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.[2][3] Notothenioids constitute approximately 90% of the fish biomass in the continental shelf waters surrounding Antarctica.[4]
The Southern Ocean has supported fish habitats for 400 million years; however, modern notothenioids likely appeared sometime after the Eocene epoch.[3] This period marked the cooling of the Southern Ocean, resulting in the stable, ice-cold conditions that have persisted to the present day.[3] 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.[3]
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.[5] [6] Their adaptive radiation is characterized by depth related diversification.[3] 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.[3]
They are distributed mainly throughout the Southern Ocean around the coasts of New Zealand, southern South America, and Antarctica.[7] An estimated 79% of species reside within the Antarctic region.[3] 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.[8] 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.[9] Notothenioids have an estimated depth range of about 0–1,500 m (0–4,921 ft).[3]
Notothenioids display a morphology 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.[3] These include the presence of three flat pectoral fin radials, nostrils located laterally on each side of the head, the lack of a swim bladder, and the presence of multiple lateral lines.[3]
Because notothenioids lack a swim bladder, the majority of species are benthic or demersal in nature.[3] However, a depth-related diversification has given rise to some species attaining increased buoyancy, using lipid deposits in tissues and reduced ossification of bony structures.[3] 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.[3]