Trill symbionts are one of the lifeforms native to the planet Trill and comprise part of the combined consciousness of a joined Trill. The symbiont does not have any extrasensory perception and communication between symbionts or symbiont and host happen via electrical impulses.

The symbiont appears worm-like in shape, although larger than most common Earth worms. It can live in pools on Trill (see Caves of Mak'ala) or within a Trill humanoid host.

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")

Isoboramine levels between the host and symbiont must be within tolerable ranges. Levels below 40% could be fatal to the host and/or the symbiont. (DS9: "Equilibrium")

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")

Though according to Iklani legends the symbiont Gaunt lived for a thousand years and joined with forty different hosts, no verifiable symbiont ever survived more than twelve hosts. (The War of the Masters: "A Good Compromise")


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