The aeolid nudibranch Berghia stephanieae belongs to the group of nudibranchs that are able to associate with dinoflagellates (Symbiodiniaceae) and establish a photosymbiosis. 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. Therefore, it serves as a model organism for studying genomic adaptations needed to establish a photosymbiosis