Meet the Antarctic icefish: the only vertebrate with clear, colourless blood

In a 2006 commentary titled ‘When bad things happen to good fish: the loss of haemoglobin and myoglobin expression in Antarctic icefishes’, published in the Journal of Experimental Biology, researchers Bruce D. Sidell of the University of Maine and Kristin M. O’Brien laid out one of the strangest survival stories in vertebrate biology: a fish that lives without haemoglobin.

The Antarctic icefish, part of the family Channichthyidae, is the only known vertebrate that loses haemoglobin once it reaches adulthood. Its blood carries no red blood cells and, in some species, no myoglobin either, leaving it pale, watery and almost clear. Sidell and O’Brien’s paper makes clear this was not an advantage the icefish evolved toward. It was a genetic mutation, and it came with serious costs: an icefish’s blood can carry less than 10% of the oxygen that closely related red-blooded fish carry in theirs.

To offset that loss, the icefish’s body restructured itself. Its heart grew to four to five times larger relative to body size than the hearts of related red-blooded fish, its total blood volume increased by up to four times, and its blood vessels widened significantly, all so that more of its thin, oxygen-poor blood could move through the body faster. That adaptation is not free: icefish use about twice as much energy pumping blood as their red-blooded relatives.

The paper also documents how several icefish species independently lost myoglobin, the protein muscles use to store oxygen, each through a different genetic mutation at a different point in time. This finding puzzled researchers, since myoglobin is known to help hearts perform better under physical stress. Losing it should have made the icefish’s heart less efficient, not more, yet the species persisted regardless.

Part of the explanation lies in where the icefish lives. The Southern Ocean around Antarctica is extremely cold, and cold water holds far more dissolved oxygen than warmer water does. Near the Ross Ice Shelf, where the water can drop to around -1.9°C, oxygen saturation is close to total. That abundance lets icefish absorb oxygen directly through their blood plasma and their scaleless skin, easing their dependence on haemoglobin. Their slow-moving evolutionary ancestors also had naturally low energy requirements to begin with, which made surviving on far less oxygen-carrying capacity more manageable.

Sidell and O’Brien’s work also points to nitric oxide, a small signalling molecule, as a likely piece of the puzzle. Losing haemoglobin and myoglobin also meant losing one of the body’s main routes for breaking nitric oxide down, causing its levels to rise throughout the icefish’s system. The researchers suggest this rise in nitric oxide may have driven many of the physical adaptations now seen in the species, as the icefish’s body gradually reorganised itself around a loss that, in most other vertebrates, would be fatal.

Wikimedia Commons/by Ambiederman