In deep-sea hydrothermal vent - deprived of light - primary energy production is ensured by chemosynthetic microbial communities. These microorganisms are free-living in the environment or forming associations with metazoan hots which are called holobionts. Among them, two endemic shrimp Rimicaris exoculata and Rimicaris chacei from the Mid-Atlantic Ridge (MAR) have been well described. Even if these two species exhibit very contrasting biological traits, they harbor similar microbial communities in various organs. One is found in the cephalothoracic cavity, another in the foregut and the last one colonizes the midgut. The midgut symbiotic community is mainly composed of Candidatus Microvillispirillaceae, long and thin unicellular filamentous organisms that are inserted between the microvilli of the intestinal epithelium in the ectoperitrophic space. Recent studies have also revealed the presence of a second major lineage acquired during juvenile metamorphosis: Lachnospirales. We decided to study these bacteria from shrimp from two sites (TAG and Snake Pit) using a combination of three approaches: Fluorescent in situ Hybridization (FISH), metabarcoding and metagenomics. Firstly, a specific FISH probes targeting Lachnospirales was developed, tested and optimized. It was used to detect and identify cocobacilli and filamentous Lachnospirales inside the midgut tube of adult and juvenile specimens of Rimicaris exoculata and Rimicaris chacei. To complement these data, an analysis of two MAGs (Metagenomic Assembled Genomes) has revealed a new lineage of Lachnospirales, whose metabolism seems to be close to that of the Candidatus Microvillispirillaceae with which they co-occur. Lachnospirales are mixotrophs, capable of degrading complex polymers and supplying nutrients to their host. Finally, to better characterize these lineages, an analysis of taxonomic diversity (metabarcoding) during the life cycle of the two species on the two MAR sites was carried out. This analysis enabled us to formulate hypotheses concerning the acquisition and transmission strategies of these symbionts within the holobiont.