Taxonomy, tissue, and habitat influence mollusk microbial communities.

Microbes play a crucial role in the health, development, and resilience of mollusks, yet the ecological and evolutionary factors shaping their microbial communities remain poorly understood. To uncover the drivers of microbial community composition of mollusks, we conducted a systematic review of 85 studies, including 45 on bivalves from marine and freshwater habitats; 33 on gastropods from marine, freshwater, and terrestrial habitats; and seven on cephalopods. Our synthesis reveals that both intrinsic (e.g. host phylogeny, tissue type) and extrinsic (e.g. environment, geography, and seasonality) factors influence microbial community structure, but the effects are highly taxon- and context-dependent. Although studies of bivalves often emphasize environmental drivers, those of cephalopods more frequently highlight intrinsic host features. Despite growing interest in molluscan microbiomes, we identified significant taxonomic and methodological biases, including a predominant focus on economically important species and gut tissues. We advocate for a broader, integrative approach that includes underrepresented molluscan groups, diverse tissue types, and testing of both intrinsic and extrinsic variables across spatial and temporal gradients. This review highlights the need for standardized, multi-factorial research to better understand and predict microbial community responses to environmental change of one of the most diverse and ecologically important invertebrate phyla.