The Eastern oyster Crassostrea virginica is an ecological keystone species, and a critical fisheries resource for some coastal U.S. economies. Genetic and functional variation persisting in highly fragmented populations constitutes a critical adaptive resource in the face of anthropogenic environmental change. Regional genetic homogeneity is well documented along the Atlantic U.S. coast, suggesting high gene flow within and among estuaries. While strong gene flow makes adaptation to local salinities unlikely, developmental plasticity and responses to within-generation selection are two mechanisms that can structure phenotypic variation across the estuary. Specifically, gene expression responses to osmotic stress represent many molecular phenotypes potentially influenced by both mechanisms. Following on studies of gene expression plasticity and evolution in adult Delaware Bay oysters, here wild spat stage oysters were collected from moderate and low salinity regions in Delaware Bay and exposed to moderate (22 ppt) and low (6 ppt) salinity conditions in the lab. After seven weeks, the number and ontology of differentially expressed genes under these contrasting conditions was quantified in gill and mantle tissue to assess phenotypic plasticity in response to osmotic stress (comparing each source across treatments) and test for a significant effect of spat source region. Using environmental data, we link observed patterns of differential gene expression to conditions likely experienced by spat since settlement, influencing their development and the potential for within-generation selection. This research aims to elucidate the life-stage at which patterns of differential gene expression emerge across an estuarine salinity gradient.