Oculi are small creatures which can crawl on walls and generate a damaging electric field in order to deter predators. While active, the Oculus exposes its single eye. However, when it senses a real threat, it retracts into its impenetrable shell, making it difficult to kill.
Oculi are only found in Metroid Prime. Because of their invulnerability, they are used mainly as moving obstacles for the player. For example, while obtaining the Wavebuster, the player must destroy target weak points in stone columns with Missiles. Oculi crawl directly on the blocks, obstructing an otherwise clear shot.