How does a fish survive Antarctica’s ice without any red blood?

Every other vertebrate on Earth relies on haemoglobin, the protein inside red blood cells that carries oxygen through the body. Without it, survival should not be possible. The Antarctic icefish is the exception that has puzzled scientists for decades: a fish that loses haemoglobin as an adult and, in some species, loses myoglobin too, yet still thrives in the freezing Southern Ocean.

The icefish belongs to the family Channichthyidae, and its blood is pale, watery and nearly clear rather than red. A 2006 commentary paper published in the Journal of Experimental Biology, by Bruce D. Sidell of the University of Maine and Kristin M. O’Brien, explains that this was never an adaptive advantage. It happened because of a genetic mutation, and that mutation left the icefish’s blood able to carry less than 10% of the oxygen carried by closely related red-blooded fish.

So how does it cope? The icefish’s body compensates through drastic physical changes. Its heart is four to five times larger relative to body size than that of related red-blooded fish, it holds up to four times more blood, and its blood vessels are considerably wider, all to keep a larger volume of oxygen-poor blood circulating quickly. That comes at a real cost: icefish burn about twice as much energy pumping blood as their red-blooded relatives do.

Some icefish species also lost myoglobin, the protein that stores oxygen inside muscle, through separate mutations occurring at different times across the family’s history. This was unexpected, since myoglobin normally helps a heart perform better under stress, meaning its loss should have made the icefish’s heart less efficient rather than more.

Environment turns out to be a major part of the answer. Cold water holds far more dissolved oxygen than warm water, and near the Ross Ice Shelf, where temperatures fall to around -1.9°C, the surrounding water is nearly saturated with oxygen. Icefish take advantage of this by absorbing oxygen directly through their blood plasma and their scaleless skin, reducing how much they need haemoglobin in the first place. Their ancestors were also slow-moving fish with low natural energy needs, which made surviving on a much smaller oxygen budget more feasible.

Researchers also point to nitric oxide, a small signalling molecule, as a likely factor. Losing haemoglobin and myoglobin meant the icefish also lost one of its main ways of breaking nitric oxide down, causing its levels to rise throughout the body. Scientists believe that rise may have triggered many of the physical adaptations seen in icefish today, with the animal’s body gradually reshaping itself around the absence of two proteins that almost every other vertebrate depends on.

Wikimedia Commons/by Ambiederman