The Antarctic and its bordering waters  exhibit unique species adapted against the freezing water temperatures, one of which being  is the Antarctic crocodile ice fish. The species is classified under the family called Channichthyidae, comprising the group known as Notothenioidi. Which is broken up into several species, that are unable to produce haemoglobin. Except for all but one subspecies, the Neopagetopsis ionah, which is a result of mutation in the evolution of the species. Most species inhabit the Antarctic and South American shorelines, making up 90% of the fish populations. The species based in the Antarctica continental shelf, withstand water temperatures as below as -2 degrees.

Figure 1, Three individuals of the Antarctic crocodile icefish.

The crocodile shaped head and snout of the icefish shown in figure 1, is the morphological feature which gives the classification for its name. Primarily a predator of other fish, it has been known to eat crustaceans. It uses the element of surprise to ambush and capture its prey. With one meal being able to suffice for a long period of time. It consumes individuals double its body size and its low metabolic rate, helps it can slowly digest its prey. Spreading the food resource for the duration of time between prey capture, is one part of the species biological adaptions to survive their cold environment.


The first line of defence against the cooler temperatures is thermoregulation, a biological adaption within the cells of the organism. The production of proteins known as Antifreeze glycoprotein prevent the blood of the icefish freezing at temperatures past -0.8 C, which typically the blood of other fish would freeze. The component of glycoproteins prevents freezing by attaching to the ice crystals that form on the surface of the fish’s skin and prevents the development of further crystallisation by isolating the ice nuclei. The video below shows the function of antifreeze combating the crystallisation of ice on the body of a fish. The lack of haemoglobin proteins is just one biological component missing from the species body. Others include having no swim bladder organ and a light lighter skeletal structure compared to most fish species. Their light skeletal structure aids a build-up of lipid deposits, which they use to regulate weight mass to control buoyancy at different depths. The discoloration of the species body is a due to the lack of haemoglobin, but also having no scales. giving their pale appearance.

What is haemoglobin?

Haemoglobins are the red blood cells apparent in most species bodies including ourselves, which aid in the transport of oxygen around the bodies bloodstream. To survive without haemoglobins a organism would have to live in an environment with high levels of oxygen concentrations. Luckily for the crocodile icefish, the waters surrounding Antarctica within its continental shelf, contain large concentrations of dissolved oxygen due to the solubility of the compound in cooler conditions. The species only can transport less than 10% of oxygen within it bloodstream at one given time. An underwhelming transport rate compared to that of other species that have higher haemoglobin levels.

The icefish absorbs oxygen through the surface of its skin. Allow lacking the proteins of myoglobin, which consist of iron and oxygen binders within the muscle tissue of most species. The only presence of myoglobin around the heart of the species which is significantly larger then the average fish species heart, due to the volume of blood they pump and large intake of oxygen absorbed directly from the skin into the bloodstream.  The video below enables a breakdown of the structure and function of the compound within red blood cells and why it is a key factor for most like ourselves.

How did the species lose its Haemoglobin?

The loss of the ability to produce haemoglobin or reduced production rates of the cell, was thought to have originated through natural selection, environmental pressures pushing the the species to adapt to the colder conditions of the Antarctic. Trying to adapt with increased blood thickness and having to pump larger volumes of blood around the body. Making the lack of ability to produce haemoglobin or myoglobin, proving to be disadvantageous due it to the high energy levels consumed, in terms of volume of blood pumped from the heart and the inability to carry larger quantities of oxygen with its bloodstream. It is the result of extreme pressure from its environment causing the species to evolve to survive the Antarctic waters.


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