Composition and dynamics of the gill microbiota of an invasive Indo-Pacific oyster in the eastern Mediterranean Sea Dror Zurel, 1,2 Yehuda Benayahu, 2 Amitai Or, 2 Amir Kovacs 3 and Uri Gophna 3 * 1 The Porter School of Environmental Studies, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel. Department of 2 Zoology and 3 Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel. Summary Gill bacterial communities of Chama pacifica, an Indo- Pacific invasive oyster to the eastern Mediterranean Sea, were compared with those of Chama savignyi, its northern Red Sea congeneric species. Summer and winter bacterial populations were characterized and compared using 16S rDNA clone libraries, and sea- sonal population dynamics were monitored by auto- mated ribosomal intergenic spacer analysis (ARISA). Clone libraries revealed a specific clade of bacteria, closely related to marine endosymbionts from the Indo-Pacific, found in both ecosystems, of which one taxon was conserved in oysters from both sites. This taxon was dominant in summer libraries and was weakly present in winter ones, where other members of this group were dominant. ARISA results revealed significant seasonal variation in bacterial populations of Mediterranean Sea oysters, as opposed to Red Sea ones that were stable throughout the year. We suggest that this conserved association between bac- teria and oyster reflects either a symbiosis between the oyster host and some of its bacteria, a co-invasion of both parties, or both. Introduction Biological invasions in marine habitats represent a rec- ognized worldwide threat to the integrity of native com- munities, the economy and human health (e.g. Bax et al., 2003; Streftaris et al., 2005; Galil, 2007; Rilov and Crooks, 2009). Many factors have been found to affect the success of invasion, such as the lack of predators in the invaded ecosystem or physico–chemical conditions that select against or in favour of the invader (reviewed by Vermeij, 1996). One such factor is the presence of microbial pathogens or mutualists that may affect the invader’s success to survive and establish a community in its target ecosystem. For example, the microsporidian parasite Fibrillanosema crangonycis has been found to promote the invasion of its North American amphipod host Crangonyx pseudogracilis to the UK by altering the host’s sex ratio towards more females (Galbreath et al., 2004). Similarly, the shipworm Teredo navalis, hosts cellulose-digesting symbiotic gill bacteria that allowed it to bore into wooden substrates, such as the vessels that have distributed this bivalve worldwide (Norman, 1977). The eastern Mediterranean Sea (EMS) is susceptible to biological invasions due to its location between the Atlan- tic, Pontic and Erythrean regions, heavy maritime traffic, and its coastal waters that are teeming with maricultured organisms (Arvanitidis et al., 2006). The greatest influx of invaders into the Mediterranean Sea resulted from the opening of the Suez Canal in 1869, which allowed the entry of Indo-Pacific and Erythrean biota (Por et al., 1972; Galil, 2007). Several Mediterranean species have been wholly or partially displaced by these invaders (Galil, 2007; Rilov and Crooks, 2009). Such is the case for the Indo-Pacific oyster Chama pacifica (class Bivalvia, family Chamidae), first recorded in the Mediterranean Sea by Tillier and Bavay in 1905, and which has since outnum- bered its indigenous congener C. gryphoides, which is now hardly encountered due to the new invader that has successfully replaced it (Mienis et al., 1993; Galil, 2007). Chama pacifica has managed to thrive along the Levantine basin (Mienis et al., 1993). Chamidae species are commonly found attached to sub-littoral hard sub- strata, in areas with low sedimentation, where they are usually well hidden by overgrowing fouling organisms that extend equally over the natural rocks and shells (Purchon and Purchon, 1981). Previous studies on this species in the Mediterranean Sea focused mainly on the structural effects of C. pacifica on the invaded ecosystem, since it Received 1 December, 2010; accepted 24 January, 2011. *For cor- respondence. E-mail urigo@tauex.tau.ac.il; Tel. (+972) 3 6409988. Environmental Microbiology (2011) 13(6), 1467–1476 doi:10.1111/j.1462-2920.2011.02448.x © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd