We are interested in the cellular mechanisms that support wide ranging latitudinal distribution of marine ectotherms and have studied a new facet of thermal tolerance in zoarcids fish, from different climatic regions. The common eelpout, Zoarces viviparus, from the North Sea was compared to the Antarctic eelpout, Pachycara brachycephalum, with respect to cellular oxidative stress parameters and the potential to induce metabolic reorganization during functional hypoxic stress. Functional hypoxia sets on as the consequence of critical warming in marine ectotherms, and we hypothesized that survival of thermal stress involves hypoxic signalling to induce re-organisation of blood supply to central tissues.It turned out very clearly that polar eelpout maintain higher antioxidant defence levels (antioxidant enzyme activities and 3 fold higher levels of liver glutathione) to counterbalance higher tissue oxygen solubility and to protect their susceptible membranes from oxidative damage. Both species have normoxic levels of hypoxia inducible transcription factor (HIF-1α). Acclimation (11 weeks) at above habitat temperature (5°C) in the polar eelpout and at low temperature of 6°C within the natural range of North Sea eelpout led to increased HIF-1 DNA binding and dimer formation. In the talk, we will discuss new concepts of the possible function of hypoxia inducible factor in stress response of marine animal ectotherms.