The Ceti eel utilized a creature known to Khan Noonien Singh and his followers as the "Ceti bison" as a host for its young. The young would wrap themselves around the animal's cerebral cortex which would result in changes in the bison's behavior as the eel grew in size. At times the Ceti bison could appear inactive, while at other times, they seemed to stagger and move about in an odd, almost drunken, manner. Fortunately, as the two creatures had evolved on the same planet, the Ceti bison generally did not suffer any ill-effects from acting as host, although if the eel failed to vacate its host in a sufficiently timely manner the host would soon die from massive cerebral damage. Adult eels could inflict a nasty, but generally not dangerous bite on humanoids, though a large swarm of adults could kill and eat a person in short order. (TOS novel: To Reign in Hell: The Exile of Khan Noonien Singh)
When Ceti Alpha VI exploded in 2268, shifting the orbit of Ceti Alpha V, the planet went from being class M to class K; all life on the planet died except for the Augment colonists and the Ceti eel. In fact, the eel survived by turning to the colonists to serve as new hosts. Several of Khan's followers, including his wife, Marla McGivers, died after being infected with the eel.
In humans, the eel rendered its victims highly suggestable. Unfortunately, as the eel grew in size, individuals would begin to experience fits of madness and eventual death.
In 2285, when the USS Reliant visited Ceti Alpha V seeking a suitable planet on which to deploy the Genesis Device, Captain Clark Terrell and Commander Pavel Chekov beamed down to the surface to check some unusual readings. The two men came across the Botany Bay colony and were subsequently captured by Khan who had eels placed into their ears to make them more suggestable and reveal why they had come to the planet. Khan also used eels to control the Reliant engineering crew when he took control of the ship. (TOS movie, novelization & comic adaptation: The Wrath of Khan; Decipher RPG module: Worlds)
By 2287 the numbers of the species had gone into a terminal decline due to a combination of the planet's increasingly inhospitable climate, the few remaining Augments having grown adept at shielding themselves from the eels, and the lack of any other species to use as hosts. After the last Augments were removed from the planet by Captain James T. Kirk, the species likely became extinct. (TOS novel: To Reign in Hell: The Exile of Khan Noonien Singh)
The Ceti eel could also be found on the surface of the mirror Ceti Alpha V. When the Klingon-Cardassian Alliance attempted to establish a prison on the planet, several prisoners, guards and the warden died before the discovery of the Ceti eel. Although Alliance officials claimed to have exterminated the creature, it could often be found as an item for sale within the black market. Moreover, the eel was sometimes used by members of the Terran Rebellion when attempting to garner information from the Alliance and its personnel. (TNG novel: The Worst of Both Worlds)