Coconut crab
The coconut crab is a terrestrial species of giant hermit crab, and is also known as the robber crab or palm thief. It is the largest terrestrial arthropod known, with a weight up to. The distance from the tip of one leg to the tip of another can be as wide as. It is found on islands across the Indian and Pacific Oceans, as far east as the Gambier Islands, Pitcairn Islands, and Caroline Island, and as far west as Zanzibar. While its range broadly shadows the distribution of the coconut palm, the coconut crab has been extirpated from most areas with a significant human population such as mainland Australia and Madagascar.
The coconut crab is the only species of the genus Birgus, and is related to the other terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Juvenile coconut crabs use empty gastropod shells for protection like other hermit crabs, but the adults develop a tough exoskeleton on their abdomens and stop carrying a shell. Coconut crabs have organs known as branchiostegal lungs, which they use for breathing instead of their vestigial gills. After the juvenile stage, they will drown if immersed in water for too long. They have an acute sense of smell, which they use to find potential food sources, and which has developed convergently with that of insects.
Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and the pith of fallen trees, but they eat carrion and other organic matter opportunistically. Anything left unattended on the ground is a potential source of food, which they will investigate and may carry away – thereby getting the alternative name of "robber crab". Despite its name, coconuts are not a significant part of the crab's diet. Although it lives in a burrow, the crab has been filmed climbing coconut and pandanus trees. The crab has never been filmed selectively picking coconut fruit, though they might dislodge ripe fruit that otherwise would fall naturally. When a crab is not near its burrow, climbing is an immediate escape route from predators. Sea birds eat young crabs, and both humans and larger, older crabs eat crabs of all ages.
Mating occurs on dry land, but the females return to the edge of the sea to release their fertilized eggs, and then retreat up the beach. The larvae that hatch are planktonic for 3–4 weeks, before settling to the sea floor, entering a gastropod shell and returning to dry land. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years. In the 3–4 weeks that the larvae remain at sea, their chances of reaching another suitable location are enhanced if a floating life-support system avails itself to them. Examples of the systems that provide such opportunities include floating logs and rafts of marine or terrestrial vegetation. Similarly, floating coconuts can be a very significant part of the crab's dispersal options. Fossils of this crab date back to the Miocene.
Taxonomy
The coconut crab has been known to western scientists since the voyages of Francis Drake around 1580 and William Dampier around 1688. Based on an account by Georg Eberhard Rumphius, who had called the animal "Cancer crumenatus", Carl Linnaeus named the species Cancer latro, from the Latin latro, meaning "robber". The genus Birgus was erected in 1816 by William Elford Leach, containing only Linnaeus' Cancer latro, which was thus renamed Birgus latro.Birgus is classified in the family Coenobitidae, alongside one other genus, Coenobita, which contains terrestrial hermit crabs.
Common names for the species include coconut crab, robber crab, and palm thief, which mirrors the animal's name in other European languages. In Japan, the species is typically referred to as yashigani, meaning 'palm crab'.
Description
B. latro is both the largest living terrestrial arthropod and the largest living terrestrial invertebrate. Reports of its size vary, but most sources give a body length up to, a weight up to, and a leg span more than, with males generally being larger than females. The carapace may reach a length of, and a width up to.The body of the coconut crab, like those of all decapods, is divided into a front section with 10 legs, and an abdomen. The front-most pair of legs has large chelae, with the left being larger than the right. The next two pairs of legs, as with other hermit crabs, are large, powerful, walking legs with pointed tips that allow coconut crabs to climb vertical or even 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 the shell or coconut husks that juveniles habitually 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 in the cavity containing the breathing organs, inside the carapace. Some difference in color occurs between individuals found on different islands, ranging from orange-red to purplish blue, In most regions, blue is the predominant color, but in some places such as the Seychelles, most individuals are red.
Although B. latro is a derived type of hermit crab, only juveniles use salvaged snail shells to protect their soft abdomens, while adolescents sometimes use broken coconut shells for the same purpose. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead harden their abdominal terga by depositing chitin and calcium carbonate. Absent the physical constraint of living within another creature's shell, B. latro grows much larger than its relatives in the family Coenobitidae. Despite being the product of carcinization, like most true crabs B. latro bends its tail beneath its body for protection.
The hardened abdomen protects the coconut crab and reduces water loss on land, but must be periodically moulted. Adults moult annually, digging a burrow up to long in which to hide while their soft shell hardens. Depending on the size of the individual, 1–3 weeks are needed for the exoskeleton to harden. The animals remain in this burrow for 3–16 weeks, again depending on size.
Respiration
Except as larvae, coconut crabs cannot swim, and they drown if left in water for more than an hour. 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 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 the pericardium.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 Diogenidae, they are reduced in size and have comparatively less surface area.
Sense of smell
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. Crabs that live in water have specialized organs called aesthetascs on their antennae to determine both the intensity and the direction of a scent. Coconut crabs live on the land, so the aesthetascs on their antennae are shorter and blunter than those of other crabs and are more similar to those of insects.While insects and the coconut crab originate from different clades, the same need to track smells in the air led to convergent evolution of similar organs. Coconut crabs flick their antennae as insects do to enhance their reception. Their sense of smell can detect interesting odors over large distances. The smells of rotting meat, bananas, and coconuts, all potential food sources, especially catch their attention. The olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.
Lifecycle
Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August. Males have spermatophores and deposit a mass of spermatophores on the abdomens of females; the oviducts 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. The female lays her eggs shortly after mating, 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 migrates to the seashore and releases the larvae into the ocean. The coconut crab takes a large risk while laying the eggs, because it cannot swim. If a coconut crab falls into the water or is swept away, its weight makes swimming back to dry land difficult or impossible. The egg-laying 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 them within a few days.
The larvae float in the pelagic zone of the ocean with other plankton for 3–4 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 postlarval 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 5 years after hatching. They reach their maximum size only after 40–60 years. They grow remarkably slowly, and may take up to 120 years to reach full size, as posited by Michelle Drew of the Max Planck Institute.