See text for genera.
The notothenioids all lack a swimbladder, and the majority of species are therefore benthic or demersal in nature. 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. 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.
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