The Yeerks are a sentient parasitic alien species that are slug-like and grayish in color. The Yeerks crawl into the brain of another species and control everything the animal does. There are a number of species of which there are many Controllers, including:
- Gedds - a monkey-like species from the Yeerk homeworld.
- Hork-Bajir - a tall, unintelligent species.
- Taxxons - a large cannibalistic species where most are voluntary hosts.
- Sharks - The Yeerks modified the brains of some sharks for this purpose.
There is also only one Andalite-controller. The Yeerk who controls this Andalite is Visser Three, and is lucky because he has the Andalite morphing power.
Yeerks reproduce through fusion-fission. This happens when three parent Yeerks merge into a single entity, which then breaks apart into hundreds of separate grubs. This destroys the three parent Yeerks in the process. It is assumed all Yeerks are asexual because of this; meaning any three, presumebly random, Yeerks can fuse in this way. Despite this, they are usually referred to as "he" or "she", depending on their host's gender -- this usually occurs when the host is either a Human or an Andalite, both of which do exhibit multiple genders.
The Yeerk hail from a world of a dark green and yellow, lightning-torn atmosphere. The land is largely made up of dark rich soil, which is dotted with roughly circular Yeerk pools, which are believed to connect to one another somehow.
Each of these Yeerk pools are named; an example of which is Sulp Niar. These pools contain a sludgy gray liquid, which is filled with the various nutrients the Yeerks require to survive, although this does not include the Kandrona rays.
The sky is also dotted with various satellites; one of these is a moon called Madra.
Only about a hundred animal species continue to exist on this world, including the Gedd, which became the Yeerk's natural host, and eventually this evolved into a nearly symbiotic relationship.
As of September 2006, the Yeerk date was Generation 705, mid-cycle. The next mid-cycle starts sometime in August of 2007.
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