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For the TNG episode of the same name, please see "Symbiosis".

Symbiosis, also known as a joined species, symbiotic relationship or mutual dependence, was a dependent relationship between at least two dissimilar species in an arranged relationship consisting of a host and a symbiont. A symbiotic organism is known as a symbiote.


The host's body is destroyed as the parasite leaves it

The parasitic relationship was harmful to one, but beneficial to the other. Unlike normal predatory behavior, it was usually in the parasite's best interest to keep its prey alive. The parasite also lived very close to, or attached to the victim. In this case, the joining was often unwanted by the host. It was in the nature of most species to survive, thus naturally rejecting such relationships.

Neural parasites

See main article: Neural parasite

Social parasitism

Parasitic behavior (including non-organisms)

A symbiotic lifeform invades Enterprise in 2152


A Trill symbiont

The relationship is mutually beneficial to both organisms. One example of this was the relationship between Sentient vermiform lifeforms from the planet Trill and their Trill hosts. They were able to share in a mutually beneficial relationship with their hosts. (DS9: "Equilibrium") The symbiont remained protected and shared resources with the host's body. The host gained the memories of the past hosts' lives and experiences, something deemed a wanted benefit by the host. Trill were primary hosts of the symbionts, however there were other host species, including a Human host in the form of William T. Riker. (TNG: "The Host")

According to Odan, the Trill were not always symbiotic, by rather became a joined species a millennia ago. (TNG: "The Host")

After Benjamin Sisko first met Doctor Julian Bashir and Jadzia Dax in 2369, Sisko asked Dax if she had told Bashir "about that slug inside of you?" Dax assured him that Bashir knew he was a Trill, and that "he finds it fascinating. He's never met a joined species before." (DS9: "Emissary")


Sargon leaves his sphere to enter his host, Kirk

In a commensalist relationship, one organism benefits from the relationship, the other is not affected. Often, the host showed little or no benefit from such a joining.

This exhibited itself in the case of beings of disembodied form described as "matter without form," who borrowed the bodies of USS Enterprise crew. They were an advanced humanoid race whose world was destroyed. As such, they requested to use the crew's bodies as a temporary form to construct Android bodies to live in. After one of the beings, Henoch, tried to kill one of the other two, the plan was abandoned and the beings left their host. In this situation the relationship was neutral for the crew members. There was no long term harm done, however there was no personal gain as being a host. (TOS: "Return to Tomorrow")

Other references

Kevin Uxbridge was a botanist who specialized in symbiotic plant life. (TNG: "The Survivors")

As Captain Christopher Pike came to realize in his captivity, the Talosians were not doing the illusions just out of passion. They were after respect and mutual dependence. (TOS: "The Cage")

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