Two yellow butterflyfish
      with dark-blue mask on eyes and fade vertical strips on the sides

A butterflyfish with diagonal
      black and yellow stripes
Black-white mask

A pair of masked butterflyfish

( Chaetodon semilarvatus )

In result of evolution in clear sea water some fish became colorful like butterflies. They recognize each other by colors of body, by face, and even by eyes. Many coral butterefly-fish have got a mask on their eyes. This two fish have dark-blue masks.