Notothenioidei is one of 19 suborders of the order

Perciformes Perciformes (), also called the Acanthopteri, is an order or superorder of ray-finned fish in the clade Percomorpha. ''Perciformes'' means " perch-like". Among the well-known members of this group are perches and darters ( Percidae), and als ...

. The group is found mainly in

Antarctic The Antarctic (, ; commonly ) is the polar regions of Earth, polar region of Earth that surrounds the South Pole, lying within the Antarctic Circle. It is antipodes, diametrically opposite of the Arctic region around the North Pole. The Antar ...

and

Subantarctic The sub-Antarctic zone is a physiographic region in the Southern Hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46th parallel south, 46° and 60th parallel south, 60° south of t ...

waters, with some species ranging north to southern

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and southern

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

. Notothenioids constitute approximately 90% of the fish biomass in the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...

waters surrounding

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

Evolution and geographic distribution

The

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

has supported fish habitats for 400 million years; however, modern notothenioids likely appeared sometime after the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch. This period marked the cooling of the Southern Ocean, resulting in the stable, frigid 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. The earliest known notothenioids are the fossils '' Proeleginops'' and '' Mesetaichthys'' from the

La Meseta Formation of Seymour Island, the latter of which already shows close similarities with the extant '' Dissostichus''. 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 Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...

(in the family Channichthyidae) and changes in buoyancy. They are distributed mainly throughout the

around the coasts of

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, southern

, and

. An estimated 79% of species reside within the Antarctic region. They primarily inhabit seawater temperatures between ; however, some of the non-Antarctic species inhabit waters that may be as warm as 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 .

Anatomy

Notothenioids display a

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

that is largely typical of other coastal perciform fishes. They are not distinguished by a single physical trait, but rather a distinctive set of morphological traits. These include the presence of three flat

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

radials, nostrils located laterally on each side of the head, the lack of a

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

, and the presence of multiple

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...

s. Because notothenioids lack a swim bladder, the majority of species are

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

demersal The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer o ...

in nature. However, a depth-related diversification has given rise to some species attaining increased buoyancy, using

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

deposits in tissues and reduced

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

of bony structures. This reduced ossification of the skeleton (observed in some notothenioids) changes the weight and creates neutral buoyancy in the water, where the fish neither sinks nor floats, and can thus adjust its depth with ease.

Physiology

Notothenioids have a variety of physiological and

biochemical Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, ...

adaptations In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

that either permit survival in, or are possible only because of, the generally cold, stable seawater temperatures of the Southern Ocean. These include highly unsaturated membrane lipids and metabolic compensation in enzymatic activity. Many notothenoids have lost the nearly universal heat shock response (HSR) due to evolution at cold and stable temperatures. Many notothenioid fishes 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 non-Antarctic species either produce no or very little antifreeze, and antifreeze concentrations in some species are very low in young, larval fish. They also possess aglomerular kidneys, an adaptation that aids the retention of these antifreeze proteins. While the majority of animal species have up to 45% of

(or other oxygen-binding and oxygen-transporting pigments) in their blood, the notothenioids of the family Channichthyidae do not express any

globin The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myo ...

proteins in their blood. As a result, the oxygen-carrying capacity of their blood is reduced to less than 10% that of other fishes. This trait likely arose due to the high oxygen solubility of the Southern Ocean waters. At cold temperatures, the oxygen solubility of water is enhanced. The loss of hemoglobin is partially compensated in these species by the presence of a large, slow-beating heart and enlarged blood vessels that transport a large volume of blood under low pressure to enhance cardiac output. Despite these compensations, the loss of globin proteins still results in reduced physiological performance.

Systematics

Naming

Notothenioidei was first described as a separate grouping, as a "division" he named Nototheniiformes, by the British ichthyologist Charles Tate Regan in 1913., this subsequently has been considered as a suborder of the Percifomes. The name is based on the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''Notothenia'', a name coined by Sir John Richardson in 1841 and which means "coming from the south", a reference to the Antarctic distribution of the genus.

