Every year in late spring, Australia’s Christmas Island becomes covered in crabs when the more than 40 million red crustaceans that call the northwest island territory home start their annual migration to the sea, covering the landscape in a mass of crimson claws.

The migration starts with the first heavy rains in October, November or December. At that point, there’s enough moisture in the air for the large crustaceans, which can reach up to 11cm across, to make the arduous, five-day journey from their homes in wet inland forests to the Indian Ocean, covering up to 9km along the way.

With so many of the creatures on the move, Parks Australia works before and during the migration to protect the crustaceans by closing roads, building fences and constructing underground tunnels. Drivers are encouraged to stop for the crabs.

Upon reaching the sand, the male crabs dig burrows and fight each other for ownership of the shelters. When the female crabs arrive (usually five to seven days after the first males), they begin to mate, and the females stay in their beachside burrows until the last quarter of the lunar cycle. The females always wait for the first day of the last quarter – regardless of when they started the migration – to spawn and release their eggs into the sea. Researchers speculate that since this phase of the moon has the least sea level change between high and low tides, the eggs have higher chances of survival.

This year, the possible spawning dates (and dates of the quarter moon) are 28 November or 28 December, so the initial migration will happen seven to 18 days before, depending on the weather. The crabs tend to be on the move in the morning and early evening when the air is cooler, but any dry spells will halt the migration until wetter weather prevails.

Follow the Parks Australia blog or the Christmas Island Tourism Facebook page to get an alert at the first signs of the cruising crustaceans.