Once, flounder posed a puzzle for specialists. While studying how fish adapt to their surroundings, they placed a flat glass tank on a chessboard. Over time, the fish became so mottled that you could practically set up kings, bishops, and pawns on its back and start a match.
For a long time, they couldn’t figure out how the flounder changed its colors. But then, one day in the Sivash, they caught a fish that was completely black. What was going on? All flounders have a greenish protective coloration, yet this one was entirely black. Upon closer inspection, they discovered it was completely blind. The inability to perceive any color had caused the fish to turn black.
This led scientists to conclude that visual perception is crucial for fish. To test this theory, they began outfitting the flounder with glasses. When they put on green lenses, the fish turned green; with red-tinted glasses, it darkened. And with yellow glasses, it took on a color almost like a canary!