The gill tissue of bivalve mollusks hosts rich symbiotic microbial communities that may contribute to the animal's metabolism.Spondylus spinosusis an invasive oyster that has become highly abundant along the eastern Mediterranean Sea (EMS) coastline, but is scarce in the northern Red Sea (NRS), its indigenous region. The composition and seasonal dynamics of the gill microbial communities ofS. spinosuswere examined in both regions, using 16S rRNA gene amplicon sequencing. Additionally, two Red SeaSpondylusspecies,S. avramsingeriandS. pickeringae, were investigated using the same approach. Significant differences were found between microbial communities of the EMSS. spinosusand the three NRS species. Bacteria from the familyHahellaceaedominated the communities of the EMSS. spinosusand the NRSS. avramsingeri, oysters that are dominant in their habitat, yet were rare in the NRSS. spinosusandS. pickeringae, which are only seldom encountered. Bacterial communities of EMSS. spinosuswere more similar to those of NRSS. spinosusthan to those of other NRSSpondylusspecies, indicating that either part of the microbiota had co-invaded with their host into the Mediterranean Sea, or that there are species-specific selective constraints on microbial composition.