Genetic Structure in a Trans‐Oceanic Hot Vent Mussel Reveals Four Metapopulations With Implications for Conservation

Aim Genetic diversity and connectivity are crucial to informing the conservation strategy for deep‐sea organisms, especially those threatened by deep‐sea mining. The vent mussel Bathymodiolus septemdierum has an extraordinary trans‐oceanic distribution range across the Indo‐West Pacific including numerous sites eyed for exploitation. Here, we aimed to assess the connectivity of B. septemdierum across its entire range, for the first time. Location Hydrothermal vent fields on the Carlsberg Ridge (CR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), as well as those in the Mariana Trough and the Southwest Pacific. Taxon Bathymodiolus septemdierum Methods Combining new and published data, we studied the genetic structure, historical demography and gene flow of B. septemdierum using three mitochondrial and two nuclear genes from a total of 444 individuals across 16 vent fields. Results Bathymodiolus septemdierum populations are structured with four regional metapopulations including (1) the whole Western Pacific, (2) CR in the northern Indian Ocean, (3) vents on the SWIR and southern CIR (CIR + SWIR) and (4) Onnuri field on the northern CIR. IMa3 analysis shows divergence first occurred between WP and the Indian Ocean, and then between CR and CIR + SWIR metapopulations. The gene flow towards CR from both CIR + SWIR and WP is higher than the reverse directions, while Onnuri received exceptionally high migrations from both CIR + SWIR and CR metapopulations. Main Conclusions Our findings support a large‐scale dispersal scenario where larvae are carried westwards from the Pacific Ocean into the western Indian Ocean through Indonesia, and then separated into northern and southern Indian Ocean clades. The CR acts as an isolated gene pool for this species among others, and the Onnuri field serves as a key transition zone in the Indian Ocean, and we urge for the prioritised protection of these vents currently targeted by deep‐sea mining.