The human sex determination system is shared across mammals and likewise, all birds likely share the same major sex determining gene. In contrast, the molecular mechanisms of sex determination in non-avian reptiles, amphibia, and fish varies, sometimes even within a species. In zebrafish, domesticated laboratory strains lack a strong genetic sex determining locus, but zebrafish in nature possess a single strong sex-determining locus. All domesticated zebrafish individuals first develop an ovary, but then about half the juvenile population switches sex to become males in the absence of a distinguishable genetic difference. The question is: what mechanisms cause sex determination in the two recently diverged zebrafish populations? In zebrafish, some other fish, some amphibia, and unfeathered reptiles, temperature regulates sex determination. Temperature is unlikely to be a major driver of sex determination in Antarctic fish, however, because the water temperature has remained unchanged for millions of years and the major clade of Antarctic fish has many different sex chromosomes. The question is: how did sex determination mechanisms evolve as the Southern Ocean cooled to almost 2°C below zero?