The strange fish swimming through Antarctica on almost colourless blood

Beneath the ice of the Southern Ocean, in some of the coldest waters on the planet, swims a fish unlike any other vertebrate known to science. The Antarctic icefish, from the family Channichthyidae, has no red blood cells and no haemoglobin as an adult, and in some species, no myoglobin either. Instead of red, its blood runs pale, watery and nearly clear.

The Southern Ocean is key to how this is even possible. Cold water holds far more dissolved oxygen than warm water, and in places near the Ross Ice Shelf, where the water can fall to around -1.9°C, it is nearly saturated with oxygen. That abundance lets icefish absorb oxygen directly through their blood plasma and their scaleless skin, reducing how dependent they are on haemoglobin to move oxygen through the body. Icefish also evolved from slow-moving fish with naturally low energy needs, which makes surviving with far less oxygen-carrying capacity more manageable.

Still, the loss of haemoglobin was never an advantage in itself. A 2006 commentary published in the Journal of Experimental Biology by Bruce D. Sidell of the University of Maine and Kristin M. O’Brien explains that it happened through a genetic mutation, and that mutation came with real costs: an icefish’s blood can carry less than 10% of the oxygen carried by the blood of closely related red-blooded fish.

To survive that shortfall, the icefish’s body changed dramatically. Its heart is four to five times larger relative to body size than the hearts of related red-blooded fish, it holds up to four times more blood, and its blood vessels are considerably wider, all to push more oxygen-poor blood around the body faster. The cost of that adaptation is steep, with icefish using about twice as much energy to pump blood as their red-blooded relatives.

Some icefish species also lost myoglobin, the protein muscles use to store oxygen, each through a separate genetic mutation occurring at a different point in time. This surprised researchers, since a heart containing myoglobin usually performs better under stress, meaning the loss should have made the icefish’s heart less efficient, not more.

Scientists also point to a small signalling molecule called nitric oxide as part of the explanation. Losing haemoglobin and myoglobin meant the icefish also lost one of its main ways of breaking nitric oxide down, and researchers believe this caused its levels to rise throughout the body. That rise, they suggest, may have triggered many of the physical changes seen in icefish today, as the species’ body gradually reshaped itself around the loss of two proteins that almost every other vertebrate depends on to stay alive.

Wikimedia Commons/by Ambiederman