Speculative Biology Sunday: Gympie Crab

Birgus hortusportens – Gympie Crab

The Gympie Crab was actually created for a competition being held on the speculative evolution forum which I regularly partake in since I started doing these. The competition in question to create the best symbiosis between two organisms. Instead of going for the beyond clichéd plant-feeds-animal, animal-protects-plant, I thought I would do a little bit of role-reversal.

The gympie crabs modern ancestor, the coconut crab, is a terrestrial hermit crab (but carries no shell as adults) which is the largest land arthropod in the world living in tropical islands of the world, mostly in the pacific. Human populations on the islands which they live throughout the world have significant negative effects on the crabs due to habitat loss and predation from people, rats and pigs. The Gympie Gympie which the Gympie crabs carry are completely identical genetically to modern Gympie Gympie, except for a slight founder effect meaning that they are generally smaller and reproduce earlier in their life history. It is a shrub native growing one to two metres tall and native to Australasia. The plants are covered with silica tipped hairs filled with powerful toxins, which commonly kills dogs and horses, and has claimed at least one human life.

At some point in the 21st century Gympie Gympie was accidentally introduced to a large pacific island inhabited by coconut crabs and humans and a mutualistic symbiosis evolved between the crab and the plant. The crab receives protection against predators as its thick exoskeleton protects it from the stinging hairs, shade allowing it to forage in the open during the day, and fruit when the plant produces it. The plant gains from this relationship because plants are always taken from shaded positions where the crab can hide while changing over his dirtball and give the plant plenty of time out in full sun where it can prosper and is also fed and watered by the excrement of the crab, and the seeds of any one plant are distributed over a wider area than if it was growing in the ground.

The crabs hold a ball of dirt between its abdomen and its cephalothorax where the roots of the plant grow, and the stem grows upwards out from the side of the crabs body. Should the shrub grow too tall the crabs will drop the plant and either harvest a new one or tear the top of the plant off, encouraging more lateral growth which is better for the crab.

The Gympie Crab as a seperate species from Coconut Crab is hotly contested as they seem to be able to produce viable hybrids. However most of the areas where Gympie Crabs are found, Coconut Crabs are not, as they have been preferentially preyed upon to the point of extinction of the ancestral form. It has also been reported that Gympie Crabs will preferentially breed with other Gympie Crabs, reinforcing the behaviour in their offspring.

TYFYT