Red crabs are a species of land crab that live exclusively on Christmas Island and Cocos Islands. The crabs spend most of their time below ground, where they can maintain their body humidity. Without moisture, the crabs will die. Despite being land crabs, they breathe with gills. The need for moisture drives the crab’s reproductive cycle and what leads to the amazing migration that they do every year on Christmas Island.

When do they migrate?

The crabs come out of their hiding places when the wet season comes in October or November. The humid conditions make it the safest time for them to emerge, as they would suffocate in drier conditions. 

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Red crab migration is also connected to the lunar schedule. They choose this time because it is when the sea level varies the least. During the high tide between the last quarter and new moon, the red crabs will release their eggs into the sea. 

The timing of the migration is so essential to the species. If there is bad weather, the crabs will wait until the next lunar month to spawn. The process of migration goes on for around 9 to 18 days.

What does the migration look like?

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When the crabs march towards the seaside, they come in large groups. Their numbers are so large that they manage to even stop traffic. Despite getting attacked by predators on their journey, it does little to lower their numbers. 

When the crabs do reach the sea, it is quite a sight to behold. The crabs enter the water, careful not to stray too far from the shore. Since the crabs can not swim, they must be cautious to avoid waves. However, the eggs will not hatch unless they are in the sea. Once in the water, the crabs release their eggs into the water. 

They have so many eggs that they could turn the otherwise clear water black.

What happens to the eggs?

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Once the eggs hatch and turn into larvae, they live in the sea. When they develop the ability to breathe air, they move inland to repeat the same cycle their parents did. Like their parents, the baby crabs form a large migration on their journey to the forest floor.

Red crabs are indeed a testament to the complexity of nature. Their migration alone is a unique occurrence. Furthermore, their reproduction depends on many factors, such as the humidity and sea levels. Their numbers are huge despite the various obstacles they must face.