Flounder once posed a puzzle for scientists. To study how fish adapt to their surroundings, researchers placed a flat glass tank over a chessboard. Over time, the fish became so mottled that you could practically set up kings, bishops, and pawns on their backs and start a match.
For a long time, scientists couldn’t figure out how flounder changed color. But then, one day in the Sivash, they caught a fish that was completely black. What was going on? All flounder usually have greenish protective coloration, yet this one was completely black. Upon closer inspection, researchers discovered it was completely blind. The fish’s inability to perceive color had caused it to turn black.
This led scientists to conclude that visual perception is crucial for color change in fish. To test that idea, they began fitting flounder with colored lenses. When researchers fitted the fish with green lenses, the fish turned green; with red-tinted lenses, it darkened. With yellow lenses, it took on a canary-like color.
