show Abstracthide AbstractWe combined a laboratory common garden experiment with RNA-sequencing (RNA-seq) to characterize the response of the euryhaline diatom Skeletonema marinoi to low salinity. We collected eight genotypes of S. marinoi along the Baltic Sea salinity cline, exposed them to a range of salinities, and compared gene expression between high and low salinity treatments. The results help shed light on how ancestrally marine diatoms adapt to, and ultimately diversify in, freshwater environments.