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Coconut crab

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

                  iCoconut crab

                             Conservation status

   Data deficient (DD)
            Scientific classification

   Kingdom:     Animalia
   Phylum:      Arthropoda
   Subphylum:   Crustacea
   Class:       Malacostraca
   Order:       Decapoda
   Suborder:    Pleocyemata
   Infraorder:  Anomura
   Superfamily: Paguroidea
   Family:      Coenobitidae
   Genus:       Birgus
   Species:     B. latro

                                Binomial name

   Birgus latro
   Linnaeus, 1767
   Coconut crab distribution
   Coconut crab distribution

   The coconut crab (Birgus latro) is the largest terrestrial arthropod in
   the world. It is a derived hermit crab which is known for its ability
   to crack coconuts with its strong pincers in order to eat the contents.
   It is sometimes called the robber crab or palm thief (in German,
   Palmendieb), because some coconut crabs are rumored to steal shiny
   items such as pots and silverware from houses and tents. Another name
   is the terrestrial hermit crab, due to the use of shells by the young
   animals (although terrestrial hermit crab also applies to a number of
   other hermit crabs — see Australian land hermit crab). The coconut crab
   also has different local names as for example ayuyu in Guam, or unga or
   kaveu.

Physical description

   Print of a coconut crab from the Dictionnaire D'Histoire Naturelle of
   1849.
   Enlarge
   Print of a coconut crab from the Dictionnaire D'Histoire Naturelle of
   1849.

   Reports about the size of Birgus latro vary, and most references give a
   weight of up to 4 kg (9 lb), a body length of up to 400 mm (16 in), and
   a leg span of around 1 m (3 ft), with males generally being larger than
   females. Some reports claim weights up to 17 kg and a body length of 1
   m. It is believed that this is near the theoretical limit for a
   terrestrial arthropod. However, when the body is supported by water,
   larger sizes are possible (see Japanese spider crab). They can reach an
   age of up to 30–60 years (references vary). The body of the coconut
   crab is, like all decapods, divided into a front section (
   cephalothorax), which has 10 legs, and an abdomen. The front-most legs
   have massive claws used to open coconuts, and these claws (chelae) can
   lift objects up to 29 kg (64 lb) in weight. The next three pairs have
   smaller tweezer-like chelae at the end, and are used as walking limbs.
   In addition these specially adapted limbs enable the coconut crab to
   climb vertically up trees (often coconut palms) up to 6 m high. The
   last pair of legs is very small and serves only to clean the breathing
   organs. These legs are usually held inside the carapace, in the cavity
   containing the breathing organs.

   Although Birgus latro is a derived type of hermit crab, only the
   juveniles use salvaged snail shells to protect their soft abdomens, and
   adolescents sometimes use broken coconut shells to protect their
   abdomens. Unlike other hermit crabs, the adult coconut crabs do not
   carry shells, but instead harden their abdominal armor by depositing
   chitin and chalk. They also bend their tails underneath their bodies
   for protection, as do most true crabs. The hardened abdomen protects
   the coconut crab and reduces water loss on land, but has to be moulted
   at periodic intervals. Moulting takes about 30 days, during which the
   animal's body is soft and vulnerable, and it stays hidden for
   protection.

   Coconut crabs cannot swim and will drown in water. They use a special
   organ called a branchiostegal lung to breathe. This organ can be
   interpreted as a developmental stage between gills and lungs, and is
   one of the most significant adaptations of the coconut crab to its
   habitat. The chambers of this breathing organ are located in the rear
   of the cephalothorax. They contain a tissue similar to that found in
   gills, but suited to the absorption of oxygen from air, rather than
   water. They use their last, smallest pair of legs to clean these
   breathing organs, and to moisten them with seawater. The organs require
   water to function, and the crab provides this by stroking its wetted
   legs over the spongy tissues nearby. Coconut crabs may also drink salt
   water, using the same technique to transfer water to their mouths.

   In addition to this breathing organ, the coconut crab has an additional
   rudimentary set of gills. However, while these gills were probably used
   to breathe under water in the evolutionary history of the species, they
   no longer provide sufficient oxygen, and an immersed coconut crab will
   drown within a few hours or minutes (reports vary, probably depending
   on the levels of stress and exercise and the resulting oxygen
   consumption).

