With the start of the rainy season, Christmas Island undergoes a remarkable transformation. The roads close, the forest floor becomes a bustling hub, and millions of red crabs swarm the island. This remote Australian territory, situated around 190 nautical miles south of Indonesia, plays host to a unique natural phenomenon. Every year, the red crabs emerge from their burrows in large numbers and make their way towards the Indian Ocean, guided by the rhythm of rainfall and lunar phases. These thick-legged creatures, with hard shells measuring about five inches or less in width, blanket the 52 square mile island as they journey from land to sea.
Brendan Tiernan, the senior field program coordinator for threatened species at Christmas Island National Park, describes this event as a spectacular sight that few are fortunate enough to witness. Having lived and worked on the island for 17 years among its 1,500 residents, Tiernan remains a keen observer of this incredible animal migration each year.
Despite facing challenges such as the impact of a destructive invasive species and a dry spell in 2023 that disrupted their reproductive cycle, Tiernan reports that the red crab population is thriving. In 2015, there were an estimated 45-50 million red crabs on Christmas Island. Today, that number has more than doubled, with over 100 million crabs calling the island home. Their sheer numbers and unique life cycle play a significant role in shaping the island’s ecosystem.
The red crabs of Christmas Island, scientifically known as Gecarcoidea natalis, are a species with a limited habitat range. They are native to Christmas Island and a smaller population resides in the Cocos Islands, located approximately 600 miles southeast. For most of the year, these crabs inhabit the island’s forests, feeding on decaying vegetation and young seedlings as detritivores.
In areas where the red crabs are abundant, the forest floor is clear of leaf litter and pocked with their burrows, creating a unique landscape. While such a barren forest floor might signal trouble in other tropical regions, on Christmas Island, it is a natural state shaped by the crabs’ activities. These crustaceans have a significant impact on their environment, even though they are not always visibly active. During the dry season, they remain sheltered in their burrows, emerging only when the rains arrive.
As the wet season approaches, the red crabs await the first significant rainfall in late October or November, signaling the start of their migration to the ocean. They must reach the coast in time to spawn around the last quarter moon, when the tides are calmest. Males reach the coast first to construct mating burrows, followed by females who select a suitable mate and lay their eggs in coastal burrows. After hatching, the larvae return to land as tiny crablings, completing the cycle.
While the red crabs have faced challenges in the past, such as the invasion of yellow crazy ants that decimated their numbers, recent efforts to control the ant population have led to a resurgence in the red crab population. The introduction of a biocontrol agent in 2016 has helped reduce ant numbers, allowing the red crabs to thrive once more. Despite occasional setbacks due to anomalous weather patterns, the red crabs continue to persevere.
Looking ahead, climate change poses a long-term threat to the red crabs and the ecosystem of Christmas Island. With projections indicating longer dry periods and more intense wet seasons, the red crabs may face challenges in finding food and water during the dry season, as well as navigating the wet season migration and breeding period. While the red crabs have shown resilience in the face of adversity, the future remains uncertain.
For now, Tiernan anticipates another successful migration this year, with millions of red crabs painting Christmas Island red once again. As the natural wonder of the annual red crab migration unfolds, visitors and residents alike have the opportunity to witness this extraordinary event that defines the island’s identity.
