Mechanism of segregation of gastropods corresponding to phylogeny of endosymbiotic bacteria in hydrothermal vent fields in the southwestern Pacific

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Survey of chemosynthesis-based communities in the deep-sea hydrothermal vent fields in the Manus and North Fiji Basins in the southwestern Pacific was carried out using the submersible SHINKAI 6500 from Sept. 8 to Oct. 4, 2006. Changes of mode of hydrothermal venting and fauna containing Alviniconcha snails since 1990s were recognized. Population size of Alviniconcha snails in the southwestern Pacific was shown to have been maintained without rapid expansion in comparison with those in the Mariana Trough and the Indian Ocean. Chemical analyses of sea water samples collected around snails showed that Alviniconcha snails inhabit areas at 10-20°C where H_2S content is relatively high, while habitats of Ifremeria nautilei are areas at 4-8°C where H_2S content is at the undetectable level. At all habitats of Alviniconcha snails, epsilon-proteobacteria closely related to chemosynthetic bacteria with carbon dioxide fixation system using reverse TCA cycle, were dominated among planktonic free-living bacteria. Only at the Viena Woods site in the Manus Basin, where only Alviniconcha snails endosymbiosing with gamma-proteobacteria were distributed, bacteria related to the endosymbionts were detected in the water samples. Distribution of Alviniconcha snails endosynbiosing with gamma-proteobacteria is restricted by mode of hydrothermal venting rather than tectonic backgrounds of hydrothermal venting and physico-chemical characteristics of hot water. While they are thought to be sensitive to changes in biological, physical and chemical factors in the environments of their habitats, Alviniconcha snails endosynbiosing with epsilon-proteobacteria are probably able to adapt themselves to wider range of such factors.