Temperature as a key environmental factor shapes the physiology of ectothermic marine animals and thereby their biogeography and mode of life in various climates and ecosystems. Our research addresses the genetic underpinning of climate dependent physiological and ecological functions. Molecular mechanisms defining thermal limits as well as acclimation or adaptation to temperature are being studied at the level of individual proteins or protection mechanisms. Since thermal windows of individual molecules largely exceed the thermal limits of the whole animal, thermal limits are most probably set at the level of integration of molecular into functional units and networks at higher levels of organisation. Therefore we also aim to investigate how molecular functions contribute to climate sensitivity as well as acclimation and adaptation processes from a whole organism point of view. Evolutionary adaptation to various climates is addressed in comparative studies carried out with populations of the same species in climatic gradients or with congeneric species living in different climatic zones. For example, member species of the fish family Zoarcidae (eelpouts) inhabit temperate, subpolar and polar waters and represent a model system for the study of evolutionary adaptation versus seasonal acclimatisation to temperature. Our previous studies have demonstrated the high conservation level of functional genes in different eelpouts from boreal and Antarctic waters, corroborating that these species are excellent models in comparative functional genomics studies.