Christmas Island Crabs: A Spectacular Sight
January 10, 2014

Christmas Island Crabs: A Spectacular Sight

In the northern part of the US, and in higher altitudes, people have to be aware of snow covering the roadways during the months of October through December. In the southern hemisphere, maybe towards the end of November, there is a possible chance of snow covering the roads.

On Christmas Island in the Indian Ocean during this time, residents have to contend with a totally different type of road hazard. Crabs! Yes that’s right, migrating red crabs.

There are as many as 43.7 million adult red crabs that live on the Island. For the better part of the year the live in the forest, but at the beginning of the wet season, usually October/November they migrate in great numbers to the coast to breed. The females lay the eggs in the ocean and three or four weeks later, millions of juvenile crabs return to the forest.

During the migration there is painstaking care given to protect the adults as well as the juvenile crabs. Detours for the crabs are set to direct them to underpasses for safe passage to the ocean. On roads where detours cannot be set up, workers spend hours using rakes to sweep the crabs across. Also, signs are erected warning drivers of the crabs ahead.

The journey for the crab to reach its ocean destination will take five days. But with 40 million or more of the clawed, side-stepping creatures making the trek, it is a month long process.

The males arrive first and dig burrows for the females, who will arrive five to seven days later. After the breeding is complete, the males will leave for their journey back to the forest, but this time it only takes one to two days. The female will stay another two weeks, laying eggs and waiting for them to develop.

The eggs are released in coordination with high tide and when they hatch they are swept out to sea. In three or four weeks they come ashore and start their own journey home to the forest.

The eggs are released in a ballet-like manner. The female will raise her claws high, shaking them as the eggs are dispersed into the water. This will be done nightly for up to five consecutive days with around 100,000 eggs laid. They will hatch immediately upon contact with the water.

Normally the crab can only move for a few minutes at a time. But during the migration period, scientists have discovered they are energized by the release of a hormone giving them energy to complete their journey quickly.

Compared to the staggering amount of crabs, there are only 1,600 humans who call Christmas Island home. Even though there are precautions taken to protect the crabs, thousands of them are crushed by passing cars. Even with the erection of detours and signs.

Another threat to the crab is the yellow crazy ant that was accidentally introduced to the Island from China. It is believed the ants have killed around 15-20 million crabs in recent years.

Image Credit: Thinkstock

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