Thermal and Salinity Performance Limits in a Shallow-Water Shrimp Under Climate-Driven Change

Climate change is driving increasingly large fluctuations in temperature and salinity in the world’s oceans, particularly in shallow, semi-enclosed ecosystems such as saltpans and coastal lagoons. Species inhabiting these dynamic environments often rely on phenotypic plasticity to cope with such shifts; however, given the rapid pace at which these changes are occurring, their plastic limits may eventually be exceeded. Although efforts have been made to predict how species respond to these environmental alterations, important aspects, such as sex-specific differences, are frequently overlooked. In this study, we examined the performance of the widespread caridean shrimp Palaemon varians across two experimental gradients: salinity (10, 20, 30, 40) and temperature (10, 15, 20, 30 °C). We assessed cumulative survival, whole-organism traits (Critical Thermal Maximum, CTmax; Routine Metabolic Rate, RMR), and sex-specific components of the Cellular Stress Response (CSR) to evaluate acclimation capacity and potential biochemical trade-offs. Survival remained unchanged across all salinity treatments, whereas temperature had a significant effect, with individuals kept at 30 °C showing the lowest survival. RMR was unaffected by both salinity and temperature. CTmax increased by more than 5% under warmer conditions, indicating partial thermal acclimation, but did not vary with salinity. CSR biomarkers were also unaffected by salinity; however, clear sex-related differences emerged. Males showed nearly 60% higher superoxide dismutase activity and roughly 40% higher total protein content than females, resulting in distinct biochemical performance profiles. While CSR differences between sexes did not vary with temperature, both cold and warm extremes induced elevated antioxidant activity, despite antioxidants exhibiting low thermal sensitivity (Q10). Overall, P. varians demonstrated physiological plasticity in response to environmental gradients but also exhibited sex-specific trade-offs between antioxidant capacity and energy reserves, potentially linked to reproductive investment. The species’ narrow thermal safety margin (2.7–6.7 °C), combined with divergent energy allocation strategies between males and females, may constrain the resilience of shallow-water shrimp populations as climate-driven temperature variability intensifies.