   Another distinctive organ of the coconut crab is its nose. The process
   of smelling works very differently depending whether the smelled
   molecules are hydrophilic molecules in water or hydrophobic molecules
   in air. As most crabs live in the water, they have specialized organs
   called aesthetascs on their antennae to determine both the
   concentration and the direction of a smell. However, as coconut crabs
   live on the land, the aesthetascs on their antennae differ
   significantly from those of other crabs and look more like the smelling
   organs of insects, called sensilia. While insects and the coconut crab
   originate from different evolutionary paths, the same need to detect
   smells in the air led to the development of remarkably similar organs,
   making it an example of convergent evolution. Coconut crabs also flick
   their antennae as insects do to enhance their reception. They have an
   excellent sense of smell and can detect interesting odours over large
   distances. The smell of rotting meat, bananas and coconuts catch their
   attention especially, as potential food sources.

Reproduction

   Coconut crabs mate frequently and quickly on dry land in the period
   from May to September, especially in July and August. The male and the
   female fight with each other, and the male turns the female on her back
   to mate. The whole mating procedure takes about 15 minutes. Shortly
   thereafter, the female lays her eggs and glues them to the underside of
   her abdomen, carrying the fertilized eggs underneath her body for a few
   months. At the time of hatching, usually October or November, the
   female coconut crab releases the eggs into the ocean at high tide.
   These larvae are called zoeas. It is reported that all coconut crabs do
   this on the same night, with many females on the beach at the same
   time.

   The larvae float in the ocean for 28 days, during which a large number
   of them are eaten by predators. Afterwards, they live on the ocean
   floor and on the shore as hermit crabs, using discarded shells for
   protection for another 28 days. At that time, they sometimes visit dry
   land. As with all hermit crabs, they change their shells as they grow.
   After these 28 days, they leave the ocean permanently and lose the
   ability to breathe in water. Young coconut crabs that cannot find a
   seashell of the right size also often use broken coconut pieces. When
   they outgrow even coconut shells, they develop a hardened abdomen.
   About 4 to 8 years after hatching the coconut crab matures and can
   reproduce. This is an unusually long development period for a
   crustacean.

Diet

   Coconut crabs vary in size and colouring.
   Enlarge
   Coconut crabs vary in size and colouring.

   The diet of coconut crabs consists primarily of fruit, including
   coconuts and figs. However, they will eat nearly anything organic,
   including leaves, rotten fruit, tortoise eggs, dead animals, and the
   shells of other animals, which are believed to provide calcium. They
   may also eat live animals that are too slow to escape, such as freshly
   hatched sea turtles. During a tagging experiment, one coconut crab was
   observed to catch and eat a Polynesian rat . Coconut crabs often try to
   steal food from each other and will pull their food into their burrows
   to be safe while eating.

   The coconut crab climbs trees to eat coconuts or fruit, to escape the
   heat or to escape predators. It is a common perception that the coconut
   crab cuts the coconuts from the tree to eat them on the ground (hence
   the German name palm thief and the Dutch Klapperdief). However,
   according to the late German biologist Holger Rumpf (sometimes spelled
   Rumpff) the animal is not intelligent enough for such a planned action,
   and rather accidentally drops a coconut while attempting to open it on
   the tree. Coconut crabs cut holes into coconuts with their strong claws
   and eat the contents; this behaviour is unique in the animal kingdom.

   It was doubted for a long time that the coconut crab could open
   coconuts, and in experiments, some have starved to death surrounded by
   coconuts. However, in the 1980s Rumpf was able to observe and study
   them opening coconuts in the wild. The crab has developed a special
   technique to do so: if the coconut is still covered with husk, it will
   use its claws to rip off strips, always starting from the side with the
   three germination pores, the group of three small circles found on the
   outside of the coconut. Once the pores are visible, the crab will bang
   its pincers on one of them until they break. Afterwards, it will turn
   around and use the smaller pincers on its other legs to pull out the
   white flesh of the coconut. Using their strong claws, larger
   individuals can even break the hard coconut into smaller pieces for
   easier consumption.

Habitat

   Coconut crabs live alone in underground burrows and rock crevices,
   depending on the local terrain. They dig their own burrows in sand or
   loose soil. During the day, the animal stays hidden, to protect itself
   from predators and reduce water loss from heat. While resting in its
   burrow, the coconut crab closes the entrance with one of its claws to
   create the moist microclimate within the burrow necessary for its
   breathing organs. In areas with a large coconut crab population, some
   may also come out during the day, perhaps to gain an advantage in the
   search for food. Coconut crabs will also sometimes come out during the
   day if it is moist or raining, since these conditions allow them to
   breathe more easily. They live almost exclusively on land, and some
   have been found up to 6 km from the ocean.

