In some scientific literature, the term "cod icefish" is used to identify members of this family. This should not be confused with the term "icefish," which refers to the "white-blooded" fishes of the family Channichthyidae. See Icefish (disambiguation).
The notothens or cod icefishes are the familyNototheniidae of acanthopterygianfishes. They are traditionally placed in the order Perciformes together with their relatives, but like every lineage in the order, actual phylogenetic relationships among these species have not yet been determined with certainty.
Notothens are largely found in the Southern Ocean and are particularly abundant off the shores of Antarctica. As the dominant Antarctic fish taxa, they occupy both sea-bottom and water-column ecological niches. Although lacking a gas bladder, they have undergone a depth-related diversification, such as increased fatty tissues and reduced mineralization of the bones, resulting in a body density approaching neutral, to fill a variety of niches. The spleen may be used to remove ice crystals from circulating blood. As the chilly Antarctica and sub-Antarctic waters of the Southern Ocean average −1 to 4 °C (30–39 °F), most species of these regions produce antifreeze glycoproteins to prevent the formation of ice crystals in blood and other body fluids.
Some species exhibit polymorphism, for example, the circum-Antarctic Trematomus newnesi exists as two morphs in the Ross Sea, the typical morph and a large-mouthed/broad-headed morph.
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