The icefish: life without respiratory pigments

Wherever you look for life on Earth, you find it. From free-floating bacteria in the upper atmosphere to tiny organisms that make their home deep within the solid crust of its surface. Animals have been found thriving in deserts hot enough to cook them alive and, similarly, have been recorded living abundantly in environments that are cold enough to freeze their bodies solid!

Living in such hostile conditions places huge strains on the organisms that live there, which, as a necessity, have had to evolve specialised physiological adaptations if they are to survive for long enough to reproduce and pass on their genes. There are many examples of extreme and unique adaptations for this purpose, with those of the Antarctic icefish being among the most bizarre.

Icefish are found throughout the cold waters that surround Antarctica and South America, belonging to the Channichthyidae family - a small group of fish that have no respiratory pigments in their blood. 

Rather uniquely among species of animals, Antarctic icefish do not rely on respiratory pigments to carry oxygen in their blood and lack them altogether! Such respiratory pigments were once believed to be essential for multicellular life to exist since they chaperone oxygen so that it can be removed from the air and absorbed by the blood, where it is at a much higher concentration than in the surrounding atmosphere. This is an important 'law' that life must overcome if it is to grow bigger than a single-celled organism because molecules naturally diffuse from an area of high concentration to one of a low concentration and not the other way around! So, simply put, without respiratory pigments the oxygen content of blood would be too low to fuel life as we know it!

Thus, the vast majority of multi-celled organisms have blood that is packed with respiratory pigments. These pigments are usually constructed from 1 of 2 key metals, with which one being used depending on an organism's evolutionary history and can be used to synthesise a number of different pigments:

• Iron, which is the most common basis of respiratory pigments, can be used to produce:
• Haemoglobin, which is found in humans and turns red when oxgenated.
• Hemethryins, which are found in terrestrial worms and brachiopods, and turn violet when oxygenated.
• Chlorocruorin, which is found in aquatic worms and turns green when oxygenated.
• Copper, which is used to make hemocyanins. Hemocyanins are blue when oxygenated and are usually found in families of molluscs and arthropods.

The fact that icefish lack such respiratory pigments is puzzling at first glance, but actually makes sense when it is considered carefully since it provides the fish with a huge advantage in their ability to survive in the harsh cold of Antarctica. Basically this advantage stems from the fact that the colder water is, the more oxygen can dissolve in it. Thus, the frigid Antarctic waters carry much more oxygen than warmer waters do so icefish breathe in more oxygen with each 'breath'. This, coupled with the fact that they have twice as much blood fluid in their body than another species of fish their size, means that they can still provide their muscles with enough oxygen to work effectively.

But why is this advantageous to their survival? Well, the fact that icefish can supply enough oxygen to their muscles in order to survive without respiratory pigments means that they do not have to make any; saving them huge amounts of energy each year that can be used for more useful tasks instead, such as feeding, reproducing and evading predators!

Furthermore having no respiratory pigments in their blood decreases its overall viscosity by about 25%, which means that their heart has to work far less hard to pump blood around their body than it otherwise would have. In addition to placing less strain on the organ so they can live for much longer before it gives out, having thinner blood also reduces their energy expenditure and allows them to move 4 times more blood which each beat than a typical fish their size - saving even more energy!

Thus, the icefish has evolved to possess one of nature's most bizarre and unprecedented adaptations that allows it to thrive in one of the most extreme environments on Earth! Without this adaptation, or another that gave it a similar advantage, the strange fish would have undoubtedly died out long ago, being unable to to survive and reproduce in the freezing waters of the Antarctic...