Fish
Blackfin Icefish
Chaenocephalus aceratus · also called Antarctic blackfin icefish
Pumping pale, nearly colourless blood through an enlarged heart, a blackfin icefish survives in the freezing Southern Ocean entirely without haemoglobin, the oxygen-carrying pigment found in almost every other vertebrate on Earth.
Overview
The blackfin icefish, Chaenocephalus aceratus, is a large, cold-adapted fish native to ocean and polar habitat across the Southern Ocean surrounding Antarctica. Its extraordinary lack of haemoglobin, skin-based oxygen absorption, and enlarged heart make it one of the most physiologically unusual vertebrates found anywhere in the animal kingdom.
A vertebrate that broke an ancient rule
Nearly every vertebrate on Earth relies on haemoglobin to carry oxygen through its blood, but the blackfin icefish has done away with it entirely, surviving instead on a combination of oxygen-rich cold water, direct skin absorption, and a heart built to work harder than almost any other fish’s.
Status
Near Threatened, having faced historical pressure from commercial fishing in parts of the Southern Ocean, with population trends remaining uncertain.
Blackfin Icefish: common questions
Why does the blackfin icefish have such pale blood?
This species has pale, almost colourless blood, lacking the red, oxygen-carrying pigment haemoglobin found in nearly every other vertebrate species, an extremely rare adaptation among fish.
How does the blackfin icefish get enough oxygen without haemoglobin?
This species compensates by absorbing a portion of the oxygen it needs directly through its skin and scaleless body, aided by the fact that cold Southern Ocean water holds far more dissolved oxygen than warmer water.
Does the blackfin icefish have an unusually large heart?
Yes, this species has a proportionally large, enlarged heart compared with most fish, pumping a greater volume of blood to compensate for its blood's reduced ability to carry oxygen.
How big can a blackfin icefish get?
This species is among the largest members of the icefish family, growing considerably larger than many of its relatives within the same group of white-blooded Antarctic fish.