Home OceaniaAustralia Christmas Island: Migration of Red Crabs Is Something That You Must See

Christmas Island: Migration of Red Crabs Is Something That You Must See

by Tijana

The show is unique in the world! On Christmas Island, a small piece of land between Australia and Indonesia, each year, at the start of the rainy season, millions of red crabs cross the small island to reproduce and lay their eggs in the sea.

Migration of Red Crabs

14 species of crabs and approximately 45 million red crabs live on Christmas Island. These crabs are endemic.

As the name suggests, the red crab (Gecarcoidea Natalis) is usually bright red, but there are a few orange or even purple specimens that are rarer, and usually larger than the rest.

Males have a larger shell than females, while females have a larger abdomen with small claws. Christmas red crabs grow slowly, they are sexually mature around 4/5 years old and it is at this age that they take part in migrations.

They mainly feed on green leaves, other crabs, dead birds, and household garbage.

They live in burrows and crevices, or home gardens. The forest is home to a very high density of crabs, they dig burrows in almost every square meter of the ground, as they live solitary with only one crab per burrow, they do not tolerate intruders in their “home”.

They are active during the day and inactive at night. They like humidity, hence a vigorous activity during the rainy season. During the dry season, they return to their burrows, seal themselves with leaves to keep them cool and stay there for 2 to 3 months.

The annual migration of Christmas Island red crabs

Migration begins at the start of the wet season from October to December. This migration is spectacular, thousands of red crabs move from the forest to the coast to reproduce and lay the eggs in the sea.

 The release of eggs is linked to the phases of the moon and therefore of the tides, particularly during the last quarter. Of the moon because it is the period when there is the least difference between high tide and low tide.

The main migration can take 18 days. The grounds are completely covered by millions of crabs that move around, bypassing obstacles and following the roads. They mainly do the trip in the morning or late afternoon when it is cooler. If they lose too much water, they dry up and can die.

 It is the males who go first and arrive at the seaside between 5 to 7 days later. They head towards the sea to get wet, then move away from the coast and head towards terraces.

The females heel them. The males dig burrows to accommodate the females and thus reproduce in the privacy of these crevices. The fights are sometimes tough to dig these burrows, there are 1 to 2 per square meter.

Once the mating is done, they return very quickly to the plateaus and the forest. Females lock themselves in a burrow for 12 to 13 days for gestation, they can have up to 100,000 eggs in their abdomen!

Then around the last quarter moon, they leave the burrows and head out to sea to release the eggs, usually at dawn.

This spawning can last several nights, then they return to the plateaus and the forest. If the weather is unfavorable, males and females can stay in the burrow and wait together the following month.

The eggs will then hatch into larvae and will stay for 4 weeks before becoming small crabs and emerging from the water. But before leaving the water, the millions of them  will be consumed by many species of fish including manta rays and especially by the whale shark, present at this time of year precisely to enjoy the feast!

The baby crabs, which will survive from the water, will head towards the trays. For 3 years, they will hide in rocks or under tree branches.

Few baby crabs survive the feast, but about 2 out of 10 years are excellent years, thus maintaining the survival of the species.

Human activity is harmful for this migration, such as deforestation and passaging cars. They have taken steps to reduce their extinction.

 Thus, special bridges and tunnels allowed them to continue their journey without hindrance. Some roads frequented by crabs are closed to traffic during migration. And it also prohibits the spawning grounds during the season.

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