Blackfin Icefish

• fisiologíaIt is the only vertebrate known to have no haemoglobin in its blood; its plasma is almost clear and carries oxygen dissolved directly in it.
• The Antarctic icefish are the only group of adult vertebrates without haemoglobin that is known; this species is one of the sixteen that make it up.
• To make up for it, it has a disproportionately large heart, an enormous blood volume and scaleless skin through which it absorbs oxygen directly from the water.
• Its tissues contain antifreeze proteins that prevent ice crystals from forming in the blood.
• Its blood is almost transparent. Ruud described it in 1954 after catching four specimens off South Georgia, and concluded that it was, literally, plasma with a few white blood cells.
• The oxygen-carrying capacity of that blood is less than 10% of that of a related red-blooded fish, and its iron content does not reach 5% of the normal level.
• It compensates with an enormous, hypertrophied heart, a large blood volume and a blood of low viscosity that circulates easily in freezing water. Besides, the colder the water, the better oxygen dissolves in it.
• Like the other Antarctic notothenioids, it makes antifreeze glycoproteins that stop ice crystals from growing in its body.
• Its genome, published in 2019, showed that the genes protecting against ice are heavily multiplied, as are those controlling the cell's redox state.
• Its lineage split from that of the sticklebacks some 77 million years ago, and it adapted to the cold as the Southern Ocean gradually chilled.

Skeleton type

Vertebrate - internal bony skeleton (fish)

Habitat & environment

Antarctic and subantarctic continental shelf, from 5 to 770 m

Sources consulted: Ruud (1954), Nature; Kim et al. (2019), Nature Ecology & Evolution; Sidell y O'Brien (2006), Journal of Experimental Biology; Lista Roja de la UICN