|Number of Limbs||
|Behind the scenes|
- "Yeerks are parasites. A Yeerk enters a host through the ear canal, flattens itself out on the brain, and takes over completely. The host creature can't scratch an itch unless the Yeerk wants it to. We call a being who has been taken over that way a Controller. You must be thinking the Yeerks are pure evil. But let me tell you what it's like to be a Yeerk who isn't in a host. Yeerks are basically gray slugs. No hands, no legs, no eyes, no ears. If a Yeerk wants to be free, free to really move, free to see the beauty of the world around it, free to hear music or even the sound of rain on leaves, if a Yeerk wants that, it has to have a host. If a Yeerk wants to be free, it has to make another living creature a slave. Not an easy choice, is it?"
The Yeerks are a parasitic species of slug-like creatures who require host bodies to see, walk and verbally communicate. They have made it their mission to search the galaxy and enslave other species. They are the main antagonists of the Animorphs series.
In their natural state, Yeerks resemble large slugs, and live in lakes of sludge known as Yeerk pools. Yeerks have the ability to enter another being's body through the ear canal and take over all brain functions, including basic motor and speech control, essentially "becoming" that person. Host infested by Yeerks are known as Controllers.
Every three Earth days (or 72 hours), Yeerks need to leave their hosts' body to enter the Yeerk pool and absorb Kandrona. Kandrona is naturally produced by the sun on the Yeerk homeworld, but Yeerks have developed artificial Kandrona Ray sources (which can be large and serve a large area, or small and portable) which allow them to travel away from their planet. Without Kandrona, a Yeerk will die.
Yeerks are genderless, but are generally referred to by the gender of their primary host, or the host gender they most frequently identify as. For example, Edriss 562 and Aftran 942 are referred to as female because their primary hosts were female.
- "In their natural state, Yeerks are slugs who live in a liquid pool and absorb Kandrona rays for food. But evolution played an interesting trick with the Yeerks. Slowly, over the course of millennia, they grew to be a parasite species. They found the Gedds, another species on the Yeerk home world. And over time they learned to penetrate into the very brain of the Gedds."
The Yeerks were sentient, highly intelligent, and able to learn quickly. However, they were very limited by their Gedd hosts and their need for Kandrona.
Prince Seerow led the very first Andalite expedition to the Yeerk homeworld. He pitied the Yeerks and gave them access to Andalite technology. He founded the Andalite-Yeerk Peace and Cooperation Center, and taught the Yeerks writing and astronomy.
For a brief time, the Yeerks and Andalites lived harmoniously together. This peace was broken when a group of Yeerks rebelled, attacked the Andalite-Yeerk Peace and Cooperation Center, killed several Andalites and stole a few Andalite ships. From there, the Yeerks went on a galaxy-wide conquest, fighting Andalites as they went. Afterwards, Alloran-Semitur-Corrass first created the phrase Seerow's Kindness, which was from then on taught as a lesson to young andalites as to why they should never give their technology to other species.
- "The Yeerks were parasites. In their natural state they were just big slugs who lived in a sludgy pond called a Yeerk pool. But the Yeerks have the power to take over other bodies. They have enslaved many races throughout the galaxy – the Taxxons, the Hork-Bajir, and others. And now they had come to Earth, looking for more bodies to control."
Within the last decades, humans were brought to the attention of Edriss 562. She was intrigued by the sheer numbers of potential hosts with little or no defenses. She proceeded to bring Yeerk troops in secret to Earth. For her work she was awarded the office of Visser One. The secret invasion of Earth continued under the supervision of Visser Three.
The Andalites were aware of the Yeerks' invasion of Earth, but did not realise the scale of the invasion. One Andalite Dome ship, the GalaxyTree, was sent to Earth to investigate. Unfortunately, it was quickly shot down and all but one of the crew died. As Prince Elfangor's dying act, he gave the power to morph to five human teenagers and charged them with the defense of Earth.
The Animorphs were successful in delaying the Yeerks' invasion on Earth. The were also influential in stopping the Yeerks' invasion on Leera and multiple Yeerk genetics experiments to produce Controllers.
At the end of the war, like the Taxxons, some Yeerks receive the morphing power to become nothlits, as part of the peace agreement between the Andalites and the Animorphs. The number of Yeerks who received such, the existence of other Yeerk trials like Esplin 9466's, or the fate of the Yeerks who were infesting the numerous freed troops of Hork-Bajir and humans are not precisely stated. Furthermore, in the "two years later" chapter of The Beginning, Ax mentions the remaining blockade around the Yeerk homeworld which most Andalite military officers who weren't demobilized are sent to enforce.
The Yeerks run their civilization as an empire, ruled by a Council of Thirteen. Under the Council of Thirteen are numerous Vissers and Sub-Vissers who work primarily independent of each other and often run into petty squabbles. Cunning is highly prized and rewarded among Yeerk military.
Classes of Host Species
How the Yeerks classify other species.
- Physically unfit for infestation (e.g. the Skrit Na and Hawjabran)
- Capable of infestation, but suffer from severe drawbacks (e.g. the Gedds and Taxxons)
- Physically fit for infestation, but are few in number and can't be bred quickly (e.g. the Hork-Bajirs)
- Perfectly fit for infestation, but are too great a challenge for invasion (e.g. the Andalites)
- Perfectly fit for infestation, are large in numbers and able to breed quickly, and can't resist the Yeerks (e.g. the Humans)
The species used as hosts.
- Arn - Class 1 (Were possibly either class 3, 4 or even 5, but modified themselves so that when a Yeerk attempted to enslave an Arn, the Arn would die.)
- Skrit Na - Class 1
- Hawjabran - Class 1
- Gedd - Class 2
- Ongachic - Class 3
- Hork-Bajir - Class 3
- Taxxon - Class 2
- Andalite - Class 4
- Leeran - Class 3
- Human - Class 4 (Originally Class 5, but after having found humans extremely resilient they were moved to Class 4.)
- Garad - Class unknown
- Nahara - Class unknown
- Mak - Class unknown
- Ssstram - Class unknown
- Garatron - Class unknown
- Aftran 942
- Akdor 1154
- Carger 7901
- Derane 344
- Edriss 562
- Efflit 1318
- Eslin 359
- Essak Twenty-Four-Twelve-Seven-Five
- Essam 293
- Esplin 9466
- Esplin 9466 Lesser
- Estril 731
- Iniss 226
- Janath 492
- Odret 177
- Temrash 114
- Visser Two
- Visser Four