Fish of the perciform, suborder Notothenioidei includes eight families with 43 genera and 122 species. Ninety-six species inhabit Antarctic waters and 26 are non-Antarctic. They dominate the cold shelf waters surrounding the Antarctic continent. They comprise 45% of the Antarctic fish fauna. However in many areas of the Antarctic shelf, including the highest latitudes, notothenioids make up 77% of the species and 90–95% of fish abundance and biomass.

Typically variation in the barbel, a key taxonomic character, separates the species. Hence, they are erroneously identified as cods (Gadidae). Unlike cods, they lack swim bladders. As the dominant Antarctic fish species, they occupy both sea bottom and water column ecological niches. Although lacking a swimbladder, they have undergone a depth-related diversification, such as increase in fatty tissues and body density approaching neutral, to fill a variety of water column niches. As the chilly subantarctic waters averages –1 to –4 degrees Celsius, Antarctic species have anitfreeze proteins in their blood.

As Antarctica is surrounded by currents which kept the frigid waters separate from the world's oceans, this trait favours the Notothenioidei to the virtual exclusion of other fish species which are not adapted to the subzero water temperatures. Some species also exhibit morphism, for example, the circum-Antarctic nototheniid Trematomus newnesi exists as two morphs in the Ross Sea, the typical morph and a large mouth/broad headed morph.

As the major fish resource in the Southern Ocean, notothenioids are under increasing pressure from commercial fishing.