Two Species In One: Star Trek's Trill Explained

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Welcome to Trekspertise, a series where we break down the technology, history, details, and decisions that make the Star Trek universe so complex — and so fun.

The Trill are first introduced in the "Star Trek: The Next Generation" episode "The Host," wherein Dr. Beverly Crusher (Gates McFadden) falls in love with a Trill ambassador named Odan (Franc Luz). Partway through that episode, it's revealed that the face and body that Dr. Crusher loved belongs to a mere host being, while his personality and memories all come from a small, sentient worm-like creature that has been surgically implanted in his abdomen. The Trill, Odan explains, are a joined species, and many members of his species play host to long-lived symbionts. 

Many details of the Trill would be refined on "Star Trek: Deep Space Nine," which featured a Trill character, Lieutenant Jadzia Dax (Terry Ferrell), as part of its main cast. Shortly before the events of the series, Jadzia had been implanted with a symbiont named Dax, giving her the outward appearance of a woman in her mid-20s, but centuries' worth of memories culled from the symbiont's seven previous hosts. Dax had been a wife, a husband, a mother, a father, and had many different professions. Dax's gender fluidity has served as a source of inspiration for many trans and nonbinary Trekkies, as when she changes gender and appearance, most of her old compatriots take it in stride. Her friend Commander Sisko (Avery Brooks) refers to her as "Old Man," as he last knew Dax when she was a rowdy old septuagenarian man named Curzon Dax. 

Over the years, more and more details about the Trill were provided on "Deep Space Nine," and Trekkies eventually got a great sense as to the alien species' workings and its unique biology. 

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In "The Host," it's explained that not all species are biologically compatible with symbionts, and when Dr. Crusher tries to implant one into Commander Riker (Jonathan Frakes), his body rejects it. In that episode, it appears that a symbiont actually usurps the personality of its host, but on "Deep Space Nine," that's altered to be a sharing of memories. The Trill actually take on the memories and maybe some personality traits of the symbiont, but largely retain their own personalities, only now colored by the centuries of experience and confidence they inherit. Trill hosts live about as long as humans, while the symbionts can live a millennium. 

On "Deep Space Nine," it's also explained that joining is actually quite rare in Trill society; of about 1,000 applicants, only 300 are joined. One has to go through a rigorous screening process to be joined, which is a matter of biology, yes, but also of personality. The Trill only consider those with exceptional strength of character worthy of joining. Unjoined Trills have to serve as initiates under joined Trills to get a potential letter of recommendation. The screening process serves as the plot for the episode "Playing God." 

But the symbionts seemingly have no ethics or personality of their own. In the episode "Invasive Procedures," a rogue unjoined Trill named Verad (John Glover) kidnaps Dax and steals her symbiont, implanting it in his own body. Jadzia Dax strenuously objects to this, of course. Once implanted, Verad Dax has all the wild memories of the previous Dax hosts — but not Jadzia's moral objections to his theft. If he had inherited Jadzia Dax's principles, he would have returned the symbiont immediately. The host, we see, retains 100% of their free will. 

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Of course, it's also stated that if a weak-willed humanoid Trill were implanted with a strong-willed symbiont, the symbiont's personality would, by default, take over. So maybe free will among the Trill is a little trickier than we thought.

It should also be noted that joining for a Trill is meant to be permanent (Verad's shenanigans notwithstanding), with the symbiont staying in its host until their death. In the "Deep Space Nine" pilot, we see Curzon Dax dying on the operating table as the symbiont is moved into Jadzia, lying nearby. Symbionts typically cannot survive outside of a biological host for too long. On the Trill homeworld, however, symbionts can swim around, live happily, and even reproduce in specialized cave pools. Priest-like guardians care for the symbionts, and swear an oath to never be joined. 

Trill humanoids are marked by a series of spots on their temples. This causes a bit of a canonical issue, as the Odan character had outsized alien ridges on his face, but no spots. This is one of those details that Trekkies just kind of have to ignore. One can look around online and find photos of actress Terry Farrell in the first version of the Dax makeup, which was meant to look more like Odan. The showrunners didn't like the makeup, though, and went for the spots instead. Now we just kind of look the other way at Odan.

Trills eventually became a regular part of "Star Trek: Discovery" as well with the introduction of Grey Tal (Ian Alexander), although that show altered the Trill workings again. When Grey's beloved Adira (Blu del Barrio) had to carry the symbiont, they could talk to Grey as if he were an imaginary friend. That's a wrinkle Trekkies also usually ignore.