Why Millions of Crabs March to the Sea—and Eat Their Offspring Along the Way?

Imagine a crimson tide sweeping across roads, cliffs, and forests—not of water, but of millions of marching crabs. Each year, on a remote island off the coast of Australia, an extraordinary natural event unfolds: the great red crab migration of Christmas Island. This jaw-dropping spectacle captivates biologists, tourists, and photographers alike, as these fiery crustaceans make their legendary journey from dense inland rainforests to the crashing waves of the Indian Ocean. But behind the awe-inspiring scenes lies a darker truth—cannibalism in the crab kingdom.

The annual migration, typically triggered by the onset of the wet season and perfectly timed with the lunar cycle, involves over 50 million red crabs (Gecarcoidea natalis) flooding toward the ocean to breed. It is one of the most impressive synchronized movements in the animal kingdom. These crabs, guided by ancient instinct, abandon their burrows and descend upon highways, footpaths, and even residential areas, prompting local authorities to shut down roads and build crab bridges and tunnels to protect the scuttling masses.

But the journey isn’t without peril. Besides dodging predators and the crushing wheels of unsuspecting vehicles, these crabs face their own species as one of the greatest threats. Red crab cannibalism is a harsh reality. During the return leg of the migration, adult crabs, exhausted and nutrient-depleted, have been observed consuming smaller or weaker members of their own kind—including newly hatched young. This seemingly brutal behavior is, in fact, a natural survival tactic. In a world where energy is currency, cannibalism offers an opportunistic way to refuel after the exhausting migration and reproduction process.

Once the adults arrive at the shoreline, they dig burrows and mate. Females then release their fertilized eggs into the ocean, where they hatch into tiny larvae and begin their life at sea. After about a month, only a small fraction of these larvae survive the turbulent waters and return to land as juvenile crabs. Their arrival marks the beginning of a new cycle—and another treacherous march inland, where they may one day participate in the same journey, perpetuating the ritual of survival, sacrifice, and, for some, cannibalism.

What makes this phenomenon even more astounding is not just the scale or spectacle—but the paradox it represents: life and death intertwined in one majestic cycle. The same journey that brings forth new life also demands a grim toll from within the species itself. It is nature in its rawest form—unfiltered, unsparing, and yet, breathtakingly beautiful.

As the tide recedes and the roads clear, what remains is a hauntingly poetic truth: for the red crabs of Christmas Island, survival is a march painted in shades of red—both vibrant with life and shadowed by instinct’s darker hand.