The aeolid nudibranch Berghia stephanieae belongs to the group of cladobranchs that are able to associate with dinoflagellates (Symbiodiniaceae) and establish a photosymbiosis. Here we present the first gene expression profile of a photosymbiotic slug when feeding, re-feeding and starving in aposymbiotic and symbiotic states. B. stephanieae is a relevant species to understand how photosymbiosis has evolved in Nudibranchia, because it seems to be a transitional form towards a stable photosymbiosis.