Living on the edge: Temperature and salinity performance curves across levels of biological organization in a shallow water shrimp

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Phenotypic plasticity enables organisms to cope with natural environmental variability. However, climate change is intensifying this variability, causing more pronounced fluctuations in sea surface temperature and salinity, particularly in shallow, semi-confined habitats such as coastal lagoons and saltpans. To better understand how species inhabiting these highly dynamic environments cope with temperature and salinity fluctuations - and to assess potential sex-specific responses – performance of the widely distributed caridean shrimp Palaemon varians were tested along two experimental gradients: a salinity gradient (10, 20, 30 and 40) and a temperature gradient (10, 15, 20, 30 °C). Species cumulative survival and whole-organism indicators (Critical Thermal maximum - CTmax and Routine Metabolic Rate - RMR), as well as sex-specific Cellular Stress Responses (CSR) were measured to elucidate acclimation capacity and sex-specific biochemical trade-offs. Survival remained high across all salinity levels but significantly declined at elevated temperature, particularly at 30 °C. While RMR was unaffected by changes in temperature or salinity, CTmax remained unchanged with varying salinity but increased by more than 5% with elevated temperature, indicating some degree of thermal acclimation. Salinity had no significant effect on CSR; however, sex-based differences emerged, with males displaying nearly 60% higher superoxide dismutase activity and about 40% higher total protein levels than females, with distinct biochemical performance curves. While these sex-based differences were not apparent under temperature change, both cool and warm extremes significantly increased antioxidant activity, despite antioxidants displaying low thermal sensitivity (Q 10 ). These findings suggest that while P. varians exhibits physiological plasticity in response to environmental gradients, sex-specific trade-offs between antioxidant defences and energy reserves, possibly related to reproductive investment, do occur. Moreover, shrimp displayed a narrow thermal safety margin (2.7–6.7°C), which, combined with divergent energy allocation strategies between sexes, can constrain the future viability of shallow-water shrimp populations under escalating climate change pressures.

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