A study from NY times have found that Antarctic blackfin icefish developed to have transparent blood to survive in the freezing water conditions. The Antarctic blackfin icefish, Chaenocephalus aceratus, lacks hemoglobin in their blood and have thin bones which makes the brain visible through its skull. Research have studied the genome of blackfin icefish to compare them to their close relatives and found that their genomic maps changed due to evolutionary time and gave the icefish to have unusual characteristics from their ancestors. The modern icefish no longer makes red blood cells and lacks hemoglobin to carry oxygen. Instead, the icefish is dedicated to make antifreeze for blood and ice-preventing proteins for survival. Since red blood cells become hard to pump and can freeze easily under freezing water, the icefish underwent a strategy for anaemia and developed supersize gills by losing it scales to absorb the abundant oxygen from the freezing water through its skin. Through evolution, the icefish also developed floppy bones that contained less minerals than their ancestors and enabled them to rise up in the water column to eat krill and other aquatic creatures that would not be found in deep sea level.
This is very amusing how the blackfin icefish lost their ancestor's features of having red blood cells and dense bones and developed different physiological features such as transparent blood and sloppy bones for adaptation. It is also very surprising to know that anaemia, a trait that is maladaptive to most vertebrates could be an advantageous trait for a different environment.