Within the endemic invertebrate faunas of hydrothermal vents, five biogeographic provinces are recognized. Invertebrates at two Indian Ocean vent fields (Kairei and Edmond) belong to a sixth province, despite ecological settings and invertebrate-bacterial symbioses similar to those of both western Pacific and Atlantic vents. Most organisms found at these Indian Ocean vent fields have evolutionary affinities with western Pacific vent faunas, but a shrimp that ecologically dominates Indian Ocean vents closely resembles its Mid-Atlantic counterpart. These findings contribute to a global assessment of the biogeography of chemosynthetic faunas and indicate that the Indian Ocean vent community follows asymmetric assembly rules biased toward Pacific evolutionary alliances.

Bathymodiolus azoricus and Bathymodiolus puteoserpentis are symbiont-bearing mussels that dominate hydrothermal vent sites along the northern Mid-Atlantic Ridge (MAR). Both species live in symbiosis with two physiologically and phylogenetically distinct Gammaproteobacteria: a sulfur-oxidizing chemoautotroph and a methane-oxidizer. A detailed analysis of mussels collected from four MAR vent sites (Menez Gwen, Lucky Strike, Rainbow, and Logatchev) using comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH) showed that the two mussel species share highly similar to identical symbiont phylotypes. FISH observations of symbiont distribution and relative abundances showed no obvious differences between the two host species. In contrast, distinct differences in relative symbiont abundances were observed between mussels from different sites, indicating that vent chemistry may influence the relative abundance of thiotrophs and methanotrophs in these dual symbioses.

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