Antarctic icefishes have evolved extreme phenotypes due to their life in the chronic cold of the Southern Ocean, including the loss of hemoglobin and subsequent oxygen stress. Even after this extreme specialization, a single icefish species, Champsocephalus esox, escaped the Southern Ocean and inhabits temperate waters off the South American coast. This talk will examine the genomes of icefishes — both nuclear and mitochondrial — comparing C. esox to its still cold sister species, C. gunnari, using population and comparative genomics.