Combined climate stressors constrain various mechanisms for thermal tolerance in the scallop Pecten maximus.

Unfavourable climatic conditions challenge an animal's performance and fitness. We investigated how cellular homeostasis relates to whole-animal physiology in the marine bivalve Pecten maximus under warming (W), warming plus hypercapnia (WHc) or hypoxia (WHo), and the combination of all three drivers (deadly trio, DT). Starting at 14°C, temperatures were increased stepwise by 2°C per 48 h while gill tissue was sampled from experimental exposures to test for indicators of intracellular stress, including lipid peroxidation, protein damage and degradation, apoptosis and heat shock responses. Whole-animal water filtration rate, routine metabolic rate (RMR), haemolymph PO2, pseudofaeces ejection, mantle tissue intracellular acidosis and gill tissue antioxidative capacity were measured in W and DT exposures. Filtration peaked at a lower temperature under DT, when high pseudofaeces ejection suggested that ventilation was prioritized over feeding. Warming alone doubled RMR by 22°C, whereas DT increased RMR even further, reaching higher maxima by 20°C. Haemolymph PO2 was consistently lower under DT, implying that supply was less able to meet increasing demand. Warming to 26°C stimulated a gill tissue heat-shock response, accumulated ubiquitin conjugates and apoptosis, whereas adding hypoxia or hypercapnia suppressed apoptosis. DT suppressed both heat shock and apoptotic responses, with ubiquitin conjugates and branched-chain amino acids accumulating, and gills showing visible damage. Our findings indicate that climate drivers cumulatively block protection mechanisms, increase protein damage and block protein synthesis, thereby substantially reducing passive thermal tolerance and survival under extremes. The thermal tolerance of scallops is critically reduced under DT conditions, when mechanisms defending passive tolerance are exhausted at lower temperatures.