Diel coupling between drinking rate and oxygen consumption in Gymnocypris przewalskii under saline-alkaline stress

Fish in saline-alkaline water drink to support osmoregulation and acid-base balance, but the timing and whole-animal metabolic expression of this process remain unclear. We used an in vivo system to record drinking rate and mass-specific oxygen consumption (MO₂) on a shared time base in conscious Gymnocypris przewalskii exposed to saline-alkaline lake water (salinity 15.5 g·L^-1, pH 9.2). In cannulated fish, both drinking rate and MO₂ were higher at night than during the day and showed a broadly nocturnal rhythmicity. In non-cannulated controls, hourly MO₂ profiles descriptively suggested a larger nighttime elevation in lake water than in freshwater. Across matched hourly observations, a random-intercept linear mixed-effects model showed that the drinking rate-MO₂ relationship differed between daytime and nighttime. The estimated drinking rate-MO₂ association appeared stronger at night. Exploratory intestinal peptidomics at two time points (09:00 and 21:00) showed time-dependent peptide remodeling, with oxidative phosphorylation and metabolic pathways as the main up-regulated enriched categories in the nighttime period. Together, these findings indicate a positive diel association between drinking and oxygen consumption in G. przewalskii under saline-alkaline conditions and are consistent with greater nighttime physiological demand associated with drinking.