Christmas Island red crab
Encyclopedia
The Christmas Island red crab, Gecarcoidea natalis, is a species of land crab that is endemic to Christmas Island
and the Cocos (Keeling) Islands
in the Indian Ocean
. Although restricted to a relatively small area, it has been estimated that 43.7 million adult red crabs lived on Christmas Island alone, but the accidental introduction
of the yellow crazy ant
is believed to have killed about 10–15 million of these in recent years. Christmas Island red crabs eat mostly fallen leaves
and flower
s, but will occasionally eat other animals, including other red crabs (see cannibalism
) if the opportunity arises.
is up to 116 millimetres (4.6 in) wide, rounded, and encloses the gill
s. 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.
s for shelter from the sun. Since they breathe through gill
s, the possibility of drying out is dangerous. They are famous for their annual migration to the sea to lay their eggs
in the ocean.
, Rattus macleari in 1903, which may have controlled the crab's population. Surveys have found a density 0.09–0.57 adult red crabs per square metre, equalling an estimated total population
of 43.7 million on Christmas Island. Others have estimated that about 120 million are found on this island, but the basis for that claim is unclear.
An exploding population of 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. In total (including killed), the ants are believed to have displaced 15–20 million red crabs on Christmas Island.
Christmas Island
The Territory of Christmas Island is a territory of Australia in the Indian Ocean. It is located northwest of the Western Australian city of Perth, south of the Indonesian capital, Jakarta, and ENE of the Cocos Islands....
and the Cocos (Keeling) Islands
Cocos (Keeling) Islands
The Territory of the Cocos Islands, also called Cocos Islands and Keeling Islands, is a territory of Australia, located in the Indian Ocean, southwest of Christmas Island and approximately midway between Australia and Sri Lanka....
in the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
. Although restricted to a relatively small area, it has been estimated that 43.7 million adult red crabs lived on Christmas Island alone, but the accidental introduction
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
of the yellow crazy ant
Yellow crazy ant
The yellow crazy ant is a species of ant, introduced accidentally to northern Australia and Christmas Island in the Indian Ocean, that has wreaked ecological damage in both locations....
is believed to have killed about 10–15 million of these in recent years. Christmas Island red crabs eat mostly fallen leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
and flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s, but will occasionally eat other animals, including other red crabs (see cannibalism
Cannibalism (zoology)
In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species...
) if the opportunity arises.
Description
The carapaceCarapace
A carapace is a dorsal 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 tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
is up to 116 millimetres (4.6 in) wide, rounded, and encloses the gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s. 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
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
(only apparent above 3 years of age) and usually have smaller claws.
Ecology
Christmas red crabs live in burrowBurrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
s for shelter from the sun. Since they breathe through gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s, the possibility of drying out is dangerous. They are famous for their annual migration to the sea to lay their eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
in the ocean.
Population
Early inhabitants of Christmas Island rarely mentioned these crabs. It is possible that their large population size was caused by the extinction of the endemic Maclear's RatMaclear's Rat
The Maclear's Rat was a large rat which lived on Christmas Island in the Indian Ocean. Abundant, unfamiliar with and seemingly unafraid of humans, large numbers of the creatures emerged and foraged in all directions at night...
, Rattus macleari in 1903, which may have controlled the crab's population. Surveys have found a density 0.09–0.57 adult red crabs per square metre, equalling an estimated total population
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
of 43.7 million on Christmas Island. Others have estimated that about 120 million are found on this island, but the basis for that claim is unclear.
An exploding population of the yellow crazy ant
Yellow crazy ant
The yellow crazy ant is a species of ant, introduced accidentally to northern Australia and Christmas Island in the Indian Ocean, that has wreaked ecological damage in both locations....
, an invasive species
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
accidentally introduced to Christmas Island and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
from Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, 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.