The rapid increase in critical thermal maximum (CTmax) in fish (or other animals), previously exposed to critically high temperature is termed ‘heat hardening’, which likely represents a key strategy to cope with increasingly extreme environments. The physiological mechanisms that determine acute thermal tolerance, and the underlying pathways facilitating heat hardening, remain debated. It has been posited, however, that exposure to high temperature is associated with tissue hypoxia and may be associated with increased expression of hypoxia-inducible factor-1 (Hif-1). We studied acute thermal tolerance in zebrafish lacking functional Hif-1α paralogs (Hif-1aa and Hif-1ab double knockout; Hif-1α-/-), which are known to exhibit markedly reduced hypoxia tolerance. We hypothesised that Hif-1α-/- zebrafish would suffer reduced acute thermal tolerance relative to wild-types and that the heat hardening ability would be lost. On the contrary, we observed that Hif-1α-/- and wild-type fish did not differ in CTmax, and both genotypes exhibited heat hardening of a similar degree when CTmax was re-tested 48 h later. Despite exhibiting impaired hypoxia tolerance (based on loss of equilibrium experiments), Hif-1α-/- zebrafish display unaltered thermal tolerance, suggesting that these traits are not necessarily functionally associated. Hif-1α is accordingly not required for short-term acclimation in the form of heat hardening.

Critical thermal maximum (CTmax) trials in wild-type and Hif-1a knockout zebrafish. CTmax2 was conducted 48 h after CTmax1. Hypoxia tolerance determined as time to loss of equilibrium during hypoxia expsoure.