Transcriptomics of Acartia tonsa exposed to low salinity stress

Salinity is a key environmental factor shaping marine species distributions that is rapidly changing due to global change. Yet, the mechanisms underlying salinity tolerance and adaptation remain poorly understood for many organisms. We investigated local adaptation in the calanoid copepod Acartia tonsa, a marine zooplankter that thrives in the Baltic Sea despite its steep salinity gradient. Using a common-garden experiment combined with transcriptomic profiling, we quantified differences in low-salinity tolerance between copepods from the North Sea (>25 PSU, site WH) and the Baltic Sea (<15 PSU, site SW08). For each population, 15 replicates of 50 newly matured adults were transferred into mesh cups and exposed to either low salinity (7 PSU) or control (15 PSU) treatments. Samples were collected at three time points: pre-transfer (t0), 3 hours post-transfer (t1), and 24 hours post-transfer (t2), with RNA extracted from pooled animals (N=25 per biological replicate). We found evidence for a shared osmoregulatory response across populations, but also signatures of local adaptation: Baltic copepods exhibited higher resilience to low-salinity stress, including upregulation of key osmoregulatory genes. In contrast, North Sea copepods showed suppression of metabolic and developmental processes, likely reflecting an energy conservation strategy. Our findings shed light on the physiological and molecular mechanisms underpinning A. tonsa's broad salinity tolerance and demonstrate its potential for adaptation to increasingly extreme environmental conditions.