Actually I feel like I should explain flatfish because they are such a wonderfully demented bit of evolution. Tonguefish like Symphurus thermophilus are just one kind of flatfish. Others in the order Pleuronectiformes include a lot of fish you've probably heard of: dab, flounder, sole, and halibut are just a few! Their #1 uniting feature is that they are flat fucks. Their #2 feature (not quite present in all, but in most) is eye migration.
"What do you mean eye migration?" Take it away Euskal Herriko Unibertsitatea in Bilbao, Spain!
The European flounder is not the only one doing this. Far too many of these fish have solved their desire to be flat fucks who still have both eyes placed where it's useful in the jankiest way possible. They are actually very thin TALL fish, and simply lie on their sides 24/7. I guess you can do that, fine. But that would mean one eye is buried in the sediment all the time, and that just won't do. Evolution gave them both eyeballs and they will use them. So sometime after they're born one of their eyeballs... well... migrates across their skull. Which one moves depends on species (and sometimes random luck) but the end result is both eyeballs come to sit on the dextral ("right") or sinistral ("left") side of the fish, and that becomes the side of the fish that faces up.
And then they just live like that! They're optimized to spend time sitting on or just under the sand/substrate, but they swim just fine. Here's an unID'd flat fuck "do you mind?"ing away from a diver in Greece. As weird evolutionary jank goes, flatfish seem to be doing extremely well for themselves, and I find that both funny and delightful.
BONUS TRIVIA: apparently the many flat fucks of Pleuronectiformes have such an easy time crossbreeding that scientists have at least once discovered what they thought was itself a stable population of one species was actually a hybrid of two different species. They're especially good at intergeneric hybrids, AKA not just crossing two species but crossing two species that are each from a different genus.


