Symbiosis was a dependent relationship between dissimilar organisms. Types of symbiosis included parasitism, mutualism, and commensalism. Some would argue that "symbiosis" only referred to mutualism.
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.
- Anaphasic lifeform
- Ceti eel
- Denaxian tapeworm
- Interspatial parasite
- Onaya, although this might be considered mutualism on some level
- Tarchannen III species
- Ves Alkar
- See main article: Neural parasite
Parasitic behavior including non-organismsEdit
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")
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")