Robert Goeren: Evolution Of The Coconut Crab

Evolution of the Coconut Crab

By: Robert Goeren

Evolution is the most common ideology debated of all time. Evolution has brought all creatures to their essential environment today. One species that adapted through evolution that has become the largest land-living arthropod in the world is the coconut crab.
The coconut crab also known as the robber crab or palm thief is a species of terrestrial hermit crab, which is at the upper limit for terrestrial animals. Weighing up to nine pounds it can grow up to three and a half feet from leg to leg. The crabs got a nickname for being the robber crab for climbing into coconut trees or other fruit trees and finding fruits to fill their large structure. Coconut crabs are found
on islands across the Indian Ocean and parts of the pacific Ocean as far east as the Gabier Island. The crabs adapted to its environment allowing it to grow as a population. Each year more crabs are being killed off by humans for their meat. These crustaceans meat is considered a delicacy in varies islands that they habitat. There is some difference in color between the animals found on different islands ranging from orange-red to purplish blue. These animals like all land crabs spend most of their time one the land. Mating occurs on dry land, but the females migrate to the sea to release their fertilized 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.
The coconut crab is the only species of genus Birgus.
Known as Birgus latro it is a derived type of hermit crab. Only young crabs use snails shell to protect their soft abdomens, adolescents sometimes are seen using broken coconut shells to protect their abdomens, hence the name coconut crab. Unlike all other hermit crabs, the adult crabs do not carry a shell but instead harden their abdominal terga by depositing chitin and chalk. Not being constrained by the physical confines of living in a shell allows the species to grow much larger than other hermit crabs in the family Coenobitidae. Like most crabs, the coconut crab bends its tail underneath its body for protection. The hardened abdomen protects the crab and reduces water loss on land. These animals are true crabs that shed their exoskeleton annually. The adults dig a burrow up to 1m long to hide while they are vulnerable in the process of moulting (shedding). It remains in the burrow for three to sixteen weeks, depending on the size of the animal. After it is done it takes one to three weeks for the exoskeleton to
harden. The body is divided into a front section, which has ten legs and an abdomen.
The front most pair of legs has large claws, with the left claw larger than the right. The next two pairs of legs are large and powerful walking legs with pointed tips. These are the legs that allow the crab to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezers- like chalae at the end allowing young coconut crabs to grip the inside of a shell or coconut hust to carry for protection. Adults use this pair of legs for walking and climbing. The last pair of legs is small and used by females to tend theirs 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. Coconut crabs have evolved organs known as “bryanchiostegal lungs”, which are used instead of the vestigial gills for breathing. They cannot swim and will drown if immersed in water for long. This organ can be interpreted as a developmental state between gills and lungs. It 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. 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. They have developed an acute sense of smell, which has evolved convergent with that of insects and which they use to find potential food sources. Coconut crabs flick their antennae as insects do to enhance their reception. They have an excellent sense of smell and can detect odors over large distances. The smells of rotting meat, bananas, and coconutsl 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. Adult coconut crabs feed on fruits, nuts, seeds and the pith of fallen trees, but will eat carrion and other organic matter. This species is popularly associated with the coconut and has widely reported to climb trees to pick coconuts, which it then opens to eat. 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.
Coconut crabs are unique creatures that have adapted through evolution to survive on land as a crustacean. Humans have grown side with these crabs although the constant hunt for them could intern extinct the animal. Their adaptions are crucial to the type species they have become on our earth.

Work Cited
I.W. Brown & D. R. Fielder, ed. (1991). The Coconut Crab: Aspects of the Biology and Ecology of Birgus latro in the Republic of Vanuatu. ACIAR Monograph 8. Canberra, Australia: Australian Centre for International Agricultural Research. ISBN 978-1-86320-054-7. Available as PDF: pp. i–128
Johnson, Stevens. The Coconut Crab. March 5th 2003. Web. April 13th 2013. Coconutcrab.org