- "In their natural state, I bet they're pretty stupid. Tiny little brains. But when they land on a planet, they find some smart creatures and hijack their brains for a while - get them to do some of the thinking for them. They hive off some thinking, some processing, into - well, say, otters, or people or whatever they can - and then, later, the otters or people go back to the slimeys, upload the results of all that thinking, and the slimeys repeat it again with other otters."
- ―The Doctor, Wetworld
The Swamp Creature (a.k.a. Slime Creature or "Slimey" as it is referred to by the Tenth Doctor) is an unusual life form which is able to use the neural activity of other creatures to compensate for its own limited brain power.
In its adult form, the creature is a huge amorphous mass of flesh, which remains submerged in water and uses its numerous long tentacles to interact with the world above the surface. Its nervous system is diffuse, making it a slow and inefficient thinker on its own. However, it has developed the ability to inject other species with strains of RNA containing information, and eventually uses the brains of other species for neural processing.
The victims of the Swamp Creature soon develop strange red dots on their skin. These dots contain genetic information in the form of RNA, which has been injected into them by the monster, and which synthesizes proteins that act on the victim's brain. These Proteins will tell the brain to be suppressing intelligence and stimulating aggression basically. They are able to encode specific instructions in the proteins, ordering their victims to accomplish specific tasks. They are also able to absorb the victim's knowledge upon contact.
The creature reproduces by regeneration and feeds on practically anything organic. Once it arrives on a new planet it will immediately enter in contact with the brains of the most intelligent local species, thus benefiting from first-hand knowledge about the environment it finds itself in. Eventually, the Swamp Creature grows larger and larger, controlling the minds of the local life forms and neutralizing any potential predators. If allowed to continue, it could grow large enough to cover the entire surface of the worlds it conquests. Before reaching that point, however, its prime concern is to reproduce and spread itself into new worlds, which may be accomplished by blasting itself apart in a nuclear explosion.
Naturally, the creature can survive through extremes of radiation and temperature. It can drift for decades in the vacuum of space and still remain alive; be blasted to pieces and have each piece regenerate into a whole new one. When the time comes for the creature to spread itself, it will seek the mind of some being intelligent enough to instruct it in how to build a nuclear bomb, and order its mind-controlled servants to do so. The resulting explosion will send off pieces of the Swamp Creature to orbit, where they may eventually reach out a new inhabited planet and perpetuate the cycle.
- Wetworld, by Mark Michalowski (2007)