The crocodile icefish is recognisable by its pale, ghost-like body and long, broad snout. (Julian Gutt, Alfred Wegner Institute)

In the chilly depths of the Southern Ocean, where temperatures rarely exceed 2.0 °C, an evolutionary oddity lays near the benthos. This family of fish are characterised by their colourless blood and bizarre anatomy, as well as their ability to live in one of the most extreme environments on planet Earth. The name of this family is the crocodile icefish, or Channichthyidae.

Belonging to the suborder of Antarctic fish Notothenioidei, the crocodile icefish can grow up to half a metre in length, they can live up to 1,500m deep, and with their alien looks you may suspect that they’re as strange and interesting inside as they are on the outside. You’d be correct.

Evolutionary accident

Icefish are the only vertebrates on the planet that have no haemoglobin in their blood. Haemoglobin is the protein which binds oxygen to the red blood cells in our body and is what makes them appear red. The lack of this protein means that crocodile icefish have clear blood, a trait completely exclusive to them. It’s not known for sure why icefish produce no haemoglobin, a protein which seems so important to life.

At first it was suggested that the change was adaptive, and that the lack of haemoglobin somehow help the icefish live in their cold environment, but this has proved not to be the case. On the contrary, evolutionary pressures have made the icefish compensate for their lack of haemoglobin by developing higher blood volumes and larger hearts. Now it is thought the loss of haemoglobin was an evolutionary accident, a defect only allowed to continue because of the Antarctic’s low, unchanging temperature. The cold water allows for a higher concentration of dissolved oxygen, meaning that the icefish can still absorb enough oxygen to survive, even without their oxygen binding proteins.

The brink of survival

But the lack of colour isn’t the only strange thing about the blood of the icefish, as they also have a circulatory system laced with antifreeze. This is a trait that is prevalent in the Notothenioidei, as the formations of ice crystals in the blood would be detrimental and often fatal to organisms. Instead, the icefish’s blood contains antifreeze proteins which keep these ice crystals from growing large enough to damage blood vessels and tissue. It’s essential to keeping the icefish alive in temperatures which are well below the freezing point of our own blood.

However, even this adaptation is not without its consequences. The antifreeze prevents small ice crystals from growing, but also keeps the tiny shards from melting altogether, leaving the icefish with a quantity of small ice crystals in its bloodstream all its life. Luckily, they cannot grow large enough to damage any of its organs such as its huge heart. 22% of the total energy produced by the icefish is sunk into its heart, a large muscle lacking in the usual coronary arteries which characterise the hearts of most other creatures on the planet, which has the sole task of pumping its low-oxygen, antifreeze-laden blood around its limpid body.

Despite its setbacks, however, the icefish thrives, its ambush predator lifestyle serving it well in the depths of the Southern Ocean, though how long this will last is not certain.

End of the line

All is not well concerning the temperature of the Southern Ocean. (NASA)

The fate of the icefish is dependant almost solely on the temperature of the Antarctic waters. They have been massively unchanged for millions of years, barely varying between -1.8 and 2.0. This near-static temperature allowed the icefish to evolve, and the loss of this stability could destroy them.

Climate change threatens the icefish with a sudden drastic warming of their home. Recent studies have shown the Southern Ocean is warming much faster than the rest of the world’s oceans, and with a higher temperature comes a lower concentration of oxygen in the deep waters, something which the icefish may not be able to deal with. If the warming of the Southern Ocean continues at the rate it has since the 1950s, the crocodile icefish’s evolutionary mistake could prove to be their ultimate downfall.

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