The mytilid mussel Bathymodiolus thermophilus lives in the deep-sea hydrothermal vent regions due to its symbiotic relationship with chemosynthetic Gammaproteobacteria species which reside inside specialized gill cells. The symbionts in the gill bacteriocytes oxidize the reduced sulfur amply available in the vent environment. Here we sequenced and assembled the complete genome of a gill symbiont sampled from an individual mussel from the East Pacific Rise (EPR9N), using PacBio sequencing technology. The final symbiont assembly consists of a single contig size of 2.83 Mb, with a GC content of 38.6% and encodes for 2,133 protein-coding genes. CRISPR diversity analysis confirmed this genome originated from a single symbiont strain. Comparative analysis revealed 2,554 core gene clusters were shared with other B. thermophilus thiotrophic gill symbiont genomes, whereas 125 were unique to this EPR9N strain. In addition, we found that EPR9N strain has a unique hydrogenase operon among Bathymodiolus mussels consisting of additional H2-sensing hydrogenase subunits and a histidine kinase gene. Also, we found methylated regions sparsely distributed throughout the EPR9N genome, mainly in the transposases regions and densely present in the rRNA gene regions. Variation in genome size, gene content and genome re-arrangements across individual hosts suggest multiple symbiont strains can associate with B. thermophilus. This complete mussel symbiont genome will be invaluable for further comparative genomic analyses studying structural genome evolution, symbiont population diversity, and symbiont ecology in deep-sea chemosynthetic environments.

Plasticity in salt tolerance can be crucial for successful biological invasions of novel habitats by marine gastropods. The intertidal snail Batillaria attramentaria, which is native to East Asia but invaded the western shores of North America from Japan eighty years ago, provides an opportunity to examine how environmental salinity may shape behavioral and morphological traits. In this study, we compared the movement distance of four B. attramentaria populations from native (Korea and Japan) and introduced (USA) habitats under various salinity levels (13, 23, 33, and 43 PSU) during 30 days of exposure in the lab. We sequenced a partial mitochondrial CO1 gene to infer phylogenetic relationships among populations and confirmed two divergent mitochondrial lineages constituting our sample sets. Using a statistic model-selection approach, we investigated the effects of geographic distribution and genetic composition on locomotor performance in response to salt stress. Snails exposed to acute low salinity (13 PSU) reduced their locomotion and were unable to perform at their normal level (the moving pace of snails exposed to 33 PSU). We did not detect any meaningful differences in locomotor response to salt stress between the two genetic lineages or between the native snails (Japan versus Korea populations), but we found significant locomotor differences between the native and introduced groups (Japan or Korea versus the USA). We suggest that the greater magnitude of tidal salinity fluctuation at the USA location may have influenced locomotor responses to salt stress in introduced snails.