The coconut crab, commonly known as the robber crab or palm thief, is a kind of terrestrial hermit crab. It is the world’s biggest terrestrial arthropod, weighing up to 4.1 kg (9 lb). From the tip of one leg to the tip of the other, it may reach a width of up to 1 m (3 ft 3 in). It has been extirpated from most locations with a major human population, including mainland Australia and Madagascar, and sections of the Pacific Ocean as far east as the Gambier Islands and the Pitcairn Islands. Coconut crabs may also be seen near Zanzibar on the African coast.
The coconut crab
- is the sole species in the genus Birgus, and it is linked to the Coenobita group of terrestrial hermit crabs. It demonstrates a variety of adaptations to land life. Coconut crabs, like other hermit crabs, utilize empty snail shells for protection, but adults grow a robust exoskeleton on their abdomens and quit carrying a shell. Coconut crabs have branchiostegal lungs, which they use instead of their vestigial gills for breathing. If they are submerged in water for too long after the juvenile stage, they will drown. They have a keen sense of smell that they employ to locate prospective food sources and that has evolved in tandem with insect senses.
Adult coconut crabs devour fleshy fruits, nuts, seeds, and the pith of fallen trees as a main source of food, but they will also eat carrion and other organic debris when the opportunity arises. Anything left on the ground that is abandoned is a possible source of food, which they will explore and maybe carry away, earning them the nickname “robber crab.”
Even though the species is commonly connected with the coconut palm, it does not eat coconuts. The crab, although living in a burrow, has been seen climbing coconut and pandanus trees. Though they may dislodge ripe fruit that would otherwise fall spontaneously, no footage shows a crab choosing coconut fruit carefully. Climbing provides a fast escape path from huge marine birds.
Females mate on dry land, then return to the sea’s edge to discharge their fertilized eggs before retreating up the beach. For 3–4 weeks, the larvae are planktonic before settling on the seafloor, entering a gastropod shell, and returning to dry land. After roughly 5 years, sexual maturity is reached, and the overall life can exceed 60 years.
If a floating life support system is available to the larvae throughout their 3–4 weeks at sea, their chances of reaching another appropriate place are increased. Floating logs and rafts of marine or terrestrial plants are examples of systems that give such chances.
Western scientists have known about the coconut crab since the travels of Francis Drake in 1580 and William Dampier in 1688. Carl Linnaeus (1767) named the species Cancer later, from the Latin, which means “robber,” after a report by Georg Eberhard Rumphius (1705), who termed the animal “Cancer crumenatus.” In 1816, William Elford Leach created the genus Birgus, which contained just Linnaeus’ Cancer later, which was renamed Birgus latro.
Birgus is part of the Coenobitidae family, which also includes Coenobita, the genus that includes terrestrial hermit crabs.
The coconut crab, robber crab, and palm thief are common names for the species, which are all variations of the animal’s name in various European languages (e.g. German: Palmendieb). The species is known in Japan as Yashigani, which means “palm crab,” since it inhabits several of the country’s southern island systems.
Coconut crabs, except as larvae, are unable to swim and will perish if left in water for more than an hour. They breathe with a branchiostegal lung, which is a unique organ. This organ is a developmental step between the gills and the lungs, and it is one of the coconut crab’s most important adaptations to its environment.
The branchiostegal lung has tissue that is similar to that found in gills, but it is designed to absorb oxygen from the air rather than water. The surface area of this organ is increased by expanding it laterally and evaginating it; it is advantageously situated in the cephalothorax to lower both the blood/gas diffusion distance and the return distance of oxygenated blood to the pericardium.
clean and wets their respiratory organs with their hindmost, tiniest pair of legs. The coconut crab does this by rubbing its moist legs over the spongy tissues surrounding it, which allows the organs to work normally. Coconut crabs can move water from their chelipeds to their maxillipeds and sip it from little puddles.
Crabs possess a rudimentary set of gills in addition to the branchiostegal lung. Although these gills are similar in number to those seen in aquatic animals from the Paguridae and Diogenidae families, they are smaller and have a smaller surface area.
Sense of smell
They have an acute sense of smell, which it uses to seek food. Depending on whether the scented molecules are hydrophilic in water or hydrophobic in the air, the smelling process works quite differently. Crabs that live in water have specialized organs on their antennae called aesthetascs that help them estimate the density and direction of a smell. Because coconut crabs dwell on land, their aesthetascs are shorter and blunter than those of other crabs, and they resemble those of insects.
While insects and coconut crabs evolved in distinct ways, the necessity to monitor odors in the air led to the creation of strikingly similar organs. Coconut crabs use their antennas in the same way that insects do to improve their reception. Over long distances, their sense of smell may discover fascinating scents. They are particularly attracted to the aromas of decomposing flesh, bananas, and coconuts, all of which are possible food sources. In comparison to other parts of the brain, the coconut crab’s olfactory system is well-developed.
Between May and September, coconut crabs marry often and swiftly on dry land, notably between early June and late August. The oviducts open near the base of the third pereiopods, and fertilization is considered to occur on the external surface of the abdomen when the eggs pass through the spermatophore mass. Eggs are extruded on land in cracks or burrows near the coast. Shortly after mating, the female deposits her eggs on the underside of her belly and glues them there for a few months, transporting the fertilized eggs below her body.
The mother coconut crab migrates to the seaside after the larvae hatch and discharges them into the ocean. Because coconut crabs can’t swim, they incur a big risk when they deposit their eggs. If a coconut crab falls into the ocean or is washed away, its weight makes swimming back to dry ground difficult, if not impossible. The egg-laying frequently occurs at dusk on rocky coasts, especially when high tide is present. After the larvae have been discharged, the empty egg cases remain on the female’s body, which she consumes after a few days.
For 3–4 weeks, the larvae float in the pelagic zone of the ocean with other plankton, where they are consumed by predators in enormous numbers. Before molting into the postlarval glaucophane stage, the larvae go through three to five zoea stages, which take 25 to 33 days. They settle to the bottom, discover and wear a suitable-sized gastropod shell, then travel to the beach with other terrestrial hermit crabs after they reach the glaucophane stage of growth. They go on dry land on occasion during such time. They then permanently abandon the ocean and lose their capacity to breathe in water.
They alter their shells as they develop, as do all hermit crabs. When young coconut crabs can’t find a suitable seashell, they typically rely on shattered coconut chunks. They acquire a tough abdomen after they outgrow their shells. Around 5 years after hatching, the coconut crab achieves sexual maturity. After 40–60 years, they attain their maximum size. According to ecologist Michelle Drew of the Max Planck Institute, it develops extremely slowly, taking up to 120 years to reach full size.