Christmas Island red crabs (Gecarcoidea natalis) live in the rain forest and have to migrate annually to the ocean to release their eggs. The adults are terrestrial and can not survive in water. They play an important role in the forest ecology by selectively eating seeds and seedlings, fruits and flowers, cleaning up leaf debris, turning over soil, and fertilizing it with their waste. They also feed on dead animals, including other red crabs, human rubbish, and the non-native giant African land snail. Dominating the forest floor, they have almost no competition for food.
An estimated 50 million adult red crabs live on Christmas Island which was named on Christmas Day in 1643 by Captain William Mynors of the British East India Company vessel, the Royal Mary. The accidental introduction of the yellow crazy ant from Africa may have killed between ten and fifteen million crabs since about 1990.
The first rains usually start the migration in October or November, but sometimes as late as December or January, providing the moist preconditions. The timing is also linked to the moon phase so eggs are released by the females into the sea toward dawn at the turn of high tide in the last lunar quarter. It is the safest time because the sea level varies the least for the longest period.
The released eggs hatch upon contact with the sea where clouds of the crab larvae swirl around near shore before sweeping out to sea for a few weeks. They develop into megalopae which look like shrimp, then gather near shore for a day or two before they become .2 inch diameter crabs. It takes about nine days for the baby crabs to migrate to the middle of Christmas Island where they hide in rocks, tree branches and other forest debris until they are about three years old. They will moult often as they grow until at four to five years, they moult only once a year. They are then sexually mature and start their own annual migrations. Read the Christmas Island National Park website for more details on the males creating burrows and mating habits to create the eggs released.
Fish and large filter-feeders like manta rays and whale sharks congregate in the waters around Christmas Island during the red crab larval stage and consume millions of the larvae. Studies of the crabs found that a bunch of adults in the middle of the island did not migrate, in which case they did not reproduce that year.
During the land migration, the crabs must cross three or four roads between the forest and the breeding grounds. Vehicles crush many of them and sometimes the hard shells puncture tires and cause accidents. Park rangers put out crab fences along the most traveled roads to direct the crabs into underpasses or crab grids for a safer crossing.
In peak migration times, road sections are closed to vehicles and people park near Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach to watch the moving crabs. Public notice boards and local radio stations announce the crab migration movement locations.
These seemingly inconsequential creatures play their important role in this tiny island’s environment. They are an example of why the loss of any species on the planet may seriously affect a region. If they were not protected by their island habitat with the national park covering mainly rain forest over 65 percent of the island, they might have become an extinct species as well.
View pictures on The Wilderness Society website of the devastating effects on the red crab’s rain forest habitat by the yellow crazy ant. Germinating seedlings are altering the rain forest structure in the absence of the red crabs. A bait containing fipronil has been found to achieve a 99 percent reduction in worker crazy ant density at low doses within days.