Almost by definition, animals that live in and around Antarctica have to be bizarre. After all, you have to be pretty strange to want to live in frigid temperatures year round. Today’s group of animals, the crocodile icefish, are specialist fish that are particularly fond of cold waters.

There are sixteen species of crocodile icefish, all of which are found in the Southern Ocean. They range from Antarctica to southern South America, staying in waters that stay between -1.8 to 2.0 degrees Celsius year round. Because of the increased oxygen content in cold water, lower temperatures are key to icefish survival.

Icefish can reach lengths of 25 to 50 centimetres, so they get to be reasonably sized. They have protruding snouts and distinctive spiny dorsal fins. Crocodile icefish don’t have super flashy colouration; they are light coloured with darker patterns, such as stripes or blotches covering their bodies.

A crocodile icefish – they look a little funny, don’t they? Image by
Marrabbio2, Copyrighted free use, vie Wikimedia Commons

One of the strangest things about crocodile icefish is the lack of hemoglobin and red blood cells in their blood. Because of this, icefish have colourless blood, which must look very strange. They are the only known vertebrates that lack hemoglobin.

So how to these fish survive without the oxygen-carrying protein hemoglobin? Well, they have much larger blood vessels and bigger hearts than other fish. Crocodile icefish have blood volumes that are four times as large as other fish, and their hearts’ outputs are five times greater. Icefish also have very spongy ventricles in their hearts, which means the organ can absorb oxygen from the blood they pump. All these adaptations means blood flows in large quantities at low pressures, allowing oxygen dissolved in the blood plasma to be absorbed by the body.

You may think, then, that icefish have evolved a more efficient way of living without haemoglobin, but that is not the case. In fact, although icefish can live without hemoglobin, the lack of it is not adaptive. The reason these traits evolved when they were not adaptive is that crocodile icefish evolved in a very unique environment.

An ocellated icefish. Image by Valerie Loeb, Public domain, via Wikimedia Commons

The steadily cold waters of the Antarctic allow for very high concentrations of oxygen in the water. There is also limited competition in Antarctic waters — many species of fish went extinct millions of years ago, and ocean trenches and current keep icefish populations relatively isolated.  Thus, a deleterious trait that would have resulted in the extinction of the species anywhere else has been able to persist in crocodile icefish, and this unique species is the end result.

I love writing about animals that live in extreme environments; they are always some of the coolest creatures around. The fact that icefish can survive without one of the most important proteins in our bodies is crazy. The world is truly full of amazing creatures.