Trill symbionts are one of the lifeforms native to the planet Trill and comprise part of the combined consciousness a joined Trill. The symbiont does not have any extrasensory perception and communication between symbionts or symbiont and host happens via electrical impulses.
Unlike the host, the typical Trill symbiont can live for long periods of time, so, more often than not, the symbiont is transferred from one host to another once the previous host dies. With that transfer, the symbiont brings the memories of all previous hosts to the joining, allowing the host access to several life-time's worth of knowledge and experiences. If a Trill host is unavailable, the symbiont can live in stasis or in some other species for short periods of time; long periods of joining can cause the non-Trill host and the symbiont damage. (TNG: "The Host")
The Trill Symbiosis Commission matches symbiont to humanoid host. Only one in every thousand Trill humanoids have the capacity, mentally and physically, to join with a symbiont, while, in the typical year, approximately 300 symbionts become available with approximately 5,000 potential hosts (known as initiates) graduate to host status. (DS9: "Equilibrium")
- The Symbiosis Commission have lied to the general Trill public about the capacity of the humanoid population's ability to join with the symbionts. Nearly 50% of the potential population has the ability to host a symbiont.
The symbiont remains the host's top priority, and the host will give his or her life to protect the symbiont from harm or death. In 2372, a fatally-wounded Ulitania Jonar requested Dr. Marie Bourget remove the Jonar symbiont and transfer it to Tawana Killan to save the symbiont. (Star Trek: The Cantabrian Expeditions: "Oh, What a Lovely War")
- Dax symbiont
- Gotens symbiont
- Gotens symbiont (mirror)
- Jonar symbiont
- Kemis symbiont
- Lex symbiont
- Seifer symbiont
- Xen symbiont