The crocodile icefish or white-blooded fish (Channichthyidae) comprise a family of notothenioid fishes found in the cold waters around Antarctica and southern South America. Water temperatures remain relatively stable in the Southern Ocean, generally ranging from -1.8 to +2.0°C. Sixteen species of crocodile icefish are currently recognized.
Icefish blood is colorless because it lacks hemoglobin, the oxygen-binding protein in blood. Red blood cells are usually absent and if present are rare and defunct. Oxygen is dissolved in the plasma and transported throughout the body without the hemoglobin protein. The fish can live without hemoglobin because of their low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment (the solubility of a gas tends to increase as temperature decreases). However, the oxygen-carrying capacity of their blood is less than 10% that of their relatives with hemoglobin.
To compensate for the loss of hemoglobin, they have larger blood vessels (including capillaries), greater blood volumes (four times that of other fish), bigger hearts, and greater cardiac outputs (fivefold greater) compared to other fish. Their hearts lack coronary arteries and the ventricle muscles are very spongy, enabling them to absorb oxygen directly from the blood they pump. Their hearts, large blood vessels and low-viscosity (RBC free) blood are specialized to carry out very high flow rates at low pressures. This helps to reduce the problems caused by the lack of hemoglobin. In the past, their scaleless skin had been widely thought to help absorb oxygen. However, current analysis has shown that the amount of oxygen absorbed by the skin is much less than that absorbed through the gills. The little extra oxygen absorbed by the skin may play a part in supplementing the oxygen supply to the heart which receives venous blood from the skin and body before pumping it to the gills.
