Christmas Island Red Crab Facts

The Christmas Island red crab (Gecarcoidea natalis) is a land crab famous for its epic annual mass migration to the sea to spawn. Once numerous on Christmas Island, crab numbers have been devastated by the accidental introduction of the yellow crazy ant.

Description

Christmas Island red crabs are large crabs with bodies measuring 4.6 inches in width. Males tend to be larger than females, with larger claws and a narrower abdomens. They have claws of equal size, unless one has been damaged and has regenerated. The crabs are usually bright red, but orange or purple crabs sometimes occur.

Habitat and Distribution

Red crabs are endemic to Christmas Island (Australia), in the Indian Ocean. Relatively recently, the species immigrated to the nearby Cocos (Keeling) Islands, but the number of crabs on the Cocos Islands is much lower than on Christmas Island.

Diet

The crabs are omnivorous scavengers. They feed on fruit, seedlings, fallen leaves, flowers, human rubbish, the giant African land snail, and dead animals. They also cannibalize other Christmas Island red crabs.

Behavior

Most of the year, Christmas Island red crabs live in the forest. They usually hide under branches or leaves on the forest floor or inside rocky outcrops. These areas help protect them from predators and keep them moist.

Reproduction and Offspring

Christmas Island red crabs reach sexual maturity around 4 and 5 years of age. At the beginning of the rainy season (October to November), the crabs increase activity and travel to the coast for spawning. The timing is linked to the phase of the moon. Males reach the shore first and dig burrows. When the females arrive, the crabs mate in these burrows.

After mating, the males return to the forest, while the females remain another two weeks. They release their eggs into the water at the turn of high tide on the last quarter of the moon and then head back to the forest. The eggs immediately hatch upon contact with the water and are swept out to sea by the tide. The larvae remain at sea for 3 to 4 weeks, molting several times until they reach the megalopae stage. The megalopae cluster near the shore for a day or two before molting into small 0.2-inch crabs and journeying inland. Crabs molt several times as juveniles, but usually once a year as adults. Based on the life expectancy of related crabs, the Christmas Island red crab probably lives 20 to 30 years.

Conservation Status

As of 2018, the International Union for Conservation of Nature (IUCN) had not evaluated the Christmas Island red crab for a conservation status. Crab population numbers have plummeted due to invasion by the yellow crazy ant. The yellow crazy ant displaces and kills crabs. In the 1990s, the population of red crabs was estimated to be 43.7 million. Estimates of losses due to ants range from 10 million to 40 million. Researchers are hoping the introduction of a Malaysian wasp may give the crabs a chance to recover. The wasps eat the ants, so crabs in the test area can dig mating burrows in areas once infested with ants.

Threats

Ants are not the only threat Christmas Island red crabs face. They are preyed upon by coconut crabs. Entire generations of larvae may be eaten by fish, whale sharks, and manta rays, but the few times larvae survive, there have been enough to maintain the crab population.

Christmas Island Red Crabs and Humans

Red crabs cross roads during their annual breeding migration. The crab exoskeletons can puncture tires, plus the crabs die from being crushed. Park rangers have set up crab fences to direct the crustaceans to protected underpasses and bridges. The Christmas Island red crabs are protected by law and people are more aware of their plight, so drivers tend to be respectful of the animals during their migration.

Sources

• Adamczewska, A. M. and S. Morris. "Ecology and behaviour of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration." The Biological Bulletin. 200 (3): 305–320, June, 2001. doi:10.2307/1543512
• Dittrich, Stephanie. "How a Wasp Might Save the Christmas Island Red Crab." Island Conservation. January 24, 2019.
• Hicks, John W. "Red Crabs: On the March on Christmas Island." National Geographic. Vol. 172 no. 6. pp. 822–83, December, 1987.
• O'Dowd, Dennis J.; Green, Peter T. & P. S. Lake (2003). "Invasional 'meltdown' on an oceanic island." Ecology Letters. 6 (9): 812–817, 2003. doi:10.1046/j.1461-0248.2003.00512.x
• Weeks, A.R.; Smith, M.J.; van Rooyen, A.; Maple, D.; Miller, A.D. "A single panmictic population of endemic red crabs, Gecarcoidea natalis, on Christmas Island with high levels of genetic diversity." Conservation Genetics. 15 (4): 909–19, 2014. doi:10.1007/s10592-014-0588-x