Impacts of Long-Term High-Temperature and Low-Salinity Stress on the Circadian Rhythms of Antioxidant, Immune, and Endocrine Systems in Turbot (Scophthalmus maximus)

Turbot (<i>Scophthalmus maximus</i>) is an economically vital cold-water fish frequently challenged by summer heat and low salinity. However, the temporal response of physiological circadian rhythms to such long-term stress remains underexplored. This study investigated antioxidant, immune, and endocrine rhythms in turbot acclimated to control (16 °C, 30 ppt), high-temperature (23 °C), and low-salinity (10 ppt) conditions for 30 days. Subsequently, time-series sampling was performed every 4 h for 72 consecutive hours. Under optimal conditions, hepatic superoxide dismutase (SOD), serum alanine aminotransferase (ALT), and melatonin exhibited robust 24 h rhythms. Long-term stress disrupted this homeostasis through divergent mechanisms. Low-salinity stress induced "rhythmic remodeling," maintaining balance via phase shifts or novel infradian (48-72 h) oscillations in thyroid hormones (T3, T4) and ALT, without oxidative damage. Conversely, high-temperature stress triggered "rhythmic collapse," characterized by a loss of daily rhythms in SOD and ALT, sustained inflammation indicated by elevated acid phosphatase (ACP), metabolic depression (suppressed T3), and malondialdehyde accumulation. These findings demonstrate that heat stress poses a more destructive threat to circadian integrity than hyposmotic stress. Consequently, the rhythmic dynamics of ACP, ALT, T3, and T4 are identified as critical indicators of stress status, serving as potential biomarkers for screening stress-tolerant strains for selective breeding.