Gradual warming reshapes respiratory metabolism and metabolic responses in the clam Mactra veneriformis

Increasing frequency and intensity of marine heatwaves pose growing challenges to intertidal bivalves. This study investigated the effects of gradual temperature elevation on respiratory metabolism and associated physiological responses in the clam Mactra veneriformis. Oxygen consumption rate (OCR), ammonia excretion rate (AER), gill histology, Na⁺/K⁺-ATPase activity, and metabolomic profiles were examined at 17-28 °C. OCR increased significantly with temperature (p < 0.05), suggesting elevated aerobic metabolic demand. In contrast, AER showed a unimodal pattern, increasing to a peak at 26 °C and declining at higher temperatures, possibly reflecting physiological constraints under thermal stress. O:N ratios remained above 7, indicating a mixed utilization of metabolic substrates with a greater contribution of lipids and carbohydrates. Gill structure remained intact at lower temperatures but exhibited looser architecture at 26-28 °C, suggesting structural adjustment. Na⁺/K⁺-ATPase activity increased progressively with temperature, indicating higher ion-transport demand. Metabolomic analysis identified 2426 metabolites, with differential metabolites mainly enriched in lipid-related pathways. Overall, gradual warming induced coordinated physiological and metabolic responses in Mactra veneriformis, providing an integrated perspective on its response to thermal stress.