Star Trek's Trills are outwardly distinguished by the spots on their skin. Some carry a separate symbiotic organism inside of them which carries the memories of all previous Trill hosts.
The alien race known as the Trill was introduced in the early nineties, first on Star Trek: The Next Generation and then popularized on Star Trek: Deep Space Nine. Terry Farrell’s Jadzia Dax served as Deep Space Nine’s chief science officer and is without question the most well known Trill.
THE TRILL ARE A SPECIES OF ALIEN IN STAR TREK AND THEY CARRY A SYMBIONT INSIDE THEM
Answering the question of “What is a Trill in Star Trek?” is a bit more complicated than asking similar questions about Vulcans, Klingons, Andorians, etc. So, let’s use an analogy: There’s a lot of science fiction about alien beings taking over the bodies of humans and acting through them. Invasion of the Body Snatchers, The Thing, and Venom are just a few examples.
Imagine if the parasitic aliens in those movies — rather than traveling to other worlds to find hosts — had a companion humanoid species that lived with them and existed primarily to act as hosts. That humanoid species would be very much like the Trill.
Unlike in those other examples, the symbionts who bond with the Trill aren’t malevolent. When bonded or “joined” as the Trill often put it, both Trill and symbiont benefit from the experience.
In fact, only a very small percentage of Trill are allowed the privilege to bond with symbionts. Potential hosts are rigorously screened and put through multiple levels of academic and physical competition against other Trill, and only the very best of the best wind up joined with a symbiont.
Some of the losers of those competitions don’t take things so well. For example, in the season 2 Star Trek: Deep Space Nine episode “Invasive Procedures,” the Trill Verad (John Glover) takes the station hostage in the hopes of removing the symbiont from Jadzia Dax for himself.
WHAT IS A TRILL? SPOT THEM BY THEIR SPOTS
What is a trope seen in Trill that you can see in much of the TNG era of Star Trek? Take a human, add one thing, BOOM: instant alien. In many cases, the one thing added wound up being ridges on noses, foreheads, cheeks, etc.; but in the case of the Trill, it’s spots.
Trill look exactly like humans except for a series of cheetah print spots that run along their forehead, continue down both sides of their face, down the neck, and continuing down the lengths of their bodies. There are only two exceptions to this: Odan and Kareel Odan (Nicole Orth-Pallavicini) in TNG‘s “The Host” who instead of spots had ridged foreheads.
Trill symbionts on the other hand are often colloquially referred to as “slugs,” and with good reason. We haven’t seen the symbionts in the flesh as much as we’ve seen their humanoid hosts, though they have appeared a number of times.
The symbionts are shaped like teardrops and are roughly the size of a vegetable like an eggplant. While they can exist and survive outside of Trill hosts, they’ve never been shown to have anything like eyes or ears.
THE TRILL FIRST INTRODUCED ON STAR TREK: THE NEXT GENERATION WERE DIFFERENT
The Trill first appeared in the Star Trek: The Next Generation season 4 episode “The Host.” The first Trill we meet is Odan (Franc Luz) — a mediator of great renown.
While attempting to negotiate a peace treaty between two alien races, Odan has a romantic relationship with Dr. Crusher (Gates McFadden), who is later shocked to learn he has a symbiont living inside him, and even more surprised at the end of the episode when — having changed hosts — Odan appears in the body of a female Trill.
However, figuring out what is and is not a Trill in Star Trek may be best served by skipping “The Host” entirely, because of a long list of inconsistencies between the episode and how the Trill were later portrayed. Much of what this episode told us about the species, including the way they look, was changed later.
TRILL JOINING HAPPENS THROUGH SURGERY
At the end of a joined Trill’s life, the surgery is performed to transfer that Trill’s symbiont to its new host. What is a certainty is that once the new Trill host is bonded with its symbiont for 93 hours, according to Star Trek: Deep Space Nine, both Trill, and symbiont become dependent on one another.
If host and symbiont are somehow separated, the Trill host will die within a few hours. Without its host — or a new, suitable host — the symbiont will die as well.
When Trill and symbiont bond, it isn’t like Invasion of the Body Snatchers — the symbiont does not control the Trill. Together they essentially become a brand new person, with the joined Trill exhibiting their own personality along with aspects of the symbionts previous hosts.
The Trill will have access not only to all of the symbiont’s memories, but to all those of its previous hosts, and will often display qualities, quirks, skills, and talents of previous hosts. For example in the season 3 DS9 episode “Equilibrium,” Jadzia learns she can play music because a previous host was a musician.
One of the first things that happens to a joined Trill is that they change their name. Trill have first and last names just like most humans. However, it is customary that a joined Trill will keep their first name while changing their last name to the name of their symbiont.
