Contrasting influences on bacterial symbiont specificity by co-occurring deep-sea mussels and tubeworms

Relationships between deep-sea invertebrates and bacterial symbionts, fueled by sulfide and methane, are well known, yet factors influencing symbiont specificity remain cryptic. For animals that obtain their symbionts from the environment, both host identity and geographic location can impact the ultimate symbiont partner. Bacterial symbionts were analyzed for 3 co-occurring species each of Bathymodiolus mussels and vestimentiferan tubeworms, from three deep methane seeps off the west coast of Costa Rica. The bacterial internal transcribed spacer gene was analyzed via direct and barcoded amplicon sequencing to reveal fine-scale symbiont diversity. Each of the three mussel species (B. earlougheri, B. billschneideri and B. nancyschneideri) hosted genetically-distinct thiotrophic endosymbionts, despite living nearly side-by-side in their habitat, suggesting that host identity is influential in driving symbiont specificity. The dominant thiotrophic symbiont of co-occurring tubeworms Escarpia spicata and Lamellibrachia (L. barhami and L. donwalshi), on the other hand, was identical regardless of host species or sample location, suggesting lack of influence by either factor on symbiont selectivity in this group of animals. These findings highlight the specific, yet distinct, influences on the environmental acquisition of symbionts in two foundational invertebrates with similar lifestyles, and provide a rapid and precise method of examining symbiont identities.