Something has damaged the fish's ability to control the chromataphores used to adjust color and pigment based on mood. Most likely a bacterial infection. The chromataphores on this side of the fish's face have defaulted to the darkest "setting" possible.
Just like humans, specific parts of the body are seperated and controlled by specific parts of the brain. That's why you are seeing a very specific line and area affected by the malady. It is NOT genetic. If it were genetic the fish would have been born this way.
I've seen this in a variety of fish over the years and it is always a specific quadrant of the fish's body that is affected. It is always very well delineated and typically affects the face. It will never get better and from now on the fish's face wil be black on that side.
As far as I know there is no known catalyst nor no known treatment.
And despite the posts stating that it is cool or neat, it is a biological failure due to unknown reasons. Not something that I would be excited about.
