I wouldn't say the tongue-parasite is under the fish's control, but rather that the parasite is "deciding" to emulate a tongue for its own purposes. IIRC it will eventually take off and leave the fish to starve.

I feel it's worth noting that there are a lot of functions of our bodies that we don't explicitly control. When describing them, we tend to use "other" language to describe them to the extent we don't control them, e.g. "my body did this" vs "I did this". Artificial limbs will fit into that framework just fine.