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Sensitivity in the antioxidant system of discus fish (Symphysodon spp.) to cold temperature: evidence for species-specific cold resistance

The discus fish (Symphysodon spp.) is an endemic species of the Amazon that is among the most popular ornamental fish around the world, and is usually used as the model animal for studying the diversification of Amazon fish. Here, a comparative analysis of two species of discus fish, i.e., S. haraldi and S. aequifasciatus, based on several antioxidant indexes was conducted, to test the hypothesis that cold resistance might correlate with the diversification of discus fish. We set up a continuous sequence of three temperature programs, namely 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). Subordinate function (SF) combined with principal component analysis (PCA) showed that the cold hardiness of S. haraldi during cold treatment was in the order of cooling > cold maintenance ≈ recovery, but the cold hardiness of S. aequifasciatus during cold treatment was in the order of cold maintenance > cooling > recovery. Specifically, the lowest cold hardiness was observed in S. aequifasciatus during recovery, indicating that cold stress resulted in more seriously oxidative stress in S. aequifasciatus than in S. haraldi. Overall, these results show a significant interspecific variation, indicating the correlation between environmental adaptation and the diversification of discus fish.