Large mussels of the mytilid subfamily Bathymodiolinae are common inhabitants of deep-sea hydrothermal vents and cold seeps, where gill-borne symbionts allow them to utilize energy-rich compounds such as hydrogen sulfide and methane to support abundant growth. This subfamily also includes smaller symbiont-bearing mussels found on deep-sea wood and organic deposits. Phylogenetic analyses suggest that wood association is ancestral to bathymodiolin evolution. This observation led to the “wooden steps” hypothesis, which proposed that wood and other large organic deposits have acted as evolutionary steppingstones, introducing the progenitors of the modern vent and seep Bathymodiolinae to their remote environments. Although this hypothesis implies an evolutionary trajectory from shallow to deep water, no bathymodiolin species that grows and reproduces at depths less than 100 m has yet been formally described. Here we describe a new bathymodiolin genus and species, Vadumodiolus teredinicola, found growing and reproducing at a depth of 18 m in uninhabited shipworm burrows in the remnants of an ancient submerged bald cypress forest off the coast of Alabama. These results demonstrate that the bathymodiolin radiation has not been limited to deep water and that specific association with wood has led to the successful invasion of both deep and shallow marine environments.

To search for evidence of bacterial symbionts in the tissues of Vadumodiolus teredinicola, we sequenced the gill metagenomes of three specimens, assembled contigs and sorted contigs of predicted bacterial origin into bins reflecting individual predicted bacterial MAGs using SPADES and metaBAT2. Reads of bacterial origin were sparsely represented in the three individual gill metagenome datasets. However, by combining reads from all three gill metagenome sequences during SPADES assembly, a single, though highly fragmented, bacterial MAG was assembled. MetaBAT2 uses tetranucleotide composition information to perform binning. MetaBAT2 takes a metagenome assembly and the reads that produced the assembly and organizes the contigs into putative genomes, called "bins". Because metaBAT2 only uses contigs longer than 1.5 kbp during binning, we performed BLASTn searches using the 16S rRNA genes, 23S rRNA genes, and coding sequences (CDS) of Bathymodiolus thermophilus thioautotrophic gill symbiont strain EPR9N to recruit un-binned short bacterial contigs in the assembled metagenome. Because the metaBAT2 binned bacterial contigs and concatenated contigs obtained using BLAST searches are phylogenetically indistinguishable, it is reasonable to assume that they represent the same bacterial genome present in the gills, and thus can be combined to create a more complete assembly. The final draft genome of Vadumodiolus teredinicola gill bacterium was annotated using Prokka and is deposited in GenBank format (GBK extension).