Christmas Island Red Crab

Gecarcoidea natalis

The Christmas Island red crab is a species of land crab that is unique to Christmas Island, Australia, in the Indian Ocean. Christmas Island red crabs are well known for their annual mass migration in which they move in millions to venture from land to the ocean to mate and lay their eggs.


Conservation status conservation_status_image
It is estimated that 40 to 50 million red crabs currently live on Christmas island, but in the past 15 years the population has been reduced by up to 40% by attacking yellow crazy ants.
Endangered for over
15
years
Range can cover Christmas Island in the Indian Ocean
  • Scientific Name
    Gecarcoidea natalis
  • Weight
    17 ounces
  • Size
    5 inches
  • Life Span
    Over 12 years
  • Habitat
    Rainforests, coast lines and gardens
  • Diet
    Omnivorous scavengers
  • Gestation
    3-4 weeks in larval stage (A single female can lay up to 100,000 eggs)
  • Predators
    Yellow crazy ant and coconut grabs. During larval stage: manta rays and whale sharks

Summary

Christmas Island red crabs are a large crab species that are usually bright red in color. However, orange and purple colored crabs have also been found. Land crabs use their gills to breathe and avoid direct sunlight so they do not dry out. The claws of the Christmas Island red crab are of equal size unless one is injured, in which case the claw will regenerate. Males are usually larger in claw and body size than females, although females have a wider abdomen. Female Christmas Island red crabs are responsible for the laying of eggs, which they release their eggs into the ocean. The eggs then hatch immediately upon contact with the water. Apart from the breeding season, Christmas Island red crabs are solitary and will defend their burrows if approached by another red crab. Christmas Island red crabs are opportunistic omnivores, meaning they rely greatly on scavenging when it comes time to eat. They eat a wide-ranging diet, from fallen leaves, fruits, and seedlings to dead animals and even other Christmas Island red crabs. The red crab has almost no competition for food or resources due to their dominance of the islands' forest floors.
habitat-map

Habitat

Christmas Island red crabs live in the dark forests in self-dug burrows where they are sheltered from the sun. During the dry season, the red crab will cover the entrance to their burrow to maintain humidity. Juvenile Christmas Island red crabs do not live in burrows but instead hide in rock outcrops, fallen tree branches, and other forest floor litter. The famous migration of the Christmas Island red crab begins at the start of the wet season (October or November) as it allows the red crabs to travel along the forest floor to the ocean without the worry of drying out. During migration, the red crabs leave their burrow and make the journey to the coast where male red crabs dig out new burrows where the mating takes place. The males then return to the forest and the female red crabs remain in the temporary burrow until it is time to lay their eggs.

Threats

When the red crab is still in its larval stage, their main predators are fish and large filter-feeders like whale sharks. However, on the land, Christmas Island red crabs have no natural predators. Although the crabs have no natural predators, the decline in their population was noticed after the accidental introduction of yellow crazy ants. The yellow crazy ants are believed to of killed anywhere from 10 - 15 million Christmas Island red crabs since their population boom. The red crabs became prey of the yellow crazy ants during their time on the forest floor and their migration numbers were heavily impacted. It was found that although the yellow crazy ants were preying on the red crabs, they were also heavily reliant on a honey-dew produced through a symbiotic relationship with another insect, the yellow lac scale. The sugar produced in the lac scale's honey-dew supplied ample food for the yellow crazy ants, resulting in even more fertile queens and lower death rates among workers.

Conservation Effort

It is thought that decreasing the amount of access the yellow crazy ants have to the honey-dew will result in lower ant population and bring back the success of the Christmas Island red crab. 300 micro-wasps from Malaysia were brought in to help curb the dew production of the lac scale insects. The micro-wasps are a parasitic species that lay eggs within the lac scale, which eventually kills the lac scale and ceases dew production.

Outcome

Since the introduction of the micro-wasps, the Christmas Island red crab population is on the rebound. Red crabs have been able to dig their mating burrows, which would have been otherwise overtaken by yellow crazy ants, and carry through with egg-laying.

Christmas Island red crabs have also been named a protected species and harming them is illegal. The building of “crab crossings”, walls and bridges red crabs can walk over rather than cross the road, on Christmas Island is a precaution taken to avoid human and red crab interaction. Major road closures and detours are also taken during the migration season.

How You Can Help

Christmas Island red crab migration season is one of the Island’s biggest tourist attractions and although the population of Christmas Island red crabs are on the rebound, but you can still help to ensure the red crabs populations remain healthy. If you are ever in the area during migration season, please:

  • carefully walk among the sea of red crabs,
  • pay attention to road closures and redirection notices,
  • do not pick up or feed the red crabs, and
  • respect the space of the red crabs

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