The primary goal of this study was to characterize the microbial communities within reproductive stages of lucinid bivalves of the species Loripes orbiculatus in Mauritania. An additional goal was to screen these egg masses for the host's primary gill endosymbiont and thus to check whether these are vertically or horizontally transmitted. The reproductive biology of marine invertebrates is complex and we particularly lack information on the life cycle of chemosymbiotic bivalves including their symbiont transmission mode. With this study we want to provide valuable information on the biology of the bivalve family Lucinidae.