The Ceti eel was a burrowing desert animal native to the planet Ceti Alpha V, capable of surviving extremes in its environment. The Ceti eel was the only known indigenous creature that survived the orbital shift of Ceti Alpha V following the explosion of Ceti Alpha VI.
Ceti eels incubate their larvae within the plates of their jointed carapace. Upon emergence, the eel larvae can enter the ear of a larger animal, where it wraps itself around the cerebral cortex. This causes the host extreme pain and renders them extremely susceptible to outside suggestion. Over time, as the larva matures, the subject suffers from madness and eventual death.
In 2285, Khan used Ceti eels to gain the cooperation of Captain Clark Terrell and Commander Pavel Chekov of the USS Reliant. However, both men fought against the eels' control of them; Terrell even committed suicide (vaporizing himself with his phaser) rather than execute Admiral James T. Kirk as Khan had ordered him to do. (Star Trek II: The Wrath of Khan) After Terrell's death, Chekov collapsed in agony, whereupon the eel that had infected him - possibly sensing the death of its 'brother' - crawled out of Chekov's ear and fell to the ground, where it was quickly vaporized by Kirk.
- As part of its campaign to protect its exclusive right to market Star Trek-related items, Paramount sought and obtained a design patent from the US Patent and Trademark Office for the Ceti eel. Called an "ornamental design for a toy animal", it was issued in 1984.
- The name "ceti eel" never appears during the movie. Khan describes the animal without naming it, and McCoy doesn't recognize it when it exits Chekov's ear, though Chekov had told Kirk and McCoy of it when they were on Regula I.
- In Evan Carlos Somers's original draft for DS9: "Melora", Fallit Kot (then named Megsy Del) was to kill Quark by using a Ceti eel but the idea was later abandoned in rewrites. (The Official Star Trek: Deep Space Nine Magazine, p. 62)
- Roberto Orci and Alex Kurtzman, screenwriters of Star Trek, created the Centaurian slug as a homage to the Ceti eel.