The lucinid Codakia orbicularis (Mollusca: Bivalvia) is found in the shallow water seagrass beds of Thalassia testudinum in the Caribbean Sea. Their gills are colonized by a sulfur-oxidizing endosymbiotic gamma-proteobacteria. These bacteria oxidize sulfur compounds to generate energy used to fix carbon dioxide to create organic components via the Calvin Benson cycle, in order to provide metabolic substrates and energy to their host C. orbicularis. In starvation condition, the bivalves were placed in artificial seawater without sulfur and organic components. In these dramatically conditions, nutrients for the both host bivalves and endosymbiotic bacteria are limited. Interestingly, in a previous proteomic study, it has been shown an over-expression of proteins involved in glycogen metabolism during starvation condition. This suggests that bacteria produce glycogen which can be use by bivalves. In order to investigate the effects of starvation on the metabolism of endosymbiotic bacteria, we selected some genes involved in glycogen metabolism and in some key metabolic pathways. We analyzed by qRT-PCR the relative expression of genes in 5 bivalves in normal condition compared to 5 bivalves kept 1 or 2 months in starvation respectively. We aimed glgA, glgC, galU, glgP for glycogen metabolism; rbcl1, rbcl2 for glucose synthesis metabolism; and aprA, dsrA, soxB, fccB for sulfur metabolism. The results showed an over-expression of galU during starvation and no expression was detected for glgA and glgC suggesting an activation of glycogen producing by UDP-Glucose pathway. Nevertheless, glucose synthesis metabolism seems to be down-regulated (decreased expression of rbcl1), whereas for sulfur metabolism, only dsrA was expressed and up-regulated. These results are preliminary, and we are currently conducting a global transcriptomic study in order to identify the set of genes and metabolic pathways affected during a long-term starvation in this marine symbiotic model harboring sulfur-oxidizing bacteria.