Crocodile Icefish - SEL

Crocodile icefish or Channichthyidae belong to the suborder Notothenioidei (with a total of 122 species). There are 25 identified species of Channichthyidae which are found in South America and Antarctica's continental shelf waters where temperatures can drop to -1.9°C.

They lack swimbladders which are internal gas filled organs that help fish to control buoyancy. However, to keep at a constant depth, some species have lipid deposits within tissues in order to alter its weight. Channichthyidae have no haemoglobin in their blood resulting in its transparent colour however evidence of haemoglobin genes remain in their genome. Oxygen diffuses directly into blood plasma. They have low metabolic rates and due to the freezing waters, oxygen has a high solubility. They also have larger blood vessels to carry higher volumes of blood, low viscosity blood and bigger, specialised hearts. For example, spongy ventricles instead of coronary arteries to allow oxygen to be directly absorbed from blood as it is pumped.

Thermoregulation
The temperatures of Antarctic waters vary between 2°C and -2°C throughout the year. Channichthyidae have antifreeze proteins (AFPs) which bind to small ice crystals to inhibit growth and recrystallisation of fatal ice in their blood. Blood of typical fish would freeze at -0.8°C. AFPs do not bind to liquid water as this would cause the fish to dehydrate and die but instead bind to solid ice nuclei. Many AFPs have hydrophobic amino acids on their surface (in contrast to many globular biological proteins), these attach to gaps in the interlocking hexagonal structure of ice. This structure distinguishes solid H2O from liquid H2O.

The icefish's lack of haemoglobin was thought to be an adaptation to the extreme cold but research now reveals that the lack of haemoglobin is a disadvantage (although not lethal). The loss of red blood cells lowers the viscosity of the blood and the high solubility of oxygen reduces the demand for haemogobin. But, icefish have to spend twice as much energy circulating blood as other fish (amongst other compensations).

Commercial Use
AFPs could be used to increase freezing tolerance of crop plants and extend harvest in colder climates. They may also be used in hypothermia therapy and used in the preservation of resources from transplant organs to frozen foods.