The coconut crab (coconut crab) is a species of decapod crustaceans belonging to the family Coenobitidae. It is the heaviest arthropod  known to exist on land (the crab largest by length is the Japanese giant crab ). The coconut crab is a species of hermit crab , whose characteristic ability is to open coconuts with its strong tweezers to feed on the content.

Synonymy

In English it receives the alternative name of robber crab or palm thief, while in German it is designated as Palmendieb. These names are due to it's penchant for stealing bright objects such as cutlery and metal packaging houses and tents. In fact, the binomial name reflects this conduct.  It is also called "terrestrial hermit crab" for it's habit, in times of molting, using the shell of small animals. This can be confusing because it also applies to other species such as Coenobita variabilis. In different regions it receives local names like "Ayuyu" in Guam , "Unga" or "Kaveu".

Physical Description

Reports on the size of the species differ, although most of the references estimate a weight of more than 4 kg, body length of 40 cm and span legs of about 1 m with males being generally larger than the females. It is estimated that this is the theoretical size limit for a terrestrial arthropod; in the case of marine units where the water helps support the weight, larger sizes are achieved, for example Macrocheira kaempferi.

They have a lifespan between 30 and 60 years, taking into account that the references to it are varied. The body of the Birgus latro or the Coconut crab, like all decapods, is divided into a central section (carapace) with 10 legs and the abdomen.

The first pair of legs is armed with large claws, which are used to break coconuts with which it feeds, and also carry objects up to 29 kg of weight . The next pair of legs, as in all hermit crabs, are powerful legs to walk and climb up the trunks of coconut trees, to reach upto 6 m high. The fourth pair is smaller, with tweezers in the ends used by the young to make the shell or coconut shell as protection; adults use them to walk and jump. The last pair of legs is very small and serves only to clean the breathing organs. These legs are usually carried inside the carapace , in the cavity containing such organs.

Abdomen protection

Although the species is a derived type of hermit crabs , only the juveniles use shells of snail or shells to protect their soft abdomen, and sometimes use coconut shells with the same objective. Unlike other hermits, adult crabs shells do not employ others, but harden their abdominal armor with deposits of calcium and keratin.

Another protection strategy is to double its tail below the abdomen, as most species of crabs. Hardened abdomen protects the animal and reduces moisture loss, but must be changed at regular intervals. Molting takes about 30 days, during which the body of the crab is soft and vulnerable, so it remains hidden to protect itself.

Breathing

In adulthood, coconut crabs can not swim and will drown in the water. Breathing uses a special organ called branchiostegal lung, which can be defined as an intermediate evolutionary stage between gills and lungs, and is one of the adaptations most characteristic of the species to the ecosystem.

Branchiostegal lung is located behind the cephalothorax and contains a biological tissue similar to that found in the gills , but adapted to the absorption of oxygen from air instead of water. The last pair of legs serves as a cleaning tool and to moisten them with seawater, since the respiratory organs require water to function. To this end, the crab frequently strikes its wet paws against the spongy tissues; with the same technique the crab can drink seawater.

In addition to this breathing organ, the species has a rudimentary set of gills. Although it is likely that they were used to breathe underwater in an earlier evolutionary stage, no longer provide enough oxygen, so a crab submerged in water will drown in a short time.

Nose

Another distinguishing organ of the crab is its nose . The process of smell operates differently if molecules is hydrophilic in water or hydrophobic in the atmosphere. As most crabs live in the water, they have specialized organs called aesthetascs on their antennae to determine both the concentration and direction of a smell.

However, as coconut crab lives on soil, aestetas on their antennae differ significantly from those of other crabs, and more closely resemble the olfactory organs of insects, called Sensillum . Although insects and the crab come from different evolutionary patterns, the same need to detect smells in the air led to the development of organs of remarkable similarity in what is a clear example of evolutionary convergence . Like insects, coconut crab rapidly moves their antennae to improve their smell. It has an excellent sense and can detect interesting odors over long distances; for example, the smell of roast meat, bananas and coconuts especially call its attention, as potential food sources.

Reproduction

The coconut crab is paired rapidly and frequently on dry ground from May to September, especially in July and August. Male and female fight until the male manages to overturn the female on his back to mate; the whole process takes about 15 minutes. Soon after, the female lays her eggs and adheres under its abdomen, carrying the fertilized eggs for a few months. Upon hatching, usually October to November, the female leaves the larvae ( zoeas ) in the sea at high tide. It has been documented that females do this all on the same night, saturating the beach animal.

The larvae float in the ocean for 28 days, during which a large number are eaten by predators. Another 28 days remaining they spend in the bottom of the sea and on the beach as hermit crabs, using discarded shells for protection by other animals. In this state, sometimes they venture to the mainland. Young crabs that can not find a shell in disuse use pieces of shell coconut.

After these 28 days it will definitely leave the sea as they lose their ability to breathe in the water. Crabs reach reproductive maturity between 4 and 8 years old, one of the longest periods between the crustaceans.

Food

The coconut crab diet consists mainly of fruits, especially coconuts and figs. it also devours any organic matter, including leaves, rotting fruit, eggs of turtle, dead animals and their shells, that provide calcium. They are also capable of eating animals that are slow to flee, like sea turtles youth. During an experiment it was observed that they devoured Polynesian rats. It often steals food from other animals and takes it to its lair, to stay safe while eating.

The crab climbs trees to eat coconuts or other fruits, and so does to escape the heat or other predators.  The crab perforates holes in the coco with its strong tweezers to eat the content: this conduct is unique in the animal kingdom.

For a long time doubts surrounded the ability of the animal, and in several experiments some specimens in captivity came to die of hunger surrounded by coconuts. However, in the 1980s Rumpf was able to observe and study them in their natural environment, where they open coconuts without difficulty.

The crab has developed a special technique to open them: if the coconut still has its outer cover, it will use tweezers to remove it in strips, starting from the point of germination, a group of three small circles where the coconut attached to the palm tree. Once these pores are visible, the crab strikes one with its pincers until it pierces. Then it turns and uses its small tweezers to extract the pulp of coconut. Larger crabs can also use tweezers to break the biggest coconut into pieces and thus feed more easily.

Habitat

The coconut crab lives in burrows or rock crevices, depending on soil type. They dig their own burrows in sand or soft soil. During the day the animal remains hidden to protect themselves from predators and reduce the loss of moisture from the heat. While resting in its burrow the animal closes the entrance with one of its pincers to achieve the microclimate wet that is needed for their respiratory organs.

In areas with large populations of crabs you can see some during the day, perhaps to take advantage in the search for food. They usually leave their burrows if the day is humid or rainy, since these climatic conditions allow them to breathe easier. They live exclusively in mainland, and they have been sighted more than 6 km inland.


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