This project studied population structuring and host-symbiont associations in the gutless annelid Olavius algarvensis, using metagenomes derived from individual worms obtained in the western Mediterranean locations. For host population genetic analyses based on exome-capturing, a reference transcriptome is used, which were assembled from 12 individual transcriptomes. For the mitochondrial phylogeny, an individual of Olavius ilvae included in this study.