The hemoglobins of polar fish: evolutionary and physiological significance of multiplicity in Arctic fish

In fish, haemoglobin adaptations govern survival and biodiversity under specific environmental conditions and physiological requirements. The study of haemoglobins is aimed at identifying key links between molecular and ecophysiological adaptations. Differences in haemoglobin properties arise from the interaction of the heme with specific residues in the primary structure, but modulation in regulatory processes and altered protein synthesis may be additional adaptations in response to environmental changes. Fish experience variations in oxygen availability, salinity, ionic composition, pH and temperature. Compared to the dominant suborder Notothenioidei in Antarctica, many Arctic species possess multiple haemoglobins with different functional properties, which might be selectively and differentially expressed to secure oxygen unloading in peripheral tissues including swim bladder and retina, suggesting a mechanism of labour sharing. Haemoglobin polymorphism may further increase the repertoire of oxygen-binding tetramers, as documented in Atlantic cod widely distributed in Arctic and temperate North Atlantic waters. This ecologically and economically important species copes with fluctuating physico-chemical conditions affecting oxygen availability, demand and binding properties by displaying novel regulatory and structural polymorphisms in the main 1 subunit. The tetraploid salmonid genome harbours more and globin genes than any other teleost examined, and thermal stress activates several subunits in Arctic charr, the most northern-living freshwater fish. Thus, the success of Arctic fish seem governed by evolutionary adaptation to waters having variable low temperatures and by plasticity to environmental and human-induced habitat changes. Current knowledge of the molecular and functional diversity of haemoglobins in Arctic fish is summarised and compared with Antarctic fish in this review.