Christmas Island red crab | |
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Megalopae of the crab | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Malacostraca |
Order: | Decapoda |
Suborder: | Pleocyemata |
Infraorder: | Brachyura |
Family: | Gecarcinidae |
Genus: | Gecarcoidea |
Species: | G. natalis |
Binomial name | |
Gecarcoidea natalis Pocock, 1888 | |
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Distribution map of Christmas Island red crab |
The Christmas Island red crab (Gecarcoidea natalis) is a species of land crab that is endemic to Christmas Island and Cocos (Keeling) Islands in the Indian Ocean. [1] [2] Although restricted to a relatively small area, an estimated 43.7 million adult red crabs once lived on Christmas Island alone, [3] but the accidental introduction of the yellow crazy ant is believed to have killed about 10–15 million of these in recent years. [4] Christmas Island red crabs make an annual mass migration to the sea to lay their eggs in the ocean. [5] Although its population is under great assault by the ants, [6] as of 2020 the red crab had not been assessed by the International Union for Conservation of Nature (IUCN) and it was not listed on their Red List. [7] The crab's annual mass migration to the sea for spawning is described as an "epic" event. Millions emerge at the same time, halting road traffic and covering the ground in a thick carpet of crabs. [8]
Christmas Island red crabs are large crabs with the carapace measuring up to 116 millimetres (4.6 in) wide. [9] 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. [9] Bright red is their most common color, but some can be orange or the much rarer purple.
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. [9] 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. [10] [9]
For most of the year, red crabs can be found within Christmas Islands' forests. Each year they 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. [3] The timing of their migration is also linked to the phases of the moon. [5] 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. [3] 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. [3] [5] 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. [3]
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. [5] During this time, the larvae go through several larval stages, eventually developing into shrimp-like animals called megalopae. [9] 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. [5] 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. [5] Red crabs grow slowly, reaching sexual maturity at around 4–5 years, at which point they begin participating in the annual migration. [9] 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. Their lifespan is about 12 years. [11]
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 cannibalising other red crabs), and human rubbish. The non-native giant African land snail is also another food choice for the crabs. [9] Red crabs have virtually no competition for food due to their dominance of the forest floor.
Adult red crabs have no natural predators on Christmas Island. [12] 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. [4] In total (including killed), the ants are believed to have displaced 15–20 million red crabs on Christmas Island. [13] 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 (alternatively known as robber crabs) have also been filmed on Christmas Island preying on red crabs. [14]
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. [15]
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. [3] Less information is available for the population in the Cocos (Keeling) Islands, but numbers there are relatively low. [16] Based on genetic evidence, it appears that the Cocos (Keeling) red crabs are relatively recent immigrants from Christmas Island, and for conservation purposes the two can be managed as a single population. [17]
During their annual breeding migration, red crabs will often have to cross several roads to get to their breeding grounds and then back to the forest. As a result, thousands of red crabs are crushed by vehicles and sometimes cause accidents due to their tough exoskeletons which are capable of puncturing tires. To ensure the safety of both the 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. [9] [5] Other infrastructure to assist the crab migration includes a five-metre-high "crab bridge". [18] In recent years,[ when? ] 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. [5] Their small size, high water content and poor meat quality mean they are not considered edible by humans. [19]
The yellow crazy ant, also known as the long-legged ant or Maldive ant, is a species of ant, thought to be native to West Africa or Asia. They have been accidentally introduced to numerous places in the world's tropics.
The red knot or just knot is a medium-sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia. It is a large member of the Calidris sandpipers, second only to the great knot. Six subspecies are recognised.
The Jefferson salamander is a mole salamander native to the northeastern United States, southern and central Ontario, and southwestern Quebec. It was named after Jefferson College in Pennsylvania.
Abbott's booby is an endangered seabird of the sulid family, which includes gannets and boobies. It is a large booby and is placed within its own monotypic genus. It was first identified from a specimen collected by William Louis Abbott, who discovered it on Assumption Island in 1892.
Gecarcoidea lalandii is a large species of terrestrial crab. It is dark purple in colour, with long legs and short pincers. It is nocturnal, so spends most of the day hiding in burrows. Compared to the related G. natalis, G. lalandii is relatively widespread, being found in the Indo-Pacific from the Andaman Islands and eastwards. Adults mainly occur in forest, but can sometimes be found in more open habitats. When carrying eggs, females migrate to the coast, where they release the eggs in the tidal zone. Small adults sometimes fall prey to the crab Geograpsus crinipes.
The Christmas imperial pigeon or Christmas Island imperial pigeon, also known as Black imperial pigeon, Dusky imperial pigeon, Wharton's imperial pigeon, or burong pergam, is a large imperial pigeon endemic to Christmas Island in the northeastern Indian Ocean. It has an overall grey-blue colouration, and juveniles are duller than adults. It makes a soft purring coo sound and a deeper whoo sound comparable to a cow mooing. It lays one glossy white egg per brood, and is possibly somewhat colonial.
Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The cause of migration may be local climate, local availability of food, the season of the year or for mating.
