Notothenioidei is one of 19 suborders of the order Perciformes. The group is found mainly in Antarctic and Subantarctic waters, with some species ranging north to southern Australia and southern South America. Notothenioids constitute approximately 90% of the fish biomass in the continental shelf waters surrounding Antarctica.
The Southern Ocean has supported fish habitats for 400 million years; however, modern notothenioids likely appeared sometime after the Eocene epoch. This period marked the cooling of the Southern Ocean, resulting in the stable, ice-cold conditions that have persisted to the present day. Another key factor in the evolution of notothenioids is the preponderance of the Antarctic Circumpolar Current (ACC), a large, slow-moving current that extends to the seafloor and precludes most migration to and from the Antarctic region.
These unique environmental conditions in concert with the key evolutionary innovation of Antifreeze glycoprotein promoted widespread radiation within the suborder, leading to the rapid development of new species. Their adaptive radiation is characterized by depth related diversification. Comparison studies between non-Antarctic and Antarctic species have revealed different ecological processes and genetic differences between the two groups of fish, such as the loss of hemoglobin (in the family Channichthyidae) and changes in buoyancy.
They are distributed mainly throughout the Southern Ocean around the coasts of New Zealand, southern South America, and Antarctica. An estimated 79% of species reside within the Antarctic region. They primarily inhabit seawater temperatures between −2 and 4 °C (28 and 39 °F); however, some of the non-Antarctic species inhabit waters that may be as warm as 10 °C (50 °F) around New Zealand and South America. Seawater temperatures below the freezing point of freshwater (0 °C or 32 °F) are possible due to the greater salinity in the Southern Ocean waters. Notothenioids have an estimated depth range of about 0–1,500 m (0–4,921 ft).
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The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-smallest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian oceans but larger than the Arctic Ocean. Since the 1980s, the Southern Ocean has been subject to rapid climate change, which has led to changes in the marine ecosystem.
The Antarctic silverfish (Pleuragramma antarctica), or Antarctic herring, is a species of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefishes. It is native to the Southern Ocean and the only truly pelagic fish in the waters near Antarctica. It is a keystone species in the ecosystem of the Southern Ocean. While widely distributed around the Antarctic, the species appears to have largely disappeared from the western side of the northern Antarctic Peninsula, based on a 2010 research cruise funded by the National Science Foundation under the US Antarctic Program.
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). Nototheniidae, the notothens or cod icefishes, is a family of ray-finned fishes, part of the suborder Notothenioidei which is traditionally placed within the order Perciformes. They are largely found in the Southern Ocean.
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