Metabolic potential structures gill symbiont communities in two common shipworm species

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 have sympatric distribution in the wild, and which were reared together in laboratory co-culture. From these metagenomes, we assembled 90 metagenome-assembled genomes representing seven distinct symbiont species. The metagenome of each host specimen contained between one and five 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 patterns 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.