Trill

Trills are a sapient, spacefaring humanoid species from the planet of the same name, and share their homeworld with a sapient species of worm-like creatures known as the Symbionts. A small percentage of Trills are joined to and act as hosts to Symbionts.

Biology
Trills are a humanoid species, and have numerous spots running on each side of their bodies. Compared to Klingons, Trills have a greater tolerance for cold and a lesser tolerance for heat. They're also known for having cold hands: a biological peculiarity that they themselves acknowledge.

Their lifespan seems to be comparable to, or slightly longer than Humans, with some individuals reaching over 100 years of age. However, Trills don't live nearly as long as their Symbionts, which live for several centuries and inhabit a succession of different Trill hosts over their lifespans.

According to Jadzia Dax, Trills are strongly allergic to insect bites, although the justification she gives is that the biochemical connections between host and Symbiont can't tolerate the insects' toxins, suggesting that only joined Trills may have this vulnerability.

Some unjoined Trills, known as Guardians, have telepathic abilities which allow them to connect with the Symbionts, even though they aren't host to any. A Trill Guardian's psychic abilities allow them to perform a ritual known as zhian'tara, temporarily transferring the personalities of previous hosts into other people, so that the current host can interact with them. The similarity with the Vulcan ritual of fal-tor-pan indicates that Trill Guardians' telepathic abilities are comparable to those of Vulcans.

Joined Trills
Nearly 50% of all Trills are biologically capable of hosting Symbionts. However, the Symbiosis Commission on their planet intentionally perpetuates the myth that only one in every thousand Trills could become a suitable host, in order to prevent disputes, since the number of available Symbionts is in fact several times smaller than the number of potential hosts. If biological rejection is to take place, it will take between three and four days, and both the host and the Symbiont will die.

Once a Trill host is joined to a Symbiont, their bodies are linked in such a way that, after an initial period of 93 hours, removal of the Symbiont would be fatal for the host. The Symbiont can survive by being placed into another host, and the host can avoid death by having the Symbiont restored in time. It's not known whether joining with another Symbiont would also work to save the host's life in such situation.

After joining, the Symbiont's brain and the host's brain are interconnected, and their minds are merged to form a new personality, distinct from its individual components, which no longer operate separately. Since the Symbionts live for centuries and accumulate memories from multiple hosts, new hosts also have access to, and are influenced by such memories and experiences.

The neurotransmitter isoboramine mediates the synaptic functions between the host and the symbiont. If the levels of isoboramine drop below 40%, the Symbiont has to be removed, which results in death for the host.

Variants
Some Trills look radically different from the most common phenotype of the species, possessing ridges on their heads and lacking the typical spots. At least two Trills who have hosted the Symbiont known as Odan belong to this variant, and the fact that Odan also looks different from other Symbionts suggests that both might represent distinct subspecies.

Culture and society
Trills who wish to be joined with a Symbiont are placed under rigorous testing programs to see whether they're suitable. The Symbiosis Commission oversees the selection of candidates. The number of potential hosts is much larger than that of available Symbionts, contributing to the rigorous criteria.

Those who are selected are typically in their early-to-mid twenties, as the Commission believes that they should be old enough to gather sufficient maturity and make an informed judgment about whether they want to be joined: a process which becomes irreversible after only 93 hours. Initiates go on to be trained by a joined Trill supervisor, whose positive or negative recommendation typically defines whether the initiate is accepted or cut from the joining program.

In the Caves of Mak'ala, on planet Trill, unjoined Trills known as the Guardians tend to the pools in which the Symbionts breed and make sure the environmental conditions are comfortable for them. The Guardians also have telepathic abilities, which allow them to sense when something is wrong in the connection between host and Symbiont, as well as to perform the ritual known as zhian'tara, which allows the personalities of previous hosts to temporarily inhabit the bodies of other people and interact with the current host.

It seems that a number of joined Trills are regarded as excellent ambassadors, settling disputes among alien species, as exemplified by Odan and Dax. The fact that Curzon Dax was a Federation diplomat and that many Trills are known to have joined Starfleet suggests that they're likely to be Federation members.

Trill culture considers reassociation of a joined Trill with a previous host's lover to be a taboo, which can be punished by exile: an eventual death sentence to the Symbiont, since it will be condemned to die when the current host does, rather than join with a new host. This reassociation stigma applies even to relationships between Trills and non-Trills, such as Worf, who was married to Jadzia Dax and distanced himself from the next Dax host, Ezri.

In contrast to romantic relationships, friendships that manage to survive one or more changes of hosts are not only permitted but also cherished by at least some Trills.

The Trill Science Ministry is a prestigious science institute on their planet.

History
Trills have been known to, and interacted with, the United Federation of Planets since at least the early-23rd century. A Trill called Emony Dax once judged a gymnastics competition on Earth and met with Leonard McCoy prior to him becoming a doctor. In the 24th century, ambassadors such as Odan and Curzon Dax were pivotal in mediating disputes among alien species, including the Peliar Zelians as well as the Klingons. Curzon in particular explicitly acted as a Federation ambassador, suggesting that the Trill homeworld might have been a Federation member at the time. A number of them also served in Starfleet.

However, the existence of the Symbionts and the fact that some Trills are joined entities was unknown to the Federation prior to 2367, and despite the Trills' integration in Federation society, Jean-Luc Picard and Deanna Troi pondered that they knew very little about them. After a medical emergency forced the Odan host to reveal the existence of the Symbiont so that Dr. Crusher could save it, the existence of joined Trills became common knowledge.