The frigid seas of the polar regions both of the Antarctic and the Arctic have water temperatures below freezing. For example the in the Antarctic seas, temperatures between -1 and -4 degrees Celsius are common, well below freezing point. The water temperature varies very little from season to season and the waters are often filled with ice. Without specialised adaptation, fishes would be frozen to death because of high water content of blood and flesh.
Fishes such as the Polar cod of the Arctic and the Antarctic cod and its relatives in the family Nototheniidae are adapted to these frigid seas. In their blood they have specialised proteins called antifreeze glycoproteins or AFGPs. The AFGPs bind with ice crystals preventing them from growing to hazardous sizes, thereby preventing tissue damage and continue to allow free flow of the blood.
The AFGPs in the Polar cod and the Antartic cod and its relatives are very similiar in their molecular structure. AFGPs are made in the liver and secreted into the blood circulatory system. They are present in all bodily fluids and are absent from the urine.