They suggested ND6 modifications may improve protein conformation and therefore complex I subunit interactions at subzero temperatures, and/or a role in modulating mitochondrial complex I redox potential and reactive oxygen species production. As ROS production has been attributed to complex I, the down regulation of complex I activity under thermal stress could alter reactive oxygen species production. The functional consequences of positioning and the structural change in nototheniid ND6 have never been analysed, and therefore form the central focus of this study. We addressed the function and thermal sensitivities of the ETS with particular emphasis on the relative contributions of complex I and complex II and the enzymatic capacities of selected proteins in two Antarctic nototheniids. At different temperatures from 0 to 15uC, we measured the oxygen consumption rates and membrane potential in isolated liver mitochondria respiring on several substrates. In addition, we determined enzymatic capacities within membrane fraction enriched protein extracts. We further compared the amino acid content of eight notothenioid and non-notothenioid fish species from sub-Antarctic, temperate, tropical and Arctic waters in order to identify possible differences that may provide notothenioid ND6 with unique biochemical properties and underline their adaptation to the cold. As behavioural and morphological differences between species can also relate to mitochondrial plasticity and capacities, we compared two endemic and sympatric Antarctic nototheniid species from King George Island, Notothenia coriiceps and Notothenia rossii. Both species have a wide circum-Antarctic distribution, particularly in shelf areas of the Scotia Arc, extending to the Antarctic continental shelf in the case of N. coriiceps. Functional capacities may differ according to lifestyle requirements, which in turn contribute to defining the width of the thermal tolerance window. The two selected species show different adaptations to life in the water column or in benthic habitats, in line with their respective external morphologies: N. coriiceps is demersal and sedentary, undergoes winter dormancy AZD2281 associated with metabolic suppression, and feeds mainly on benthic organisms. N. rossii is semipelagic and migratory, and in addition feeds on water column prey during the summer months. The differences in hepatosomatic indices and condition factors, both of which were higher in N. coriiceps, relate to the morphological and physiological differences consistent with the differential adaptations to inhabit the water column of the benthic N. coriiceps and the bentho-pelagic N. rossii. This becomes evident by a greater density, expressed as mean percentage buoyancy of N. coriiceps over N. rossii. N. coriiceps is a heavy rugged fish, and field observations with underwater cameras showed that it is a rather inactive GDC-0449 sit-and-wait predator.