Crabs of Christmas Island
Infra-order Brachyura, Family Gecarcinidea
Christmas Island Red crab | Gecarcoidea natalis (Pocock, 1888)
Red crabs are normally fire-engine red with black and white patterning on upper surface of the body shell (or carapace). Occasional yellow/orange specimens occur. An adult red crab carapace may measure up to 120 mm across. Males grow larger overall than females. Females usually have smaller claws than males. Young non-breeding crabs all have the characteristic narrow abdomen of the male. The broader female abdomen only becomes apparent in the third year of growth. Endemic to Christmas Island.
Red crabs are found all over the island from sea level to the highest points. Although most common in the moist environment of the rainforest, red crabs live in a variety of habitats including limestone pinnacle areas on the coastal shore terraces, and even domestic gardens. The only habitat they are not found in are the areas cleared of rainforest and stripped of soil for phosphate mining. Tall rainforest on deeper soils has the highest crab densities. They dig burrows in almost every square metre of available soil or live in deep crevices in rock outcrops. Estimates of the population size in the 1980s and 1990s were around 120 million individuals, although current estimates reduce this to about 50–60 million.
Red crabs are diurnal (active during the day) and almost inactive at night except for periods during their annual breeding migration. They take great care to conserve body moisture; an important factor influencing their activity.
Red crabs’ diet consists mainly of fallen leaves, fruits, flowers and seedlings. They prefer fresh green leaves but will eat any fallen leaves. They are not solely vegetarian however. They will eat dead crabs and birds, the introduced Giant African snail, and palatable human refuse.
The annual breeding migration of the Christmas Island red crabs has been acknowledged by many as one of the most spectacular animal migrations on Earth. It usually occurs during October–December.
It is estimated that since 1995, introduced yellow crazy ants have killed 10-15 million red crabs, about one quarter to one-third of the total island-wide population; a catastrophic decline in this keystone species. Strategic direction for the response to the yellow crazy ant incursion has been provided by long term researchers and a committee comprising scientists with expertise and knowledge of invasion biology, ants and the island’s ecosystems. Baiting with a Fipronil® poison bait matrix has proven extremely effective in reducing yellow crazy ant numbers.An ongoing baiting program is underway and research has been initiated to find a biological control agent that will be able to be used to combat the high densities of the introduced scale insects that produce the prime food source for the yellow crazy ant on the island. The idea being that by both poisoning the ant, and also removing the providers of their major food source, their numbers will be able to be drastically reduced and the ant’s capability to sustain high populations substantially reduced. Red crabs, robber crabs and other terrestrial crabs are able to cohabit with yellow crazy ants if their numbers are low.
Red crabs are a keystone species in Christmas Island’s ecology. The red crab is the major seed, seedling and litter consumer in the island’s rainforest. They eat fallen vegetation, leaves and fruit and recycle the nutrients contained in this material. Their cylindrical brown droppings scattered over the forest floor act as fertilizer. Their burrowing turns and aerates the soil and they are a major determinant of the unique structure and composition of the Christmas Island forest as a result of their selective grazing on seeds and seedlings.
More detailed and specific information on red crab physiology and biology, their annual breeding migration, migration management, the affect of the introduced yellow crazy and the ant control program, with accompanying images, is presented in the book
"Crabs of Christmas Island" by Max Orchard