Saline migrants into freshwater habitats constitute among the most destructive invaders in aquatic ecosystems throughout the globe. However, evolutionary and physiological mechanisms underlying such habitat transitions remain poorly understood. To explore mechanisms of freshwater adaptation and distinguish between adaptive (evolutionary) and acclimatory (plastic) responses to salinity change, we examined genome-wide patterns of gene expression between ancestral saline and derived freshwater populations of the Eurytemora affinis species complex, reared under two different common-garden conditions (0 vs. 15 PSU). We found that evolutionary shifts in gene expression (between saline and freshwater inbred lines) showed far greater changes and were more widespread than acclimatory responses to salinity (0 vs. 15 PSU). Most notably, many genes showing evolutionary shifts in gene expression across the salinity boundary were associated with ion transport function, with inorganic cation transmembrane transport forming the largest Gene Ontology category. Of particular interest was the sodium transporter, the Na⁺/H⁺ antiporter (NHA) gene family, which was discovered in animals relatively recently. A few key ion regulatory genes, such as NHA paralog #7, demonstrated concordant evolutionary and plastic shifts in gene expression, suggesting the evolution of ion transporter plasticity and function during rapid invasions into novel salinities. Moreover, freshwater invasions were associated with the evolution of reduced plasticity in the freshwater population, again for the same key ion transporters, consistent with the predicted evolution of canalization following adaptation to stressful conditions. Our results have important implications for understanding invasion mechanisms by some of the most widespread invaders in aquatic habitats.