Western Pacific tonguefish (Symphurus thermophilus)!
This hardy flatfish makes its homes in the active hydrothermal vents of the western Pacific Ocean! They seem to prefer hydrothermal vents with great amounts of sulfur, resting near pools of molten sulfur with no issues.
my love to flatfishes has begun from the game Stardew Valley. the first fish I got from ocean was exactly the flounder. a flat beautiful fish with eyes that looked directly in my blue soul.
from then on, when I moved around the Starfew Valley world, I always held her above my head and even went to bed with her. it’s so good that this moment from the game turned into reality
Modern flatfish are characterized by their highly asymmetrical skulls, with both eyes positioned on just one side of their head. They aren't actually born this way, but instead they undergo "eye migration" as juveniles, twisting up their skulls to bring one eye across the top of the head.
Progressive eye migration in a developing Summer Flounder, Paralicthys dentatus.
From Helfman et al (2009). The diversity of fishes. 2nd ed., Wiley-Blackwell.
This bizarre arrangement is the result of flatfish adapting to life laying flat on the seafloor, but instead of slowly widening and flattening themselves out they took an evolutionary "shortcut" by simply tipping their tall narrow bodies over onto one side. Initially this would have left one of their eyes unusable, but random mutations causing slightly asymmetrical skulls would have rapidly become highly advantageous to the earliest members of this lineage – and over time they just got wonkier and wonkier.
We've even found fossils of early flatfish in the "halfway there" stage of their evolution!
Amphistium paradoxum lived in what is now northern Italy during the Eocene, around 50-48 million years ago. About 20cm long (~8"), it had one eye partially migrated towards the top of its head, but not all the way around yet, showing a transitional state between its bilaterally symmetric ancestors and its more twisted-skulled modern relatives.
Unlike most modern flatfish Amphistium came in both "right-eyed" and "left-eyed" forms in equal numbers, suggesting that a genetic preference for a specific side also hadn't developed yet.