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Intelligent Lifeform Index

Four Letter Code SYMB
Federation Status Member
Planet of Origin Trill
Encountered In 2367 the existence of the Symbionts became common knowledge
T/E Rating T0/E0
Current Tech Level N+
List of Named Symbionts

Full ILI GalleryPermitted Species Gallery

See also: Trill

Trill symbionts are a form of parasitic life form that requires a host body in order to leave the water. Unlike other parasites, they do not take over the host but occupy the willing bodies of volunteers who meet their complex requirements in order to share the body in a symbiotic relationship.


The existence of the symbionts was for centuries a closely guarded secret and well into the 24th century, the relationships between Trill humanoids and symbionts was a source of much political and social upheaval on the planet Trill and beyond.


When a symbiont was not inside a host body, they made use of electrochemical signals to communicate with others through the liquid they lived in. A joined symbiont was also capable of communicating with its non-joined counterparts by being submerged in the fluid breeding pools on the Trill homeworld. This provided a direct form of communication between the two but not on the same level of sharing knowledge as that which a Joining was capable of providing.

While typically symbionts only communicated with and took Trill as hosts, it was possible for them to communicate with Humans at least through extended contact. This was done by matching the symbiont's bioelectric patterns with those of the other individual. However, this required direct tissue contact between the nerves of the person with those of the symbiont.

As symbionts grew older, they eventually became unable to take hosts and would sink to the bottom of the pools where they would become caretakers for the Annuated, eventually these symbionts would grow to become Annuated themselves over a period of 20,000 years or more.

Mirror Universe

In the Mirror Universe, the Trill Symbionts were infected with a hostile parasitic intelligence. Emperor Spock ordered all the symbionts, save for Dax, exterminated in order to stop them becoming a threat to the Empire. This has left Curzon Dax as the only joined Trill. In 2379, Curzon passed away, and the symbiont was joined with Ezri Dax.


Trill Symbionts, also known as swimmers, are small animals, a kind of sentient vermiform lifeform which can live either within Trill humanoid hosts or Symbiont pools on Trill. The symbionts distinguish humanoid Trill as walkers.

It was transferred from one humanoid Trill host to another upon the death of the host. Humans could also be used as a host to a Symbiont, however the Symbiont could not remain in their bodies for long without physically harming the Human host. The Symbiont took with it all memories of the previous Trill hosts. Upon joining, the new Trill host gained these memories. A host of a trill symbiont gains access to the memories of the symbiont and therefore those of all the previous hosts.

The Symbionts lived in the Caves of Mak'ala on Trill and were cared for by the Guardians. They communicated with each other via electrical impulses transmitted through the milky water they inhabited.

The Symbiosis Commission oversaw the joining of Trill and Symbionts. The general belief in the 24th century was that one in every 1,000 Trill was suitable for joining; however, the number was nearer to 50% of the population. This was a closely guarded secret because every year about 5,000 Trill became initiates while only about 500 symbionts became available.

Rites and ceremonies

Important Symbionts

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