Integrating antioxidant responses and oxidative stress of ornamental discus (Symphysodon spp.) to decreased temperatures: Evidence for species-specific thermal resistance

Abstract

Discus (Symphysodon spp.) is among the most popular ornamental fish for warm-water and species that tolerates low temperature should be preferred by discus farmers due to costs in heating aquaria. Here, we compared the antioxidant responses and oxidative stress to decreased temperatures in gills of two discus species (S. haraldi and S. aequifasciatus), by employing a continuous temperature change program, i.e., cooling (28 °C to 14 °C; −1 °C/h), cold maintenance (14 °C for 12 h) and recovery (14 °C to 28 °C; +1 °C/h). Heatmap showed distinct activities or levels of oxidative stress biomarkers in response to thermal stress between two discus species. Except for reduced glutathione content, indicators regarding antioxidant enzymatic activities (e.g., superoxide dismutase, glutathione reductase and glutathione peroxidase) and lipid peroxidation level were generally higher in S. aequifasciatus than in S. haraldi. Subordinate function analysis integrating oxidative stress biomarkers suggested that the thermal resistance of S. haraldi was in the order of cooling > cold maintenance ≈ recovery, but that of S. aequifasciatus was in the order of cold maintenance > cooling > recovery. Specifically, the lowest thermal resistance was observed in S. aequifasciatus during cold recovery, potentially indicating a more severe oxidative damage in terms of lipid peroxidation in S. aequifasciatus than in S. haraldi. These results suggest that S. haraldi had a stronger thermal resistance than S. aequifasciatus, exhibiting a significant interspecific variability under acute thermal stress.