For researchers trying to understand the physiology of blood and vascular systems, certain animals hold a special allure as a probe of their theories. Antarctic icefishes are such an animal. All other animals with backbones have red blood. This color is due to the iron containing protein hemoglobin. Hemoglobin performs the vital function of ferrying oxygen through the bloodstream from respiratory organs like lungs or gills to all parts of the body. Most of the blood’s hemoglobin is packed into red blood cells, the most abundant cell type in blood.
In 1928 biologist Ditlef Rustad caught an unusual, eerily pale fish in waters near Antarctica. Rustad discovered, to his astonishment, that the fishes’ blood was transparent, lacking mature red blood cells and hemoglobin. Such Antarctic icefishes are now known to be abundant in the waters surrounding Antarctica; occupying the subfreezing waters of the coldest environment on Earth. All sixteen species of icefishes lack hemoglobin.
How could icefish survive without hemoglobin? When water is very cold, its ability to carry oxygen is enhanced, and hemoglobin’s effectiveness is reduced. Oxygen diffuses through the icefishes’ enlarged gills, and maybe through its smooth skin, directly into its blood plasma, where it is carried through the body. Icefish blood can only carry about ten percent as much oxygen as in other fish, but this is good enough for the sluggish icefish to survive.
Why do icefish lack hemoglobin? We actually don’t know. Some think it was a helpful adaptation to the cold. Extreme cold causes red blood to thicken, making it more difficult to circulate. Others think it was, overall, a harmful mutation, but that the icefish evolved in other ways to cope.
