why do some fish have colourless blood?

Antarctic icefish have colourless blood with no red blood cells and

no haemoglobin, the oxygen-carrying pigment. This probably comes down to a genetic mutation, and means their blood carries 90 per cent less oxygen than red blood. They survive partly because
frigid Antarctic waters are oxygen-rich. Icefish also have enormous hearts that pump huge volumes of blood around their bodies,
making sure they get enough oxygen. Antifreeze in their blood stops them from freezing (the salty Southern Ocean gets down to -2°C) but, as they are so well-adapted to the cold, their future in a warming world remains uncertain.