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 land-living arthropod in the world, and is probably at the upper size limit of terrestrial animals with exoskeletons in recent Earth atmosphere at a weight of up to 4.1 kg (9.0 lb). It is found on islands across the Indian Ocean and parts of the Pacific Ocean as far east as the Gambier Islands, mirroring the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar.

The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomen and stop carrying a shell. Coconut crabs have evolved organs known as "branchiostegal lungs", which are used instead of the vestigial gills for breathing. They cannot swim, and will drown if immersed in water for long. They have developed an acute sense of smell, which has evolved convergently with that of insects, and which they use to find potential food sources. Mating occurs on dry land, but the females migrate to the sea to release their fertilised eggs as they hatch. The larvae are planktonic for 3–4 weeks, before settling to the sea floor and entering a gastropod shell. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years.

Adult coconut crabs feed on fruits, nuts, seeds, and the pith of fallen trees, but will eat carrion and other organic matter opportunistically. The species is popularly associated with the coconut, and has been widely reported to climb trees to pick coconuts, which it then opens to eat the flesh. While coconut crabs can climb trees, and can eventually open a coconut collectively, coconuts are not a significant part of their diet. Coconut crabs are hunted wherever they come into contact with people, and are subject to legal protection in some areas. In the absence of precise information, the IUCN lists the species as Data Deficient.

Description

Birgus latro is the largest land-living arthropod in the world; reports about the size of Birgus latro vary, but most sources give a body length of up to 40 cm (16 in), a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft), with males generally being larger than females. The carapace may reach a length of 78 mm (3.1 in), and a width of up to 200 mm (7.9 in).

The body of the coconut crab is, like that of all decapods, divided into a front section (cephalothorax), which has 10 legs, and anabdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right. The next two pairs, as with other hermit crabs, are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young coconut crabs to grip the inside of a shell or coconut husk to carry for protection; adults use this pair for walking and climbing. The last pair of legs is very small and is used by females to tend their eggs, and by the males in mating. This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. There is some difference in colour between the animals found on different islands, ranging from orange-red to purplish blue; in most regions, blue is the predominant colour, but in some places, including the Seychelles, most individuals are red.

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 terga by depositing chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other hermit crabs in the family Coenobitidae. Like most true crabs, B. latro bends its tail underneath its body for protection. The hardened abdomen protects the coconut crab and reduces water loss on land, but has to be moulted periodically. Adults moult annually, and dig a burrow up to 1 m (3 ft 3 in) long in which to hide while vulnerableIt remains in the burrow for 3 to 16 weeks, depending on the size of the animal. After moulting, it takes 1 to 3 weeks for the exoskeleton to harden, depending on the animal's size, during which time the animal's body is soft and vulnerable, and it stays hidden for protection.

Respiration

Except as larvae, coconut crabs cannot swim, and they will drown if left in water for more than an hour. They use a special organ called abranchiostegal 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 branchiostegal lung contains a tissue similar to that found in gills, but suited to the absorption of oxygen from air, rather than water. This organ is expanded laterally and is evaginated to increase the surface area; located in the cephalothorax, it is optimally placed to reduce both the blood/gas diffusion distance and the return distance of oxygenated blood to thepericardium. Coconut crabs use their hindmost, smallest pair of legs to clean these breathing organs and to moisten them with water. The organs require water to properly function, and the coconut crab provides this by stroking its wet legs over the spongy tissues nearby. Coconut crabs may drink water from small puddles by transferring it from their chelipeds to their maxillipeds.

In addition to the branchiostegal lung, the coconut crab has an additional rudimentary set of gills. Although these gills are comparable in number to aquatic species from the families Paguridae and the Diogenidae, they are reduced in size and have comparatively less surface area.

              The coconut crab has a well developed sense of smell, which it uses to locate its food. The process of smelling works very differently depending on whether the smelled molecules are hydrophilic molecules in water or hydrophobic molecules in air. As most crabs live in the water, they have specialised 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 are shorter and blunter than those of other crabs and look more like those ofinsects. 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 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 smells of rotting meat, bananas, and coconuts, all potential food sources, catch their attention especially. Research has shown that the olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.

Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August. Male coconut crabs havespermatophores and deposit a mass of spermatophores on the abdomen of the female; the abdomen opens at the base of the third pereiopods, and fertilisation is thought to occur on the external surface of the abdomen as the eggs pass through the spermatophore mass. The extrusion of eggs occurs on land in crevices or burrows near the shore. Shortly thereafter, the female lays her eggs and glues them to the underside of her abdomen, carrying the fertilised eggs underneath her body for a few months. At the time of hatching, the female coconut crab releases the eggs into the ocean. This usually takes place on rocky shores at dusk, especially when this coincides with high tide. The empty egg cases remain on the female's body after the larvae have been released, and the female eats the egg cases within a few days.

           The larvae float in the pelagic zone of the ocean with other plankton for three to four weeks, during which a large number of them are eaten by predators. The larvae pass through three to five zoea stages before moulting into the post-larval glaucothoe stage; this process takes from 25 to 33 days Upon reaching the glaucothoe stage of development, they settle to the bottom, find and wear a suitably sized gastropod shell, and migrate to the shoreline with other terrestrial hermit crabs. At that time, they sometimes visit dry land. Afterwards, they leave the ocean permanently and lose the ability to breathe in water. As with all hermit crabs, they change their shells as they grow. Young coconut crabs that cannot find a seashell of the right size often use broken coconut pieces. When they outgrow their shells, they develop a hardened abdomen. The coconut crab reaches sexual maturity around five years after hatching.  They reach their maximum size only after 40 to 60 years.


credis: http://en.wikipedia.org/wiki/Coconut_crab