Vent fields are hundreds of kilometres apart on average along the slow-spreading Mid-Atlantic Ridge, but typically tens of kilometres apart on the fast-spreading East Pacific Rise 4, 5. The occurrence of vent fields detected along the axes of mid-ocean ridges correlates positively with seafloor spreading rate 4, 5. Since the first investigations of hydrothermal vents in the eastern Pacific in the late 1970s, more than 250 active vent fields have been visually confirmed worldwide 1, and more than 400 new animal species have been described from vent environments 2 across at least eleven biogeographic provinces 3. Hydrothermal vents occur as “vent fields”, each typically <10 km 2 in extent and separated from each other by tens to hundreds of kilometres along seafloor spreading centres. This study provides baseline ecological observations prior to mineral exploration activities licensed at Longqi by the United Nations.Īt deep-sea hydrothermal vents, autochthonous primary production by chemosynthetic prokaryotes supports locally abundant populations of faunal species at the ocean floor. Other taxa occur at lower abundance, in some cases contrasting with abundances at other vent fields, and δ 13C and δ 15N isotope values of species analysed from Longqi are similar to those of shared or related species elsewhere.

Faunal zonation with increasing distance from vents is dominated by the gastropods Chrysomallon squamiferum and Gigantopelta aegis, mussel Bathymodiolus marisindicus, and Neolepas sp. Multivariate analysis of vent fauna across three oceans places Longqi in an Indian Ocean province of vent biogeography.

Six species are not yet known from other vents, while six other species are known from the Central Indian Ridge, and morphological and molecular analyses show that two further polychaete species are shared with vents beyond the Indian Ocean. We collected 21 macro- and megafaunal taxa during the first Remotely Operated Vehicle dives to the Longqi vent field at 37° 47′S 49° 39′E, depth 2800 m. The Southwest Indian Ridge is the longest section of very slow to ultraslow-spreading seafloor in the global mid-ocean ridge system, but the biogeography and ecology of its hydrothermal vent fauna are previously unknown.

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