The Christmas boobook, also known as the Christmas hawk owl, Christmas Island boobook or Christmas Island hawk owl, is a species of owl in the family Strigidae.
The Christmas white-eye is a species of bird in the family Zosteropidae. It is endemic to Christmas Island. Its natural habitats are tropical and subtropical moist broadleaf forests and subtropical or tropical moist shrubland. It is threatened by habitat destruction.
The Christmas Island shrew, also known as the Christmas Island musk-shrew is an extremely rare or possibly extinct shrew from Christmas Island. It was variously placed as subspecies of the Asian gray shrew or the Southeast Asian shrew, but morphological differences and the large distance between the species indicate that it is an entirely distinct species.
Christmas Island National Park is a national park occupying most of Christmas Island, an Australian territory in the Indian Ocean southwest of Indonesia. The park is home to many species of animal and plant life, including the eponymous red crab, whose annual migration sees around 100 million crabs move to the sea to spawn. Christmas Island is the only nesting place for the endangered Abbott's booby and critically endangered Christmas Island frigatebird, and the wide range of other endemic species makes the island of significant interest to the scientific community.
The Christmas goshawk or Christmas Island goshawk is a bird of prey in the goshawk and sparrowhawk family Accipitridae. It is a threatened endemic of Christmas Island, an Australian territory in the eastern Indian Ocean.
The coconut crab is a terrestrial species of giant hermit crab, and is also known as the robber crab or palm thief. It is the largest terrestrial arthropod known, with a weight of up to 4.1 kg (9 lb). The distance from the tip of one leg to the tip of another can be as wide as 1 m. It is found on islands across the Indian and Pacific Oceans, as far east as the Gambier Islands, Pitcairn Islands and Caroline Island and as far west as Zanzibar. While its range broadly shadows the distribution of the coconut palm, the coconut crab has been extirpated from most areas with a significant human population such as mainland Australia and Madagascar.
Gecarcinus ruricola is a species of terrestrial crab. It is the most terrestrial of the Caribbean land crabs, and is found from western Cuba across the Antilles as far east as Barbados. Common names for G. ruricola include the purple land crab, black land crab, red land crab, and zombie crab.
Johngarthia lagostoma is a species of terrestrial crab that lives on Ascension Island and three other islands in the South Atlantic. It grows to a carapace width of 110 mm (4.3 in) on Ascension Island, where it is the largest native land animal. It exists in two distinct colour morphs, one yellow and one purple, with few intermediates. The yellow morph dominates on Ascension Island, while the purple morph is more frequent on Rocas Atoll. The species differs from other Johngarthia species by the form of the third maxilliped.
A number of lineages of crabs have evolved to live predominantly on land. Examples of terrestrial crabs are found in the families Gecarcinidae and Gecarcinucidae, as well as in selected genera from other families, such as Sesarma, although the term "land crab" is often used to mean solely the family Gecarcinidae.
Cryptoblepharus egeriae, also known commonly as the blue-tailed shinning-skink, the Christmas Island blue-tailed shinning-skink, and the Christmas Island blue-tailed skink, is a species of lizard in the family Scincidae that was once endemic to Christmas Island. The Christmas Island blue-tailed skink was discovered in 1888. It was formerly the most abundant reptile on the island, and occurred in high numbers particularly near the human settlement. However, the Christmas Island blue-tailed skink began to decline sharply outwardly from the human settlement by the early 1990s, which coincided with the introduction of a predatory snake and also followed the introduction of the yellow crazy ant in the mid-1980s. By 2006, the Christmas Island blue-tailed skink was on the endangered animals list, and by 2010 the Christmas Island blue-tailed skink was extinct in the wild. From 2009 to 2010, Parks Australia and Taronga Zoo started a captive breeding program, which has prevented total extinction of the species.
The wildlife of Christmas Island is composed of the flora and fauna of this isolated island in the tropical Indian Ocean. Christmas Island is the summit plateau of an underwater volcano. It is mostly clad in tropical rainforest and has karst, cliffs, wetlands, coasts and sea. It is a small island with a land area of 135 km2 (52 sq mi), 63% of which has been declared a National park. Most of the rainforest remains intact and supports a large range of endemic species of animals and plants.
The Christmas and Cocos Islands tropical forests ecoregion covers forested areas of Christmas Island and North Keeling Island, two small seamount islands south of the Indonesian island of Java. The forests of these two islands share tree species of the Indo-Pacific and Melanesian types on nearby islands, the forests of Christmas Island and North Keeling Island are unique in how they reflect the effects of large populations of terrestrial red crabs. Because of the remoteness of the islands, there are many endemic species.
The annual migration of red crabs in Australia begins in October/November each year. Millions of red crabs Gecarcoidea natalis migrate from the Australian islands to the Indian Ocean during this one to two-week-long period. The purpose of migration is to go underwater and lay eggs and breeding has to be made possible. During this migration season, the routes of arrival and departure of crabs are closed with barriers so that they can be protected from any kind of damage.