show Abstracthide AbstractMicrobial strain variability is virtually universal, from deep-sea environments to the human gut, but the complexity of most natural microbial communities poses an immense challenge for understanding its functional ecological and evolutionary consequences. Bathymodiolus mussels are ideal for understanding strain variability because they naturally host only a few species of symbiotic bacteria. We sequenced metagenomes from 19 Bathymodiolus individuals from different hydrothermal vents in the Atlantic and developed a metagenomics workflow to tease apart co-existing symbiont strains. Metatranscriptomic data were produced to test whether strain-specific genes were expressed.