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The Great Christmas Island Red Crab Migration

These fearless red crabs have a big reputation

Christopher Andrew Bray Wikimedia
© Christopher Andrew Bray/Wikimedia

About 932 miles northwest of Perth, Australia, a renowned atoll resides: Christmas Island. The region’s namesake traces back to 1643, when an English voyager sailed past it on Christmas Day. Today, nearly two-thirds of this incredibly biodiverse island is protected as a national park. While Christmas Island contains wetland, rainforest and marine ecosystems that host many remarkable creatures, there’s one species that steals the spotlight each year: Gecarcoidea natalis, appropriately nicknamed the Christmas Island red crab. Wondering what makes these vibrant little crabs so special? Join me on a virtual trip to the Indian Ocean and learn all about these little crustaceans’ claim to fame: the annual Christmas Island crab migration!

Every year as the first notable shower of the rainy season begins, a truly awe-inspiring event happens on Christmas Island: Millions of red crabs begin their annual migration across the island, moving with unwavering determination to reach the shoreline where mating and spawning occur. It’s estimated that 40 to 50 million of these crabs participate in the migration each year, braving tough terrain and prowling predators to play their part in establishing the species’ next generation.

As you can imagine, this yearly mass migration makes for quite the sight to behold.

Once the migration begins, it will continue for around three weeks until the optimal spawning time when female crabs propel their eggs into the sea. The actual calendar dates for this event vary each year, but they usually occur sometime in October or November. This year, scientists estimated the most likely dates to be November 28 through November 30, but the rain came earlier than anticipated. The migration has officially begun!

The lunar cycle is why this migration, mating and spawning happens so consistently within the same time frame year after year. Without fail, the red crabs always spawn together before the sun rises during the final quarter of the moon as the high tide begins to turn. However, depending on how close the first rainfall occurs to this optimal lunar timeframe, the crabs may have to dash to their destination faster in some years than others … and somehow, they always know exactly how fast they need to move to make their deadline.

This mission to the sea isn’t an easy one, either. The journey across the island requires the crabs to avoid the threat of traffic as they move across roads (though some wildlife bridges have helped with this), and the heat of the sun can cause them to become dehydrated and easily exhausted. Although adult red crabs have no natural predators on land, their populations have been greatly affected by an invasive species known as “yellow crazy ants” (Anoplolepis gracilipes). These invasive insects blind the crabs with acid, and scientists estimate they’ve killed tens of millions of crabs since they first arrived on the island.

The challenges don’t end when the crabs reach their destination, though. First, male crabs who complete the journey must dig their own breeding burrows, and since millions of crabs are looking for space to burrow at the same time, this can become quite the competitive task. Once a male and female crab have successfully mated within a burrow, females will stay put, incubating their broods for a couple of weeks as the eggs develop. An amazing fact about mommy red crabs: They can produce up to 100,000 eggs per brood! While the mother crabs stay safe, male crabs begin their journeys back across the island, retracing their steps to find their way home.

Once the moon reaches its last quarter phase, all the mother crabs know: It’s time to move! As the tide moves out before the sun breaks the horizon in the early morning, the females gather at the water’s edge and release their eggs into the waves. It is only after spawning that female crabs begin their trips home.

But of course, this isn’t where the story of red crabs ends … it’s where it really begins.

As soon as the eggs are released into the water, larvae are triggered to hatch from the eggs, eventually developing to their final larval stage known as megalopae. For a couple of days, these tiny “almost baby crabs” will group together near the shore until they finally grow into their full form as baby crustaceans.

While they do have the same general appearance as what most people think of when they hear the word “crab,” let me clarify: These babies are tiny! Only about half a centimeter when they first arrive onshore, they’re so tiny that as millions of them emerge onto the shore, the unassuming eye may mistake them for a reddish algae covering the rocks and sandy shoreline. It will take these tiny trekkers a little more than a week to reach the protection of the edge of the forest, where they live and grow for the first few years of life. Once they reach ages four or five, the young crabs will participate in the migration that their species is famous for.

Unfortunately, while so many eggs are released into the water, the majority of red crab larvae never get the chance to begin the trip home. These millions of larvae are an important food source for marine animals like manta rays and whale sharks that gather near Christmas Island each year for a festive seasonal feast. Most years, few baby crabs ever come out of the sea, and some years, no crabs make it out at all. But fear not: one to two times each decade, a massive number of baby crabs somehow make it to the beach, establishing a troop of enough survivors to keep the population at a healthy level.

All in all, this species’ yearly crimson tide across Christmas Island demonstrates just how crucial seasonal migrations are, not only in ensuring a species’ survival but also in providing nourishment for other creatures in and around their native ecosystem. Yet, as arduous as the red crabs’ annual journey to lay the foundation of the next generation is, there’s another danger to their survival that’s becoming increasingly threatening each and every year: climate change. Research notes that because these animals rely on the seasonal natural cycle of a wet season, anything causing potential changes in rainfall can throw off the entire process (or even eliminate the chance a migration will happen at all).  As such, both the red crabs and animals that depend on them for sustenance face new and greater risks to their survival.

But we can’t lose hope.

Ocean Conservancy is on the front lines of ocean protection, working to create sustainable, ocean-based climate solutions and investing in research, leadership and advocacy that advances these solutions … but we can’t do it without you. For our ocean, the spectacular red crabs and all the other creatures that depend on a healthy blue planet, we need your help in the fight to confront today’s greatest climate challenges. Donate today to join us on this mission, and be sure to visit the Ocean Conservancy Action Center for all the latest ways to advocate for our ocean and planet!

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