For example, before she was joined with the Dax symbiont, Nicole de Boer’s character was named Ezri Tigan. Afterward, she became Ezri Dax. This helps to let others who may have encountered — or know of — previous hosts of specific symbionts who they are speaking to.
When Worf (Michael Dorn) first meets Jadzia Dax in the DS9 season 4 premiere, he knows nothing of Jadzia but is well aware of the symbiont Dax, because her previous host Curzon was the Federation’s ambassador to the Klingon Empire.
What is a sad but understandable tradition of the Trill in Star Trek is that when a symbiont moves from one host to another, the joined Trill is expected to keep a respectful distance from past associations. This is meant to prevent confusion and complications.
It isn’t a law, but if violated the joined Trill will be considered an outcast. We see this in “Rejoined” of DS9‘s fourth season, when Jadzia starts a romance with Lenara Kahn (Susanna Thompson), whose symbiont was previously joined with a woman who was married to Dax’s previous host.
At some point, each joined Trill is expected to complete the zhian’tara ritual. With the help of a Guardian — a non-joined Trill whose primary job is to care for symbionts before they are joined — the consciousnesses of the symbiont’s previous hosts are telepathically transferred to the bodies of willing friends of the Trill. This way, the current host gets to speak directly to the previous ones. Jadzia undergoes the ritual in DS9‘s season 3 episode “Facets,” though it’s hardly a normal version of the ceremony.
Through the body of Ben Sisko (Avery Brooks), one former hosts tries to murder Jadzia, and another tries to permanently stay with the body of Odo (Rene Auberjonois).
TRILL SYMBIONTS MAY LIVE FOREVER
While the Trill have lifespans comparable to humans, the symbionts can live for centuries, and possibly even millennia. This is one of the reasons symbionts are so highly valued among the Trill — their centuries’ worth of experience is understandably considered more precious than just about any other resource.
While we know a lot about what is and is not a Trill in Star Trek, we actually don’t know exactly how long Trill Symbionts can live. When a Jem’Hadar asks Jadzia Dax how old she is in DS9 season 4’s “To the Death,” she tells him, “I stopped counting at 300.”
When the crew encounters their descendants from 200 years in the future in season 5’s “Children of Time,” the Dax symbiont is still alive with its new host Yedrin (Gary Frank). So we know that symbionts can live well past 500 years, we just don’t know where the ceiling is.
INCONSISTENCIES IN TRILL STORIES
The question of what is and is not a Trill in Star Trek is very much complicated by inconsistencies; most — but not all — which arise between the TNG episode “The Host” and the introduction of Jadzia Dax on DS9.
The first and most obvious inconsistency is the physical appearance. The Trill of “The Host” have ridged foreheads which are replaced with spots in DS9. According to the 2018 documentary What We Left Behind, the change was made because producers didn’t like how the ridges looked on Terry Farrell. So that’s it, right? No more inconsistencies?
Yeah, you wish. What is a Trill in Star Trek continues to be obscured. In “The Host” Odan refuses to be teleported to sickbay after being injured because he says using transporters can hurt the symbiont, while other joined Trill use transporters in the franchise without issue.
In “The Host,” it’s a surprise to the Enterprise crew when they learn Odan has a symbiont, but in DS9 it’s made clear that the Trill have been a part of the Federation at least as far back as the era of Star Trek: The Original Series. Jadzia’s predecessor Curzon Dax was the Federation ambassador to the Klingons and was friends with at least three Klingons who actually met Captain Kirk.
So presumably knowledge of the Trill and their symbionts would be common knowledge in the Federation. Finally, in TNG Odan is portrayed as completely taking over the body of their hosts, while in DS9 it’s made clear that the joining creates a new personality blending that of the host and symbiont.
But the inconsistencies don’t stop with “The Host.” When Jadzia dies and Dax is joined with Ezri, the latter is stationed at Deep Space Nine as a counselor, in spite of the Trill taboo of having too much contact with the associations of former hosts.
Jadzia married Worf before her death, and being stationed at DS9 — where Ezri will be expected to work closely with Worf and perhaps even act as his counselor — seems to be a pretty clear break with that tradition.
SYMBIONTS AREN’T SUPPOSED TO BOND WITH NON-TRILL, BUT IT HAS HAPPEND
It was previously believed that symbionts could only join with non-Trill for very short periods. This happens in TNG’s “The Host” when Will Riker (Jonathan Frakes) takes on Odan’s symbiont briefly to keep it alive.
However, what is a Trill became more complicated with season 3 of Star Trek: Discovery. Shortly after meeting Adira Tal (Blue del Barrio), it’s discovered that — in spite of being a human — they are joined with a symbiont. Their partner was a joined Trill who died and they successfully joined with the symbiont afterward. What this means for the future of the Trill remains to be seen.