Trill (Star Trek)

The Trill are a fictional species of symbiotic life forms, depicted in the Star Trek media franchise. First introduced in an episode of Star Trek: The Next Generation, the species became a major part of the spin-off series, Star Trek: Deep Space Nine, which featured a Trill named Dax as one of its main characters.

Trill are depicted as comprising a humanoid host, and a worm-shaped symbiote that is implanted in the host's abdomen. Their personalities are a combination of the host and symbiote, with the symbiote's memories providing continuity between hosts. Their home world, also named Trill, is a planet in the Alpha Quadrant, the primary setting of most of the Star Trek series.

Depicted biology
The hosts, as they are called, are humanoids that appear much like humans, except with distinctive black or brown spots visible from the top of the head, down each side of the face and neck, and down across the shoulders and the sides of the chest, thighs, legs to their feet. Their hands are noted for being unusually cold to the touch ("A Man Alone", DS9 Season 1). Trill are strongly allergic to insect bites, because the toxins interfere with the biochemical connections between host and symbiont ("The Siege", DS9 Season 2).

The symbionts are helpless, worm-shaped lifeforms who contain the memories of their previous hosts, and who inhabit the abdomens of the humanoid hosts. When a host and a symbiont are joined, the resulting individual is considered a new being. When a host dies, the symbiont is transplanted into a new host. Ninety-three hours after the joining, the host and symbiont are completely interdependent, but up to then, the joining may be reversed without killing the host. A symbiont who is neither implanted into a new host nor returned to their habitat (pools of nutrient-rich liquid on the Trill homeworld) will quickly die, as will a joined Trill host within one to two days of the symbiont being removed.

In rare cases, Trill symbionts can be joined with non-Trill humanoids, but the differences in biology means the results are often unstable ("The Host", TNG). Commander William Riker was briefly joined to the Odan symbiont so that Odan could complete peace negotiations, and to keep Odan alive until a new Trill host could arrive. While this effort saved Odan's life, it nearly caused Riker's death.

Development of appearance and characterization
In the first appearance of Trill in the TNG episode "The Host", Trill were depicted physically quite differently than their later appearances. Also, they were unable to be safely transported via transporter, a weakness not shown in later appearances when Trill became a regularly used race. No explanation of the differences is ever given onscreen, but according to an article on startrek.com, humanoid Trills are actually composed of at least two races that can be used as hosts for the symbiont. It is also revealed in this episode that humans can also serve as temporary hosts to the symbionts when Commander William Riker hosted Odan's symbiont. The crew's general unfamiliarity with the race is a minor contradiction with later episodes, which stated that the Trill (such as Curzon Dax) had been working with the Federation regularly long before their first appearance in Star Trek: Deep Space Nine; "Trials and Tribble-ations" implied that a host of Dax was romantically involved with Leonard McCoy when the original Star Trek series character was still a college student. Other differences included the change in makeup style from a prosthetic forehead to a series of spots, although some makeup tests and sketches were made in the preproduction of Deep Space Nine using the earlier makeup style, but the actress who would play Jadzia Dax, Terry Farrell, proved allergic to the extensive prostheses, and so the new style was created. The Trill naming style, of using their host name as a first name and their symbiont name as a surname, differs from their first appearance, in which the Trill Odan was only known by that one name.

"The Joining" and Trill culture
It was originally assumed that only a small fraction (0.01%) of the Trill population was capable of being joined, a myth that the Trill government continues to perpetuate. This is because the joining of the Dax symbiont with the host Joran (Curzon's predecessor) was a disaster — Joran was insane and a murderer. Joran's joining with Dax was covered up and all records of it were erased. The Trill government attempted to blame the failure on Joran's unsuitability as a host, but the truth was that Joran had passed the host screening tests and was technically a stable host — as was almost half of the Trill humanoid population. The government dares not let this information become public, lest the symbionts become a commodity to be bought, sold and fought over by the public; as long as it is widely believed that only a small fraction can become hosts, the government reasons, then such widespread hysteria can be avoided ("Equilibrium", DS9).

A joined Trill is known by the given name of the host followed by the name of the symbiont: for example, when Ezri Tigan was joined with the symbiont Dax, her name became Ezri Dax.

Trill society has a taboo against resuming relationships with loved ones from one's past joining, on the principle that each life must be unique ("Rejoined", DS9). This taboo apparently does not extend to non-romantic relationships (or perhaps non-Trills), since Jadzia Dax renewed Curzon Dax's friendship with Benjamin Sisko and others without comment, as Ezri Dax renewed Jadzia's friendship with the crew of Deep Space Nine. This taboo also was not yet developed at the time of the TNG episode "The Host", which featured a romantic relationship between Odan and Dr. Crusher, which Odan wished to continue through several hosts.

Depiction in non-canonical novels
In the non-canon Deep Space Nine post-finale novels (known as the "relaunch"), it is revealed that the Trill symbionts are related to the evil parasitic species seen in the Next Generation episode "Conspiracy". The parasites resulted from genetic tampering upon the symbionts to make them resistant to disease. However, when their creations turned evil, the Trill tried to eradicate them, leading to the parasites' vendetta against the symbionts. The appearance of Odan — Trill hosts who have no spots, but ridges on their foreheads — is also explained (in the novel Forged in Fire, by Andy Mangels and Michael A. Martin), where a strain of the Klingon Augment virus (the disease responsible for the Human-like appearance of Klingons in the Original Series) managed to infect a Trill colony when it was visited by Klingon traders.

Also in the relaunch novels, it is revealed that after a few hundred years, the symbionts become incapable of any further joinings with humanoids and return to the breeding pools and the massive underground lakes that they lead to. There they can live to be many thousands of years old and of enormous size, whereupon they are referred to as the Annuated. The Trill humanoid population at large do not know this, however.

A later novel sees word getting out that almost half of the Trill population was suitable for joining and the joined Trill who had led the planet's culture and society for hundreds of years were no more special than the others. A subsequent terrorist attack by non-joined Trills using an EMP-like bomb kills many symbionts and joined Trills. The planet's leader then announces a moratorium on new joinings and, as a sign of good faith, has her own symbiont taken out using new technology capable of removing it without killing the host.

Joined Trill characters

 * Dax
 * Joran Dax
 * Curzon Dax
 * Jadzia Dax
 * Ezri Dax
 * Lenara Kahn
 * Odan