Christmas Island red crab
The Christmas Island red crab, Gecarcoidea natalis, is a species of land crab that is endemic to Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean. Although restricted to a relatively small area, it has been estimated that 43.7 million adult red crabs lived on Christmas Island alone, but the accidental introduction of the yellow crazy ant is believed to have killed about 10–15 million of these in recent years. Christmas Island red crabs are well known for their annual mass migration to the sea to lay their eggs in the ocean.
Christmas Island red crabs are rather large crabs with the carapace measuring up to 116 millimetres (4.6 in) wide. The claws are usually of equal size, unless one becomes injured or detached, in which case the limb will regenerate. The male crabs are generally larger than the females, while adult females have a much broader abdomen (only apparent above 3 years of age) and usually have smaller claws. Bright red is their most common colour, but some can be orange or the much rarer purple.
Ecology and behavior
Red crabs are land crabs which are specially adapted to living on land. Like all land crabs, red crabs use gills to breathe and must take great care to conserve body moisture. Although red crabs are diurnal, they usually avoid direct sunlight so as not to dry out. Despite lower temperatures and higher humidity, red crabs are almost completely inactive at night. Red crabs also dig burrows to shelter themselves from the sun and will usually stay in the same burrow throught the year. During the dry season, they will cover the entrance to their burrows with a loose wad of leaves to maintain high humidity in their burrow and will virtually disappear for 3 months until the start of the wet season. Apart from their breeding season, Red crabs are solitary animals and will defend their burrow from intruders.
For most of the year, red crabs can be found within Christmas Islands' forests, however, each year they must migrate to the coast to breed. The beginning of the wet season (usually October/November) allows the crabs to increase their activity and stimulates their annual migration. The timing of their migration is also linked to the phases of the moon. During this migration red crabs abandon their burrows and travel to the coast to mate and spawn. This normally requires at least a week, with the male crabs usually arriving before the females. Once on the shore, the male crabs excavate burrows, which they must defend from other males. Mating occurs in or near the burrows. Soon after mating the males return to the forest while the females remain in the burrow for another two weeks to lay their eggs. At the end of the incubation period the females leave their burrows and release their eggs into the ocean. This occurs precisely at the turn of the high tide during the last quarter of the moon. The females then return to the forest while the crab larvae spend another 3–4 weeks at sea before returning to land as juvenile crabs.
The eggs released by the females immediately hatch upon contact with sea water and clouds of crab larvae will swirl near the shore until they are swept out to sea for 3–4 weeks. During this time, the larvae go through several larval stages, eventually developing into shrimp-like animals called megalopae. The megalopae gather near the shore for 1–2 days before changing into young crabs only 5 mm (0.20 in) across. The young crabs then leave the water to make a 9-day journey to the centre of the island. For the first three years of their lives, the young crabs will remain hidden in rock outcrops, fallen tree branches and debris on the forest floor. Red crabs grow slowly, reaching sexual maturity at around 4–5 years, at which point they begin participating in the annual migration. During their early growth phases, red crabs will moult several times. Mature red crabs will moult one a year, usually in the safety of their burrow.
Christmas Island red crabs are opportunistic omnivorous scavengers. They mostly eat fallen leaves, fruits, flowers and seedlings, but will also feed on dead animals (including other red crabs, see cannibalism), and human rubbish. The non-native giant African land snail is also another food choice for the crabs. Red crabs have virtually no competition for food due to their large numbers.
Adult red crabs have no natural predators on Christmas Island. During their larval stage however, millions of red crab larvae are eaten by fish and large filter-feeders such as manta rays and whale sharks which visit Christmas Island during the red crab breeding season.
Early inhabitants of Christmas Island rarely mentioned these crabs. It is possible that their large population size was caused by the extinction of the endemic Maclear's rat, Rattus macleari in 1903, which may have limited the crab's population. Surveys have found a density of 0.09–0.57 adult red crabs per square metre, equalling an estimated total population of 43.7 million on Christmas Island. Others have estimated that about 120 million are found on this island, but the basis for that claim is unclear.
An exploding population of the yellow crazy ant, an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 10–15 million red crabs (one-quarter to one-third of the total population) in recent years. In total (including killed), the ants are believed to have displaced 15–20 million red crabs on Christmas Island.
Relationship with Humans
During their annual breeding migration, red crabs will often have to cross roads, sometimes as many as 3 or 4, to get to their breeding grounds and then back to forest. As a result red crabs are frequently crushed by vehicles and sometimes cause accidents due to their tough exoskeletons which are capable of puncturing tires. To ensure both the safety of crabs and humans, local park rangers work hard to ensure that the crabs can safely cross the island to the coast. Park rangers set up aluminum barriers called "crab fences" along heavily traveled roads. The crab fences funnel the crabs towards small underpasses called "crab grids" so that the crabs can safely cross under the roads. In recent years, the human inhabitants of Christmas Island have become more tolerant and respectful of the crabs during their annual migration and are now more cautious while driving, which helps to minimise crab casualties.
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- Tim Flannery & Peter Schouten (2001). A Gap in Nature: Discovering the World's Extinct Animals. Atlantic Monthly Press, New York. ISBN 0-87113-797-6.
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- "Red crabs". Parks Australia. December 1, 2013. Retrieved December 3, 2013.