Antarctic icefish belong to the perciform suborder Notothenioidei and are the largely endemic, dominant fish taxa in the cold continental shelf waters surrounding Antarctica. At present, the suborder includes eight families with 43 genera and 122 species. Although the Southern Ocean is relatively well sampled, new species of notothenioids are still being described.
The majority of notothenioids live at seawater temperatures between -2 and 4 ºC (28 and 39 ºF), but some subpolar species inhabit waters that may be as warm as 10 ºC (50 ºF) around New Zealand and South America. Seawater temperatures below the freezing point of fresh water (0 ºC or 32 ºF) are possible due to the dissolved salts. Notothenioids have a depth range of about 0-1,500 m (0-4,921 ft).
The notothenioids are a relatively rare example of a marine fish species flock.
Members of the family Channichthyidae (crocodile icefish), which is part of Notothenioidei, are the only known vertebrates that do not produce red blood cells and consequently their blood lacks the normal red colour. Other families in the suborder, such as the Nototheniidae (cod icefish), do have red blood cells and studies comparing the two families have revealed genes needed to make these.
The notothenioids lack a swimbladder, and the majority of species are therefore benthic or demersal in nature. However, a depth-related diversification has given rise to some species attaining increased buoyancy, using lipid deposits in tissues and reduced ossification of bony structures. Reduced ossification of the skeleton in the notothenioids changes their weight and henceforth has created a sort of neutral balance in the water, where the notothenioid neither sinks nor floats, and can thus adjust depth with ease.
Notothenioids have evolved a variety of interesting physiological and biochemical adaptations that either permit survival in, or are possible only because of, the generally cold, stable seawater temperatures of the Southern Ocean. Many notothenioid fish are able to survive in the freezing, ice-laden waters of the Southern Ocean because of the presence of an antifreeze glycoprotein in blood and body fluids. Although many of the Antarctic species have antifreeze proteins in their body fluids, not all of them do. Some subpolar species either produce no or very little antifreeze, and antifreeze concentrations in some species are very low in young, larval fish.
While the majority of animal species have up to 45% of hemoglobin (or other oxygen-binding and oxygen-transporting pigments) in their blood, the notothenioids of the family Channichthyidae have only 1%. They can still flourish in part because of the high oxygen content of the cold waters of the Southern Ocean and in part because oxygen is absorbed and distributed directly by the plasma. These fish must expend twice as much energy in cardiac output per second than the notothenioids with higher hemoglobin concentration. At a cold temperature, oxygen solubility is greatly increased.