Christmas Island red crabs are large crabs with the carapace measuring up to 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 and usually have smaller claws. Bright red is their most common color, but some can be orange or the much rarer purple.
Ecology and behaviour
Behaviour
Like most 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, and, despite lower temperatures and higher humidity, they 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 through the year; during the dry season, they will cover the entrance to the burrow to maintain a higher humidity inside, and will stay there for 3 months until the start of the wet season. Apart from the breeding season, red crabs are solitary animals and will defend their burrow from intruders.
Migration and breeding
For most of the year, red crabs can be found within Christmas Islands' forests. Each year, however, they migrate to the coast to breed; the beginning of the wet season 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. During this period they lay their eggs and incubate them in their abdominal brood pouch to facilitate their development. At the end of the incubation period the females leave their burrows and release their eggs into the ocean, 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.
Life cycle
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, where they remain 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 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 once a year, usually in the safety of their burrow.
Diet
Christmas Island red crabs are opportunistic omnivorous scavengers. They mostly eat fallen leaves, fruits, flowers and seedlings, but will also feed on dead animals, 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 dominance of the forest floor.
Predators
Adult red crabs have no natural predators on Christmas Island. An exploding population of the yellow crazy ant though, which is an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 10–15 million red crabs in recent years. In total, the ants are believed to have displaced 15–20 million red crabs on Christmas Island. During their larval stage, 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. Coconut crabs have also been filmed on Christmas Island preying on red crabs. Early inhabitants of Christmas Island rarely mentioned these crabs. It is possible that their current 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.
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. Less information is available for the population in the Cocos Islands, but numbers there are relatively low. Based on genetic evidence, it appears that the Cocos red crabs are relatively recent immigrants from Christmas Island, and for conservation purpose the two can be managed as a single population.
Relationship with humans
During their annual breeding migration, red crabs will often have to cross several roads 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 tyres. To ensure the safety of both populations, crabs and humans, local park rangers work to ensure that crabs can safely make their journey from the centre of the island to the sea; along heavily travelled roads, they set up aluminium barriers whose purpose is to funnel the crabs towards small underpasses so that they can safely traverse the roads. Other infrastructure to assist the crab migration includes a five-metre-high "crab bridge". 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.