Families

This classification follows Eastman and Eakin, 2000 and includes references to additional classified species.Froese, Rainer, and Daniel Pauly, eds. (2013). Species of ''Channichthys'' in

FishBase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

. February 2013 version. Most species are restricted to the vicinity of Antarctica. * Genus †'' Mesetaichthys'' Bieńkowska-Wasiluk, Bonde, Møller & Gaździcki, 2013 (mid-late

of Seymour Island) * Genus †'' Proeleginops'' Balushkin, 1994 (

early Eocene In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

of Seymour Island) * Family Bovichtidae

Gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

, 1862 ** Genus '' Bovichtus''

Valenciennes Valenciennes (, also , , ; ; or ; ) is a communes of France, commune in the Nord (French department), Nord Departments of France, department, Hauts-de-France, France. It lies on the Scheldt () river. Although the city and region experienced ...

, 1832 ** Genus '' Cottoperca'' Steindachner, 1875 ** Genus '' Halaphritis''

Last A last is a mechanical form shaped like a human foot. It is used by shoemakers and cordwainers in the manufacture and repair of shoes. Lasts come in many styles and sizes, depending on the exact job they are designed for. Common variations ...

, Balushkin & Hutchins, 2002 * Family Pseudaphritidae McCulloch, 1929 ** Genus '' Pseudaphritis'' Castelnau, 1872 * Family Eleginopsidae Gill, 1893 ** Genus '' Eleginops'' Gill, 1862 * Family Nototheniidae Günther, 1861 ** Genus '' Aethotaxis'' H. H. DeWitt, 1962 ** Genus '' Cryothenia'' Daniels, 1981 ** Genus '' Dissostichus'' Smitt, 1898 ** Genus '' Gobionotothen'' Balushkin, 1976 ** Genus '' Gvozdarus'' Balushkin, 1989 ** Genus '' Lepidonotothen'' Balushkin, 1976 ** Genus '' Lindbergichthys'' Balushkin, 1979 ** Genus '' Notothenia'' Richardson, 1844 ** Genus '' Nototheniops'' Balushkin, 1976 ** Genus '' Pagothenia'' Nichols & La Monte, 1936 P ** Genus '' Paranotothenia'' Balushkin, 1976 ** Genus '' Patagonotothen'' Balushkin, 1976 ** Genus '' Pleuragramma'' Balushkin, 1982 ** Genus '' Trematomus'' Boulenger, 1902 * Family Harpagiferidae Gill, 1861 ** Genus '' Harpagifer'' Richardson, 1844 * Family Artedidraconidae Andriashev, 1967 ** Genus '' Artedidraco'' Lönnberg, 1905 ** Genus '' Dolloidraco'' Roule, 1913 ** Genus '' Histiodraco'' Regan, 1914 ** Genus '' Pogonophryne'' Regan, 1914 * Family Bathydraconidae Regan, 1913 ** Genus '' Acanthodraco'' Skóra, 1995 ** Genus '' Akarotaxis'' DeWitt & Hureau, 1980 ** Genus '' Bathydraco'' Günther, 1878 ** Genus '' Cygnodraco'' Waite, 1916 ** Genus '' Gerlachea'' Dollo, 1900 ** Genus '' Gymnodraco'' Boulenger, 1902 ** Genus '' Parachaenichthys'' Boulenger, 1902 ** Genus '' Prionodraco'' Regan, 1914 (one species) ** Genus '' Psilodraco'' Norman. 1937 ** Genus '' Racovitzia'' Dollo, 1900 ** Genus '' Vomeridens'' DeWitt & Hureau, 1980 * Family Channichthyidae Gill, 1861 ** Genus '' Chaenocephalus'' Richardson, 1844 ** Genus '' Chaenodraco'' Regan, 1914 ** Genus '' Champsocephalus'' Gill, 1861 (two species) ** Genus '' Channichthys'' Richardson, 1844 ** Genus '' Chionobathyscus'' Andriashev & Neyelov, 1978 ** Genus '' Chionodraco'' Lönnberg, 1905 ** Genus '' Cryodraco'' Dollo, 1900 ** Genus '' Dacodraco'' Waite, 1916 ** Genus '' Neopagetopsis'' Nybelin, 1947 ** Genus '' Pagetopsis'' Regan, 1913 ** Genus '' Pseudochaenichthys'' Norman, 1937