With the first rainfall of the wet season, typically in October or November, Christmas Island red crabs begin their great migration towards the ocean to mate and spawn.

The precise timing of the migration that allows the simultaneous migration of over an estimated 40-50 million animals, is mediated by the lunar cycle. Each year, before the sun rises during the final quarter of the moon and at the turn of high tide, the mating season of the Christmas Island red crab begins.

The island’s most recent migration event has been of dramatic proportion.

The 2021’s annual migration “has just been absolutely epic,” said the Christmas Island National Park natural resource manager, Brendan Tiernan.

“The roads have been a seething mass of red crabs. It’s caused traffic jams on the small island and people having to get out of their cars and rake them out of the way.”

The migration of red crabs is a great asset to Christmas Island’s ecosystem, economy, and tourism and as such, the protection of these critters is of the utmost importance.

Some protective measures have been put in place all over Christmas Island to ensure the safe arrival of red crabs to their spawning sites. These include shutting off sections of roads during peak migration time, public updates on the movement of the crabs and even crab bridges built specially to allow for safe travel across busy roads.

A red crab bridge on Christmas Island. Picture from Wondrous World Image

Upon arrival at the breeding sites, a great competition begins. Millions of males scurry to dig their breeding burrows on the jam-packed beaches. Upon successfully finding a mate and securing a site to breed, the mating can begin.

After mating, the males retrace their steps and head back for the jungle. Meanwhile, females stay close to the water’s edge and begin to burrow and lay eggs.

Each female can lay as many as 100,000 eggs which are then incubated in their abdominal brood pouch to facilitate growth.

At the last quarter phase of the moon, the mother crabs release their eggs into the sea and then embark on the long journey home.

As the eggs are swept out to sea and contact is made with the salty ocean water, the eggs begin to hatch, and larvae are released. For the next 3-4 weeks the larvae go through several larval stages, until they are developed into a megalopa.

The megalopae crowd near the shore and breathe through gills, until they have grown into fully formed baby crustaceans.

Only half a centimetre in size, the baby crabs begin their own journey to the refuge of the forest in which they will live and grow until it is time for the breeding migration of their own.

This year’s large-scale migration has reminded us of the importance of this species on the island as a whole. Christmas Island red crabs provide an abundance food source for an array of life, as well as supporting the island’s residents through tourism.

Due to the specie’s reliance on the arrival of the wet season, research has suggested that any changes in rainfall could undermine the entire migration and potentially lead to a trophic cascade.

Once again, we must all do our part in protecting our oceans and combating climate change. Visit donate.oceanconservancy.org to become part of the solution today.

Written by Lucas King