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The recognition of the important role of the polar habitats in global climate changes has awakened great interest in the evolutionary biology of polar organisms. They are exposed to strong environmental constraints, and it is important to understand how they have adapted to cope with these challenges and to what extent adaptations may be upset by current climate changes. We present an introductory overview of the evolution of the Antarctic fish fauna with emphasis on the dominant perciform sub-order Notothenioidei, as well as some specific comments on the biogeography of the three phyletically basal notothenioid families. The wealth of information on the ecology and biodiversity of the species inhabiting high-Antarctic and sub-Antarctic regions provides a necessary framework for better understanding the origin, evolution and adaptation of this unique group of fish. Notothenioidei offer opportunities for identification of the biochemical characters or the physiological traits responsible for thermal adaptation. The availability of phylogenetically related taxa in a wide range of latitudes has allowed to look into the molecular bases of environmentally driven phenotypic gain and loss of function. In the process of cold adaptation, the evolutionary trend of notothenioids has produced unique specialisations, including modification of hematological characteristics, e.g. decreased amounts and multiplicity of hemoglobins. The Antarctic family Channichthyidae (the notothenioid crown group) is devoid of hemoglobin. This loss is related to a single deletional event removing all globin genes with the exception of the inactive 3' end of adult alpha-globin. In reviewing hemoglobin structure, function and phylogeny, the evolution of the fish Root effect is analysed in detail. Adaptation of the oxygen-transport system in notothenioids seems to be based on evolutionary changes involving levels of biological organisation higher than the structure of hemoglobin.

Given the unique thermal history of the Antarctic continent, fishes of dominant suborder Notothenioidei offer a remarkable opportunity to study the physiological and biochemical characters gained and, conversely, lost during their evolutionary history and to map this information on the species phylogenetic trees. The availability of phylogenetically related notothenioid taxa living in a wide range of latitudes (in the Antarctic, sub-Antarctic and temperate regions) allows to look into the molecular bases of environmentally driven gene birth and death. This evolutionary perspective has also been supported by comparison of some features of the hemoprotein devoted to the oxygen transport in fish species living in the other polar region, the Arctic. The Arctic and Antarctic marine ichthyofaunas differ by age and isolation. Fishes of the two polar regions have undergone different regional histories which have engineered the physiological diversities, so that Antarctic fish are much more stenothermal than Arctic ones. Understanding the mechanisms of phenotypic response to cold exposure in species living at different latitudes in polar habitats offers fundamental insights into environmental adaptations. This review aims at surveying the current knowledge of molecular structure, functional features, phylogeny and adaptations of the hemoglobin of fish thriving in the Antarctic, sub-Antarctic and Arctic regions (with some excursions in the temperate latitudes). Investigating the evolutionary adaptations of hemoglobins to these environments can provide new insights into adaptation currently studied merely in temperate organisms, and can shed light into the convergent processes that evolved in response to thermal adaptations.