Sex differences in stress response in the marine copepod, <i>Calanus finmarchicus</i>

As climate change intensifies, marine organisms face multiple environmental stressors that challenge their survival and adaptability. Oxidative stress occurs when environmental conditions deviate far from an organism's optimal range, and responses vary due to diverse molecular and physiological pathways. Stress responses can differ by sex and be influenced by multiple stressors, yet the underlying mechanisms remain poorly understood. This study examined sex differences in stress response in copepods <i>Calanus finmarchicus</i>, one of the most ecologically important animals in the marine ecosystem. Adult males and females were exposed to paraquat, an oxidative stress inducer, to assess survival, metabolic rate, and expression of antioxidant genes and chaperones. In addition, females were subjected to combined heat and paraquat stress to explore interactive effects on stress tolerance. Males exhibited greater upregulation of antioxidant enzymes but had lower survival than females in response to paraquat exposure. Despite similar metabolic rates between sexes, females were larger and had greater lipid sac volume than males, which may contribute to their higher resilience and survival under paraquat exposure. In females, combined paraquat and heat stress had a synergistic detrimental effect on survival but only heat stress impacted metabolic rate. In addition, heat stress modulated female's gene expression, antagonizing glutathione-S-transferase III and enhancing superoxide dismutase expression. A potential threshold for superoxide dismutase and catalase fold change was identified in paraquat-exposed females. Understanding the variability in stress responses is crucial for predicting species resilience to climate change and environmental disturbances, ultimately informing conservation and ecosystem management strategies.<b>NEW & NOTEWORTHY</b> This individual-based study reveals sex-specific differences in stress responses of the copepod <i>Calanus finmarchicus</i> to oxidative stress, with males showing higher sensitivity but no significant different metabolic strategies compared with females. It also identifies the antagonistic and synergistic effects of heat and paraquat-induced oxidative stress on antioxidant gene expression, and a potential maximum threshold for superoxide dismutase and catalase fold change in females.