burrows are very close together, and males will often fight each other for possession of a burrow.
The female crabs then join the males on the terraces to mate in or near the burrows. After mating, male crabs have a second dip in the sea before starting their journey back home.
The female crabs stay behind in the moist burrows. They produce eggs within three days of mating and will remain in the burrows for about another two weeks as the eggs develop,
Each female crab can produce up to 100,000 eggs, which she holds in a brood pouch.
When the moon reaches its last quarter, the egg-laden crabs leave their burrows and amass on the shoreline, packing into shady spots above the waterline. In some areas you might see up to 100 crabs per square metre of beach or rock.
When the high tide starts to turn before dawn, the crabs move into the sea and release their eggs before returning to the forest. Spawning may occur on 5–6 consecutive nights during the migration.
Hatching and the return of the baby crabs
Red crab larvae hatch from the eggs as soon as they make contact with the water. Clouds of larvae swirl near shore before being taken out to sea by waves and the receding tide.
They grow through several larval stages over the next month, eventually developing into prawn-like animals called megalopae. The megalopae gather in pools close to the shore for one or two days until they become fully formed baby crabs and emerge from the water.
Measuring about 5 mm across, the tiny crabs begin marching inland, taking around 9 days to reach the safety of the plateau. There they will stay hidden in rocky outcrops and forest debris for the first three years of their life.
However, the vast majority of larvae never make it out of the water – instead they are eaten by fish, manta rays, and the enormous whale sharks that visit Christmas Island to take advantage of this annual feast.
Most years, no or very few baby crabs will emerge from the sea at all. But once or twice a decade, a huge number will survive, which is enough to maintain the island’s large red crab population.
Watching the red crab migration
The red crab migration is Christmas Island’s biggest tourist attraction, drawing nature-lovers from all over the world.
The easiest places to watch the migration and spawning are Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach.