According to PBS, flatfishes (like halibut and flounder) have been deemed "the most asymmetrically-shaped vertebrate to ever live on Earth," by people like flatfish expert Alexander Schreiber.
It's not hard to see why:
Flatfish have two eyes on one side of their body, none on the other, and twisted, sideways mouths.
One side of their body is also transparent.
They can also use their fins to "walk" like millipedes as Azula previously reported.
Basically they're super-weird fish. But they don't start out that way. According to New Scientist, flatfish look like your average fish when they're born.
Their eyes are on each side of their body like all other fish.
Then, after three weeks, it all goes haywire. One eye literally moves across the fish's skull until it lands on the opposite side. NatGeo reported that this changes the bone structure in their heads.
But their mouths stay where they were, giving them that weird Picasso painting vibe.
PBS reported that baby flatfish have a really hard time swimming upright, even when their eyes are positioned normally on each side of their head. It seems their inner ear and their eyes — essential to balance in the water — give conflicting signals to the brain. So they tend to swim lopsided.
And New Scientist reported that swimming lopsided makes their sunlight intake uneven. That's a big deal because sunlight activates something in flatfish that triggers their thyroid — which has to do with building their shape.
The more sunlight the fish gets on one side, the more thyroid hormones are triggered, and the more the other eye migrates over to give them a better field of vision.