According to humans the purple marsh crab is an Earth evolved, carbon based life form, of the Kingdom Animalia. Sadly for Earth, humans are wrong. the "crabs" are not actually crabs, but small alien drones from Gliese 581 d. These aliens have had an over population crisis on their home planet and are looking for another habitable planet to bring the queen. Unfortunately humans think not enough predators are eating the purple marsh crabs, well in truth the queen is simply sending more drones. Once the number of crabs is uncontrollable, they will go through a sort of transformation much like that of a pupating insect. After three days the drones will emerge. They will begin to exterminate dangerous life forms and keep the rest as food. The queen will then land and start laying eggs. This could be prevented by exterminating every last purple marsh crab, but humans are simply to ignorant to believe this passage.