omega iris

a special iris structure found in armoured suckermouth catfishes (Loricariidae). The top part of the iris descends in a loop similar in shape to the Greek letter omega (Ω) in an upside down position. The loop can expand when light levels are high to cover the pupil centre forming a c-shaped light transmitting portion. It may also serve to break up the distinctive eye outline in a fish that is heavily camouflaged