Abstract
Mollusks are among the most ecologically and economically significant invertebrates; yet, their associated microbiomes remain understudied relative to those of other metazoans. This scoping review synthesizes the current literature on the diversity, composition, functional roles, and ecological implications of molluscan microbiomes, with an emphasis on three major groups: gastropods, bivalves, and cephalopods. Drawing on studies from terrestrial, freshwater, and marine systems, we identified the dominant bacterial phyla, including Proteobacteria, Bacteroidetes, and Firmicutes, and explored how microbiota vary across different habitats, diets, tissue types, and host taxonomies. We examined the contribution of molluscan microbiomes to host functions, including digestion, immune modulation, stress responses, and nutrient cycling. Particular attention was given to the role of microbiota in shell formation, pollutant degradation, and adaptation to environmental stressors. The review also evaluated microbial interactions at different developmental stages and under aquaculture conditions. Factors influencing microbiome assembly, such as the host's genetics, life history traits, and environmental exposure, were mapped using conceptual and graphical tools. Applications of molluscan microbiome research in aquaculture, conservation biology, and environmental biomonitoring are highlighted. However, inconsistencies in the sampling methods, taxonomic focus, and functional annotations limit the generalizability across taxa. We identify key knowledge gaps and propose future directions, including the use of meta-omics, standardized protocols, and experimental validation to deepen insights. By synthesizing emerging findings, this review contributes to a growing framework for understanding mollusk-microbiome interactions and their relevance to host fitness and ecosystem health. It further establishes the importance of mollusks as model systems for advancing microbiome science.