Tied to the lunar calendar, this massive migration usually begins during the island’s wet season, when millions of crustaceans travel from the rain forest to seaside breeding grounds to mate.
When the wet season returns, red crabs begin a legendary mass migration to their seaside breeding grounds on Christmas Island, Australia.
The red crab is a Christmas Island, Australia, originally found nowhere else in the world. But on its home turf, it is a very significant species—some 120 million individuals cover the rainforest floor and play a major role in determining the structure of the ecosystem.
Habitat and Diet
These large crabs are active during the day but prefer to stay in the shade and can die in the moisture-robbing heat of direct sunlight. They scavenge on fallen leaves, seedlings, fruits, and flowers, recycling nutrients and helping to determine the spread and composition of native flora.
Most of the year red crabs are solitary dwellers of the burrows they dig throughout the forest. During the dry season they retreat into these shelters to retain body humidity and essentially remain there for two to three months.
But when the 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.
The annual trek is also intimately tied to the lunar schedule. The crabs arrive at the coast and mate at such a time that the females can produce eggs and develop them in burrows for a dozen or so days before releasing them into the sea precisely when high tide turns between the last quarter and new moon. During this period sea level on the beaches varies the least and offers an easier approach, a factor so important that if weather delays the migration crabs will put off spawning until the next lunar month.
Red crab eggs hatch right away, and young live as larvae in the sea for a month before returning to the shoreline, molting into air breathers, and slowly returning inland to begin the cycle anew.
When does the red crab migration occur?
The migration starts with the first rainfall of the wet season. This is usually in October or November, but can sometimes be as late as December or January.
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.