Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depthdecreases from ca. 2400m in the RAINBOW vent field (36°13'N) to ca. 850m in the MENEZ GWEN vent field (37°35'N). Inthis area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along withpopulations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical andchemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trapmoorings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculataliving in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-endemicspecies live at the border of the vent field. The variations observed in structure and composition of the communitiesalong the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) andsuspended mineral particles, which render the fluids harsher for species living there. The main faunal differences observedbetween LUCKY STRIKE and MENEZ GWEN hydrothermal fields are due to an impoverishment in the hydrothermalendemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests theexistence of a succession of several biogeographic islands rather than a single province.
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