Lately there's been a lot of discussion around the many non-human cells essential to a healthy human body — called the microbiome, or metagenome. Much of this is in the gut microbiome. See this question, for example.
The Human Microbiome Project at NIH emphasizes we are just beginning to measure the specifics of this for the human case.
I assume this is similarly true for all mammal species, and I don't know how much farther. Fish have a fairly different body plan than mammals, including a fairly different gut. And coelenterates have an even more different body plan in many ways, again including the gut.
Does a typical fish depend on as large a microbiome as a human? Is there much known about that? What about jellyfish?