Contrasting cellular energy responses to regular and stochastic daily thermal oscillations in two populations of Nodipecten subnodosus scallops.

Understanding how thermal variability influences marine organisms' resilience to hyperthermia is crucial for assessing climate change vulnerability. This study investigated the effects of daily thermal variability on cellular responses and energy status in the mantle of the lion's paw scallop (Nodipecten subnodosus). Scallops from two regions in Mexico with contrasting thermal regimes-Bahía de Los Ángeles (BA) in the Gulf of California (high variability) and Ojo de Liebre Lagoon (OL) on the Pacific coast (low variability)-were exposed for 15 days to three thermal regimes: constant temperature (21 ± 2.87 °C), regular oscillation (17-25 °C circadian fluctuations, average 21.05 °C), and stochastic oscillation (random shifts between 17 °C and 25 °C, average 20.94 °C). After exposure, scallops faced an acute thermal challenge (28 °C for 17 h). Scallops under regular oscillations decreased their adenylate energy charge (AEC) accompanied by a drop in the arginine phosphate pool and an increase of ADP/ATP ratio after the acute thermal challenge, whereas animals under constant temperature and stochastic oscillation kept the cellular status unaltered. Differences in the activity of the enzymes citrate synthase and mitochondrial complex III between populations before the exposure to each thermal regime highlight metabolic differences between populations but these were not maintained after exposure to each thermal regime. The findings suggest that frequent exposure to near-upper thermal limits (28-31 °C) in regular oscillation regimes disrupts energy homeostasis during subsequent heat stress. This highlights the role of thermal variability in shaping metabolic resilience and potential vulnerabilities of N. subnodosus to climate-induced temperature extremes.