Notothenioidei


Notothenioidei is one of 19 suborders from the order Perciformes and that primarily includes Antarctic fish and Subantarctic fish, but also a few species ranging north to southern Australia and southern South America. These species, which are referred to collectively as the notothenioids, account for approximately 90% of the fish fauna biomass in the continental shelf waters surrounding Antarctica.

Evolution and geographic distribution

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 present day, excepting abrupt, rapid warming in the region in recent years. Another key factor in the evolution of notothenioids is the preponderance of the Antarctic Circumpolar Current, 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. The adaptive radiation of the notothenioids is characterized by depth related diversification. Comparison studies between non-Antarctic and Antarctic notothenioids have revealed different ecological processes and genetic differences between the two groups of fish, such as the loss of hemoglobin and changes in buoyancy.
The notothenioids are distributed mainly throughout the Southern Ocean around the coasts of New Zealand, southern South America, and Antarctica. An estimated 79% of notothenioids reside within the Antarctic region. The notothenioids primarily inhabit seawater temperatures between −2 and 4 °C ; however, some of the non-Antarctic species inhabit waters that may be as warm as 10 °C around New Zealand and South America. Seawater temperatures below the freezing point of freshwater are possible due to the greater salinity in the Southern Ocean waters. Notothenioids have an estimated depth range of about 0–1,500 m.

Anatomy

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. 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.
Because notothenioids lack a swim bladder, the majority of species are benthic or demersal in nature. 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. This reduced ossification of the skeleton 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 adaptations 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 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.
While the majority of animal species have up to 45% of hemoglobin in their blood, the notothenioids of the family Channichthyidae do not express any globin 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.

Classification

This classification follows Eastman and Eakin, 2000 and includes references to additional classified species. Except where noted, species are restricted to the vicinity of Antarctica.