Hemoglobin is a protein within mammalian red blood cells that transports oxygen for delivery to tissues throughout the body.
Antarctic icefish (Channichthyidae), like the crocodile icefish pictured above, are quite unique in being reportedly the only vertebrate that does not have circulating hemoglobin. In fact, their blood is almost transparent because of the lack of red blood cells. This translates into an ability to carry only 1/10 the amount of oxygen as red-blooded fish. In addition, not all members of the Channichthyidae family have myoglobin (which is what binds oxygen in muscle cells) in their heart muscle and all members lack this protein in their oxidative (requiring oxygen) skeletal muscle cells.
Since oxygen is freely dissolved in their blood, they have evolved blood volumes two to four times greater than red-blooded fish to increase their oxygen carrying capacity. They have also evolved large hearts to pump higher volumes of blood and more capillaries in their tissues for gas exchange. Further compensations are seen in the oxidative muscles of icefish which have dense populations of large mitochondria to help produce energy compared to red-blooded species.
As if these adaptations were not enough, these fish also have circulating antifreeze proteins that help prevent them from freezing in the chilly waters they call home. Scientists have actually created synthetic versions of these proteins in an effort to help protect frozen food and transplant organs from forming ice crystals.
O’Brien KM, Mueller IA. The unique mitochondrial form and function of antarctic Channichthyid icefishes. Integrative and Comparative Biology. 50: 993-1008, 2010.
Bilyk KT, DeVries AL. Freezing avoidance of the Antarctic icefishes (Channichthyidae) across thermal gradients in the Southern Ocean. Polar Biology. 33: 203-213, 2010.