Distribution

   Coconut crabs live in areas throughout the Indian and western Pacific
   oceans. Christmas Island in the Indian Ocean has the largest and
   best-preserved population in the world. Large populations also exist on
   the Cook Islands ( Pacific Islands), especially Pukapuka, Suwarrow,
   Mangaia, Takutea, Mauke, Atiu and smaller islands of Palmerston. Other
   populations exist on the Seychelles, especially Aldabra, Glorioso
   Islands, Astove Island, Assumption Island, and Cosmoledo, but the
   coconut crab is extinct on the central islands. They are also known on
   several of the Andaman and Nicobar Islands in the Bay of Bengal. There
   is some difference in colour between the animals found on different
   islands, ranging from light violet through deep purple to brown.

   As they cannot swim as adults, coconut crabs over time must have
   colonized the islands as larvae, which can swim. However, due to the
   large distances between the islands, some researchers believe a larvae
   stadium of 28 days is not enough to travel the distance and they assume
   juvenile coconut crabs reached other islands on driftwood and flotsam.

   The distribution shows some gaps, as for example around Borneo,
   Indonesia or New Guinea. These islands were within easy reach of the
   crab, and also have a suitable habitat, yet have no coconut crab
   population. This is due to the coconut crabs being eaten to extinction
   by people. However, coconut crabs are known to live on the islands of
   the Wakatobi Marine National Park in Sulawesi, Indonesia.

Conservation status

   According to the IUCN Red List criteria, there is not enough data to
   decide if the coconut crab is an endangered species, and therefore it
   is listed as DD (data deficient). However, according to some reports
   the populations are quite large, with one of the largest populations
   being on Caroline Island. It is believed that the coconut crab is quite
   common on some islands, but rather rare on others. Coastal development
   on many islands reduces the natural habitat of the crab.

   The juvenile coconut crab is vulnerable to introduced carnivores such
   as rats, pigs, or ants such as the yellow crazy ant. Adult coconut
   crabs have no natural predators, and are eaten only by people. The
   adults have poor eyesight, and detect enemies based on ground
   vibration.

   Overall, it seems that large human populations have a negative effect
   on the coconut crab population, and in some areas, populations are
   reported to be declining due to over-harvesting. The coconut crab is
   protected in some areas, with minimum sizes for taking and a protected
   breeding period.

Cultural aspects

   This hermit crab with its intimidating size and strength has a special
   position in the culture of the islanders. The coconut crab is eaten by
   the Pacific islanders, and is considered a delicacy and an aphrodisiac,
   with a taste similar to lobster and crabmeat. The most prized parts are
   the eggs inside the female coconut crab and the fat in the abdomen.
   Coconut crabs can be cooked in a similar way to lobsters, by boiling or
   steaming. Different islands also have a variety of recipes, as for
   example coconut crab cooked in coconut milk.

   While the coconut crab itself is not poisonous, it may become poisonous
   depending on its diet, and cases of coconut crab poisoning have
   occurred. It is believed that the poison comes from plant toxins, which
   would explain why some animals are poisonous and others not. It may
   also be possible that this poison is considered an aphrodisiac, similar
   to the highly poisonous pufferfish eaten in Japan. However, coconut
   crabs are not a commercial product and are usually not sold.

   Hunting is best on a moonless night with wet ground using flashlights.
   The best time is the three days following the new moon. Coconut crabs
   can also be hunted during the day, but this involves digging to reach
   them in their burrows or a fire to smoke them out of their hiding
   places. It is also suggested that spreading burnt coconut halves will
   attract coconut crabs.

   Children sometimes play with coconut crabs by placing some wet grass at
   an angle on a palm tree that contains a coconut crab. When the animal
   climbs down, it believes the grass is the ground and releases its grip
   on the tree, and subsequently falls.

   The coconut crab is admired for its strength, and it is said that
   villagers use this animal to guard their coconut plantations. A coconut
   crab may attack a person if it is threatened. The coconut crab,
   especially if it is not yet fully grown, is also sold as a pet, for
   example in Tokyo. The cage must be strong enough that the animal cannot
   use its strong claws to escape.
   Retrieved from " http://en.wikipedia.org/wiki/Coconut_crab"
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   with only minor checks and changes (see www.wikipedia.org for details
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