Stage matters in complex life cycles: Thermal limits across development and reproduction in a marine shrimp

Understanding how shallow marine species cope with ocean warming is essential for developing climate-smart conservation strategies. Realistic projections require detailed knowledge of thermal limits across life stages, since these can vary with development, sex and reproductive status. However, most thermal tolerance studies focus exclusively on adults, potentially underestimating species vulnerability. To address this gap, we examined the thermal tolerance of the ditch shrimp Palaemon varians, across its full life cycle through a Critical Thermal maximum (CTmax) experiment, integrating different developmental stages (Zoea 1-5, Postlarvae and Adult), while accounting for sex (Males vs. Females), and female reproductive status (Ovigerous, Spent and Immature). Early larval stages (Zoea 1- 3) exhibited lower CTmax (36.7, 37.1 and 36.8 °C, respectively) than later stages (Zoea 4- 5, Postlarvae and Adults), with Postlarvae (37.9 °C) showing the highest values. Larval development represents a bottleneck for this species' thermal tolerance. The largest increase was observed after the transition from larval life to the postlarvae stage (1.9 %), indicating that this life stage represents a physiological milestone in the acquisition of thermal tolerance in this species. Females were generally more sensitive to heat than males (37.7 °C), with those that had released their larvae recently (spent females 37.2 °C), showing the lowest CTmax values, indicating a potential energetic cost associated with reproduction. Overall, our findings show that focusing solely on adult thermal limits overlooks key vulnerabilities across development and reproduction. Integrating these factors provides a more realistic assessment of shallow-water species' tolerance to global warming.