All adult vertebrates need hemoglobin to carry oxygen in their blood.

The red color of our blood comes from hemoglobin, a protein that binds oxygen inside red blood cells. Its importance made it seem indispensable to adult vertebrate life.

Reports of Antarctic fish with colorless blood initially sounded implausible. In 1954, Johan Ruud documented the physiology of the blackfin icefish, Chaenocephalus aceratus. Its blood lacked both hemoglobin and red blood cells.

The fish still needs oxygen. It transports it dissolved directly in the liquid part of its blood, the plasma. That arrangement carries much less oxygen than hemoglobin-rich blood, but cold Antarctic seawater contains abundant dissolved oxygen. Icefishes also have unusual circulatory features, including large hearts and high blood flow, that help compensate.

Later genetic work showed that this was a genuine loss of functional hemoglobin genes, not simply an unusual way of hiding the familiar pigment.

The exception does not make hemoglobin unimportant or mean that other fish could readily do without it. It shows that a specialized lineage can survive without something that otherwise appears essential. Researchers also distinguish the loss itself from the compensatory features that made survival possible.