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
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
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