Alien (Apollo 18)

These arthropodic creatures are a deadly species native to Earth's moon. In a resting state they appear indistinguishable from rocks; they also carry a powerful venom which induces psychosis and violent aggression in humans.

Behavior and Anatomy
When not moving, the carapaces of these crab-like creatures disguise them as rocks. When mobile or hunting, they utilize six spidery legs and two large pincers by extending them through this outer shell. Sizes vary from small boulders to stones big enough to fit in one's hand. This mechanism is curious, as it implies that the creatures adapted to hide from something. However, there is no other life on the moon to speak of, nor is there in our solar system as we know. There is the possibility this camouflage is to aid in hunting prey, but they same issue remains: there is no other life on Earth's moon to hunt. This perhaps implies that they did not originate on Earth's moon.

The creatures carry a highly potent venom which they inject when biting. Symptoms of this poison include: blackening of the veins around the bite area, bloodshot eyes, antisocial behavior and finally aggression and extreme psychosis. It is unknown but highly likely that the venom is fatal, as there are only two known victims of it; one died when his space helmet was broken, the other was killed by the creatures. Inconclusive, but the rabid behavior induced by the venom indicates that it was likely a neurotoxin and would have likely caused brain and vital organ damage after prolonged exposure.

In the light these creatures remain dormant; this allows for the possibility that they gain energy through photosynthesis. This would make sense, considering the lack of resources the moon has to support life this large. They will not move or squirm even when being handled and relocated; a few were brought onto the lander as 'samples' and didn't shed their disguise until significantly later. When the sun vanishes, however, they become active. The freezing temperatures of the moon at night likely inspire the arthropods to seek warmer spots.

Smaller (perhaps juvenile) members of the species have been shown to burrow through flesh and become parasitic. It is interesting to note that this particular method of parasitic relationship is exceptionally crude and implies that the creatures do not have much experience attacking warm-blooded prey. It raises the question of whether or not the creatures had been on the moon from the start or if they were brought there by the other cosmonauts whose bodies were found scattered across the lunar surface.