Coconut Crab

General Information

The Birgus latro, or the coconut crab, is the largest terrestrial arthropod found in the Southwest Pacific or Indian oceans. They average one meter in length and weigh about ten pounds. Their coloration ranges from a light purple to a dark brown and purple color. Often, they live alone in burrows they dig themselves in soft sand or under rocks, depending on the terrain. Coconut crabs are in the order Decapoda, meaning they have five pairs of appendages considered ten legs.

The hermit crabs are a common ancestor to this species, but the coconut crab has lost dependence on shells for protection and instead uses a calcified body, removing the size restrictions as seen in hermit crabs.

Reproduction of this species involves marine and terrestrial environments. A mating fight takes plance and after insemination has occurred, females glue the eggs to her body for a few months. At hatching time, females release the eggs into the ocean at high tide, and after an average of thirty-one days, the larvae return to shore living similarly to hermit crabs. Juveniles of the species Birgus latro are similar to the adult hermit crabs, but as they reach the size of about one centimeter, taking another twenty-eight days, they give up their shells for a calcified body, and their gills for air-breathing organs (Fann-Ling Wan).

A unique feature that allows for terrestrial life are air-breathing organs. This air breathing organ is called the branchiostegal lung. These have been identified as an intermediate step between gills and lungs. The organs require water or a moist environment to function, but coconut crabs will drown if submerged in water although rudimentary gills are still present. Another interesting characteristic of these crabs compared to other crabs is their smelling capability. Their smelling organs are more comparative to insects rather than other crabs, and they are remarkable in detecting odors of rotting meat, fruit, and coconuts over extended lengths. (The Editors of Encyclopedia Britannica)

Birgus latro has been an important part of Chinese and Taiwan culture. Due to their size and easiness to capture, residents in the areas inhabited by this species have come to rely on them as a source of animal protein. With the presence of humans in their environment, the population of the coconut crab has begun to drastically decline. These declines have resulted in conservation efforts to save this species.

The common food of these crabs includes coconuts, which is how they obtained their name. They use their chelae or enlarged pincers to crack open coconuts. Another way to crack the coconuts is by dropping them from the trees, tearing strips of the husk from the shell, and beating it with their pincers. Coconut crabs are also known scavengers also feeding on carrion or the shells of other crabs believed to be their source of calcium. Not only are the crabs scavengers, but it has been found that they are predators of Polynesian rats and seabirds (Kessler, Robinet).

Pinching Forces

Due to its large size compared to other crustaceans, assumptions were made that Birgus latro exerts the greatest pinching force. This assumption was correct. Even using females, which tend to be smaller in size, researchers were able to show, using data collected from the coconut crabs, that this species has the second largest pinching force when compared to all other animals. The only other animal in comparison with a larger exerted force is Cancer pagarus also known as the brown crab, which is a strictly marine animal (Oka). The figure below shows the comparison of the force exerted from the pincers compared to the maximum force exerted by all the other animal groups. These forces came from running, jumping, swimming, flight, etc.