A globally distributed symbiont challenges host specificity in lucinid clams

We sampled lucinid clams to study their bacterial chemosymbionts at the genomic level. Lucinidae - the lucinid clam family - are very diverse. They live in many different habitats, including sediments in seagrass patches, mangrove forests, and coral reefs. The clams host bacterial chemosymbionts in their gills. The host uses its foot to pump sulfides from the anaerobic sediment into its gills. Its bacterial symbionts oxidize the sulfides and thereby produce sugars that make up to 80% of the clam’s diet. It is assumed that the host cultures its bacterial symbionts and harvests their products. Bacterial symbionts are horizontally transferred to the next generation; i.e., when lucinid clams settle in the sediment after completing their larval life stage, they pick up symbionts from their environment. While symbiont diversity is high and so far we have found many different clades of symbionts, there is one symbiont that shows a trans-Atlantic distribution. We found this symbiont across at least four main locations and host species: Florida, Cahuita (Costa Rica), Bocas del Toro (Panama), and Mauritania (Africa). The metagenome assembled genomes (MAGs) at these locations come from the same symbiont species (ANI > 96%). Subsequently, we included more samples from other parts of the world from Museum collections. Using these collections, we found this symbiont species also in East Africa and Asia. What makes this strain so widespread? What are the population genomic differences across all populations? Are there genomic differences in metabolic potential based on host or location? How varable is this symbiont at the genomic level when we zoom into the population? Can we find genomic regions under selection? How does this relate to the more localized symbionts?