Why are some fish - like the glassfish - transparent?
Some fish are transparent because it really helps them to prevent being eaten. Some fish are transparent throughout their entire lives (glass catfish, glassfish, small minnows), and others are only transparent during their early juvenile stages but then grow out of it. Being transparent helps because no matter what the fish is around, the background color shows through, and for all practical purposes, the fish is invisible. (They become visible when cought in the beam of diver`s torches and their body reflects the silvery light) Some predatory fish trigger to the presence of food, not by seeing the prey fish, but by sensing or seeing the shadow of the prey fish passing above it.
An interesting thing about the true “clear” fishes (and shrimps). If you have a regular fish, and feed it some food with a lot of pigment in it, the fish will gradually take on that color. This is the same as flamingos getting their pink color from shrimp that they eat. If you do the same with a “clear” fish, no matter how much you feed them a colored food, they will not take on that color.