In this project, we will examine the marine snail, Littorina saxatilis, which has ecotypes that have evolved repeatedly in Sweden, United Kingdom and Spain. We will examine gene expression from ecotype replicates to discover differentially expressed genes and gene co-expression modules involved in adaptation. Then, we will rear snails in a common environment to determine if the gene expression observed in the wild is due to plastic or genetic changes. To see if parallel phenotypes are the result of different regulatory mutations, we will examine the regulatory architecture by sequencing open chromatin regions of the DNA of the different ecotypes. We will conduct an expression QTL analysis to link expression variation to regulatory regions.