Local Adaptation and Osmoregulatory Mechanisms of the Copepod <scp> <i>Acartia tonsa</i> </scp> Under Low Salinity Stress

Salinity-an essential factor shaping marine species distributions-is rapidly shifting due to global change, yet the mechanisms of salinity tolerance and adaptation remain poorly understood. We investigated local adaptation in the calanoid copepod Acartia tonsa, a broadly distributed marine species that thrives in the brackish Baltic Sea. Using a common-garden design, we compared physiological and transcriptomic responses to low salinity between populations from the North Sea (> 25 PSU) and the Baltic Sea (< 15 PSU). Baltic copepods exhibited significantly higher survival under low salinity, indicating local adaptation. While both populations shared a core osmoregulatory strategy involving active ion transport and regulation of amino acids, transcriptomic profiles revealed population-specific differences. Baltic individuals showed a reduced overall gene expression response, yet maintained higher relative expression of osmoregulatory genes-suggesting a more efficient and primed transcriptomic response. In contrast, North Sea copepods exhibited broader transcriptional shifts, including downregulation of metabolic and developmental pathways after prolonged stress exposure, possibly reflecting energy conservation mechanisms. These findings reveal that A. tonsa possesses both a plastic osmoregulatory strategy and population-level adaptation that enable survival in extreme salinity conditions. While both populations tolerate short-term exposure to low salinity, local adaptation has enhanced the Baltic population's resilience. This suggests that A. tonsa is broadly tolerant of moderate climate-driven salinity declines across most of its distribution. However, our data also indicate potential range contractions in the lowest salinity zones of the Baltic Sea, underscoring the importance of identifying physiological and genetic thresholds in climate resilience studies.