Just not complete without you
Symbiosis between animals and bacteria is very common. In some cases, animals will not develop fully without their microscopic partners—sometimes, they can’t even reproduce.
Here’s a paper on the bobtail squid, and the light producing organ that doesn’t develop fully without the correct strain of bioluminescent bacteria.
Here are a couple of articles on parasitic wasps, and the effects on their reproduction caused by their microscopic hitchhikers.
And here’s an article that talks about the link between animal behavior and their microbial symbiotes.