![Christmas island red crabs - female with eggs chris bray-2](https://upload.wikimedia.org/wikipedia/commons/b/b0/Christmas_island_red_crabs_-_female_with_eggs_chris_bray-2.jpg)
The Christmas Island red crab (''Gecarcoidea natalis'') is a species of
land crab
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'', althou ...
that is
endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
to
Christmas Island
Christmas Island, officially the Territory of Christmas Island, is an Australian external territory comprising the island of the same name. It is located in the Indian Ocean, around south of Java and Sumatra and around north-west of the ...
and
Cocos (Keeling) Islands
)
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, image_map = Australia on the globe (Cocos (Keeling) Islands special) (Southeast Asia centered).svg
, map_alt = Location of the Cocos (Keeling) Islands
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in the
Indian Ocean
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
. Although restricted to a relatively small area, an estimated 43.7 million adult red crabs once lived on Christmas Island alone, but the accidental
introduction
Introduction, The Introduction, Intro, or The Intro may refer to:
General use
* Introduction (music), an opening section of a piece of music
* Introduction (writing), a beginning section to a book, article or essay which states its purpose and g ...
of the
yellow crazy ant
The yellow crazy ant (''Anoplolepis gracilipes''), 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 trop ...
is believed to have killed about 10–15 million of these in recent years.
Christmas Island red crabs make an
annual mass migration to the sea to lay their
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
s in the ocean. Although its population is under great assault by the ants, as of 2020 the red crab had not been assessed by the
International Union for Conservation of Nature
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
(IUCN) and it was not listed on their
Red List
The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biologi ...
. 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.
Description
Christmas Island red crabs are large crabs with the
carapace
A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...
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
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
(only apparent above 3 years of age) 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
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
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
burrow
An Eastern chipmunk at the entrance of its burrow
A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of sh ...
s to shelter themselves from the sun and will usually stay in the same burrow through the year; during the
dry season
The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The te ...
, 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
The wet season (sometimes called the Rainy season) is the time of year when most of a region's average annual rainfall occurs. It is the time of year where the majority of a country's or region's annual precipitation occurs. Generally, the sea ...
. Apart from the breeding season, red crabs are solitary animals and will defend their burrow from intruders.
Migration and breeding
![Christmas Island Crabs on annual migration](https://upload.wikimedia.org/wikipedia/commons/b/b1/Christmas_Island_Crabs_on_annual_migration.JPG)
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. 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
![Christmas island red crabs - megalopae stage chris bray-5](https://upload.wikimedia.org/wikipedia/commons/4/44/Christmas_island_red_crabs_-_megalopae_stage_chris_bray-5.jpg)
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
megalopa
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The ...
e. 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. Their lifespan is about 12 years.
Diet
![Gecarcoidea natalis 2](https://upload.wikimedia.org/wikipedia/commons/f/f6/Gecarcoidea_natalis_2.jpg)
Christmas Island red crabs are opportunistic
omnivorous
An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nutri ...
scavenger
Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding b ...
s. 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 Giant African land snail is the common name of several species within the family Achatinidae, a family of unusually large African terrestrial snails:
* ''Achatina achatina'', also known as the agate snail or Ghana tiger snail
* ''Lissachatina fulic ...
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
![Rhincodon typus fgbnms](https://upload.wikimedia.org/wikipedia/commons/0/0a/Rhincodon_typus_fgbnms.jpg)
Adult red crabs have no natural predators on Christmas Island.
The
yellow crazy ant
The yellow crazy ant (''Anoplolepis gracilipes''), 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 trop ...
, an
invasive species
An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
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.
During their larval stage, millions of red crab larvae are eaten by fish and large filter-feeders such as ]manta ray
Manta rays are large rays belonging to the genus ''Mobula'' (formerly its own genus ''Manta''). The larger species, '' M. birostris'', reaches in width, while the smaller, '' M. alfredi'', reaches . Both have triangular Pectoral fin#AnchPect ...
s and whale shark
The whale shark (''Rhincodon typus'') is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, Cosgrove J, D ...
s which visit Christmas Island during the red crab breeding season.
Coconut crab
The coconut crab (''Birgus latro'') is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest terrestrial arthropod in the world, with a weight of up to . It can grow to up to in width from the tip ...
s (alternatively known as robber 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
Maclear's rat (''Rattus macleari'') is an extinct large rat Endemism, endemic to Christmas Island in the Indian Ocean. It was one of two species of rat native to Christmas Island, alongside the bulldog rat. Abundant, unfamiliar with and seemingly ...
, ''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
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
of 43.7 million on Christmas Island. Less information is available for the population in the Cocos (Keeling) Islands
)
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, song_type =
, song =
, image_map = Australia on the globe (Cocos (Keeling) Islands special) (Southeast Asia centered).svg
, map_alt = Location of the Cocos (Keeling) Islands
, map_caption = ...
, but numbers there are relatively low. 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.
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 the forest. As a result, thousands of red crabs are crushed
Crushed may refer to:
* "Crushed" (''Ms. Marvel''), a 2022 episode of the American television series ''Ms. Marvel''
* "Crushed" (Roland Lee Gift song) a 2009 single by Roland Lee Gift
* "Crushed" (''The Suite Life of Zack & Cody'' episode), an ...
by vehicles and sometimes cause accidents
An accident is an unintended, normally unwanted event that was not directly caused by humans. The term ''accident'' implies that nobody should be blamed, but the event may have been caused by unrecognized or unaddressed risks. Most researcher ...
due to their tough exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
s 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. 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. Their small size, high water content and poor meat quality mean they are not considered edible by humans.[https://a-z-animals.com/animals/christmas-island-red-crab/]
References
Further reading
*
External links
*
Christmas Island National Park Website
3 minute TV clip showing crabs migrating through a town
* ttp://www.christmasislandcrabs.com Website showing the crabs of Christmas Island including the red crab migration
{{Taxonbar, from=Q1934004
Grapsoidea
Fauna of Christmas Island
Crustaceans of Australia
Terrestrial crustaceans
Crustaceans described in 1888
Christmas Island cuisine