gill symbiont community of the shipworms Teredo bartschi and Lyrodus pedicellatus

Shipworms (Bivalvia: Teredinidae) are the most prolific wood consumers in marine environments. These wormlike marine bivalves digest wood with the help of carbohydrate-active enzymes (CAZymes) produced by intracellular bacterial endosymbionts housed in a specialized tissue within their gills. Although several shipworm species are reported to harbor multiple co-occurring symbiont species, the phylogenetic identity of the symbionts, as well as the composition, variability, metabolic potential, and host specificity of shipworm symbiont communities, are incompletely understood. We sequenced the gill symbiont metagenomes of multiple individuals of two shipworm species, Teredo bartschi (22 specimens) and Lyrodus pedicellatus (14 specimens), that were reared together in laboratory co-culture. From these metagenomes, we assembled 90 metagenome-assembled genomes (MAGs) representing seven distinct symbiont species as inferred by phylogenetic analyses and average nucleotide and amino acid identities (ANI and AAI). The metagenome of each host included between 1 and 5 symbiont species, with at least one nitrogen-fixing symbiont detected in each. Six of the seven identified symbiont species were detected in both host species. The two symbiont species richest in genes encoding CAZymes occurred as sole symbionts within some individual host specimens and were numerically dominant in most others. We identified a core set of CAZyme and nitrogen fixation genes present in all metagenomes, which in some individuals were provided by single symbiont species and in others by multiple symbionts. These results suggest that, rather than host specificity, the symbiont community composition reflects the symbionts' predicted capabilities for carbohydrate degradation and nitrogen fixation.