Shipworms (Bivalvia: Teredinidae) are the most prolific wood consumers in marine environments. These wormlike marine bivalves digest wood using carbohydrate-active enzymes (CAZymes) produced by intracellular bacterial endosymbionts housed within their gills. Although several shipworm species are known to host multiple co-occurring symbiont species, the factors that influence symbiont community assembly, including the phylogenetic identity and metabolic capabilities of the symbionts, remain poorly understood. We sequenced gill symbiont metagenomes from multiple specimens of two shipworm species, Teredo bartschi (22 specimens) and Lyrodus pedicellatus (14 specimens), which were reared together in laboratory co-culture. From these metagenomes, we assembled 90 metagenome-assembled genomes (MAGs) representing seven distinct symbiont species. The metagenome of each host specimen contained between 1 and 5 symbiont species, with each including at least one nitrogen-fixing symbiont. Six of the seven identified symbiont species were found in both host species, demonstrating a lack of host species specificity in these symbioses. We identified patterns of symbiont occurrence and co-occurrence in these two hosts and used these to constrain the core set of CAZyme and nitrogen fixation gene classes necessary to support host survival. Our results indicate that, in these two host species, symbiont community composition reflects the symbionts' capabilities for carbohydrate degradation and nitrogen fixation, rather than strict species-specific mechanisms of host and symbiont sorting.

Funding provided by: Gordon and Betty Moore FoundationROR ID: https://ror.org/006wxqw41Award Number: GBMF9339 Funding provided by: International Centers for Excellence in ResearchROR ID: https://ror.org/00x19de83Award Number: 1R01AI162943-01A1 Funding provided by: Fogarty International CenterROR ID: https://ror.org/02xey9a22Award Number: U19TW008163 Funding provided by: U.S. National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: DBI 1722553 Funding provided by: National Oceanic and Atmospheric AdministrationROR ID: https://ror.org/02z5nhe81Award Number: NA19OAR0110303