Studies on the gut microbiota of animals have largely focused on vertebrates. The transmission modes of commensal intestinal bacteria in mammals have been well studied. However, in gastropods, the relationship between gut microbiota and hosts is still poorly understood. To gain a better understanding of the composition of gut microbes and their transmission routes in gastropods, a large-scale and long-term experiment on the dynamics and transmission modes of gut microbiota was conducted on freshwater snails.We analyzed 244 microbial samples from the digestive tracts of freshwater gastropods and identified Proteobacteria and Bacteroidetes as dominant gut microbes. Aeromonas, Cloacibacterium, and Cetobacterium were identified as core microbes in the guts, accounting for over 50% of the total sequences. Furthermore, both core bacteria Aeromonas and Cloacibacterium, were shared among 7 gastropod species and played an important role in determining the gut microbial community types of both wild and cultured gastropods. Analysis of the gut microbiota at the population level, including wild gastropods and their offspring, indicated that a proportion of gut microbes could be consistently vertically transmitted inheritance, while the majority of the gut microbes resulted from horizontal transmission. Comparing cultured snails to their wild counterparts, we observed an increasing trend in the proportion of shared microbes and a decreasing trend in the number of unique microbes among wild gastropods and their offspring reared in a cultured environment. Core gut microbes, Aeromonas and Cloacibacterium, remained persistent and dispersed from wild snails to their offspring across multiple generations. Interestingly, under cultured environments, the gut microbiota in wild gastropods could only be maintained for up to 2 generations before converging with that of cultured snails. The difference observed in gut bacterial metabolism functions was associated with this transition. Our study also demonstrated that the gut microbial compositions in gastropods are influenced by developmental stages and revealed the presence of Aeromonas and Cloacibacterium throughout the life cycle in gastropods. Based on the dynamics of core gut microbes, it may be possible to predict the health status of gastropods during their adaptation to new environments. Additionally, gut microbial metabolic functions were found to be associated with the adaptive evolution of gastropods from wild to cultured environments.Our findings provide novel insights into the dynamic processes of gut microbiota colonization in gastropod mollusks and unveil the modes of microbial transmission within their guts. Video Abstract.© 2023. The Author(s).