Aim: The pattern of biodiversity and biogeography is crucial to informing management and conservation strategy. But a lack of study across multiple ridge systems, especially for the Carlsberg Ridge, has hampered the conception of the overall picture for the Indian Ocean vents, a top target for deep‐sea exploration of massive sulphides. Here, we aim to characterize fauna from three new vent fields on the Carlsberg Ridge for the first time, and answer 1) what is the biogeographic pattern for vent fauna within the Indian Ocean and 2) how does this pattern guide the future environmental management on the Indian Ocean ridges.Location: Vents on the Carlsberg Ridge (CR), Central Indian Ridge (CIR), and Southwest Indian Ridge (SWIR).Taxon: Macrofauna.Methods: Samples, still images and videos were collected from vent fields by either submersible or TV‐guided grab. A comprehensive dataset of 11 fields on Indian Ocean ridges were obtained based on taxa identification and compilation. Genetic connectivity was analysed for six species using COI sequences. A framework for identifying biogeographic regions based on beta diversity measurement βsim was employed to reveal species turnover along the Indian Ocean ridges.Results: Faunal assemblages at three new vents on the CR hosted a total of 34 species. Significant genetic differentiation was detected between southern SWIR and CIR (plus Tiancheng) for three species, and between CIR (plus Tiancheng) and CR for four species. Hierarchical clustering of βsim support division of the Indian Ocean vents into three clades.Main conclusions: Species turnover along the ridges supported the separation of Indian Ocean vents into three discrete biogeographic units, boundaries between which largely corresponded to genetic breaks for shared species with lower dispersal capabilities. The results clearly show that the conservation of Indian Ocean vents must target three provinces, simultaneously.

The biodiversity of deep-sea hydrothermal vents in the Indian Ocean remains poorly characterised compared with that of their Pacific and Atlantic counterparts. Although the Longqi hydrothermal vent field is the most extensively explored vent site on the ultra-slow-spreading Southwest Indian Ridge, it is still a source of new discoveries. Here, we report and formally describe a new peltospirid snail from Longqi – Lirapex felix sp. nov. Known from only two specimens, it differs from other named Lirapex species by a depressed spire and the lack of coil loosening on the body whorl. Examinations of the external anatomy and radular characteristics agree with its placement in Lirapex, which is also supported by a molecular phylogeny reconstructed using the barcoding fragment of the mitochondrial cytochrome c oxidase I (COI) gene. This is the fifth peltospirid snail known to inhabit the Longqi field, three of which (including Lirapex felix sp. nov.) have been found nowhere else. There is growing evidence that the Longqi field represents a biogeographically unique site among Indian Ocean vents. The discovery of Lirapex felix sp. nov. adds to the unique biodiversity of Longqi field, with implications for conservation in the light of potential deep-sea mining. This work is registered in ZooBank under: urn:lsid:zoobank.org:pub:3B892762-2110-4A1E-A80D-1779CBEC5EB3

Branchinotogluma bipapillata n. sp., a new branchiate scale worm (Annelida: Polynoidae) from two hydrothermal fields on the Southwest Indian Ridge

Neolepetopsidae is a family of true limpets restricted to deep-sea chemosynthesis-based ecosystems. It is a small and little-studied family with about a dozen species in three genera, namely Eulepetopsis, Neolepetopsis and Paralepetopsis, and all named species were from the Pacific or Atlantic Oceans. Here, we describe three new species from Indian Ocean vents, namely Eulepetopsis crystallina sp. nov. found across three ridges,ŠNeolepetopsis ardua sp. nov. from the Southwest Indian Ridge and Neolepetopsis prismatica sp. nov. from the Carlsberg Ridge. Given that Neolepetopsis appears to specialize on inactive sulfide deposits, the apparent wider distribution of E. crystallina is probably attributable to bias in sampling effort at inactive chimneys. The molecular phylogeny of Patellogastropoda, reconstructed using the COI gene, supported the monophyly of Neolepetopsidae. These are the first molecular data available for Neolepetopsis, confirming that the three genera are genetically distinct. Eulepetopsis appears to be adapted to active vents, and its derived position compared with Paralepetopsis indicates a possible ‘stepping-stone’ evolutionary pathway from seeps and organic falls to vents. Our results provide new insights into this enigmatic family and highlight the importance of surveying the vent periphery, especially given that inactive vents are being eyed as a replacement for active ones in deep-sea mining.