Ory D. auteur aut IRD Gravier-Bonnet N. auteur aut IRD Chabanet Pascale auteur aut IRD Bourmaud C. A. F. auteur aut IRD Boissin E. auteur aut IRD text journalArticle eng text/pdf reformatted digital access The western Indian Ocean (WIO) is recognized as a marine biodiversity hotspot with complex oceanographic circulation resulting in limited connectivity between remote islands. This ocean region comprises several subregions of varying biodiversity, with the northern Mozambique Channel standing out as the core of this hotspot. Although the hydroids in this region are known to include cryptic species and show contrasting connectivity patterns, the mesophotic depths remain largely unexplored. The Deep Reef Refuge Hypothesis suggests that mesophotic coral ecosystems may act as refuges. However, this hypothesis is based on several prerequisites that could be affected by the presence of cryptic species. We investigated the genetic diversity and connectivity of seven hydroid species by collecting samples at euphotic and mesophotic depths around the islands of Mayotte and Reunion. Population genetic patterns were investigated using multivariate analyses and Bayesian clustering, with 8-18 microsatellite markers per species. The results revealed greater genetic diversity in Mayotte than in Reunion, even though fewer samples were collected there. This is in line with the location of the heart of the hotspot in the northern part of the WIO. In addition, all species exhibited strong genetic differentiation between samples from the two islands, supporting the "one island, one species" hypothesis previously proposed for hydroids in the region. However, contrasting values were obtained among depths depending on the species and the island, demonstrating the importance of a multi-species approach. The inclusion of mesophotic samples from the Taxella eximia/gracilicaulis and Macrorhynchia phoenicea species complexes provides new insights into the true biodiversity of these genera, revealing additional cryptic species and putative hybridization. Furthermore, the genetic connectivity estimation performed here among depths highlights several species that could be evaluated in terms of the vertical connectivity prerequisite of the Deep Reef Refuge Hypothesis in Mayotte and Reunion. specialized connectivity Mayotte microsatellite Reunion species complex twilight zone REUNION MAYOTTE MOOREA OCEAN INDIEN 036 082 020 15 12 e72665 [18 p.] 2025 2045-7758 https://www.documentation.ird.fr/hor/fdi:010095960 10.1002/ece3.72665 2045-7758 [F B010095960] https://www.documentation.ird.fr/hor/fdi:010095960 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/2026-02/010095960.pdf IRD - Base Horizon / Pleins textes 2026-02-04 2026-02-04 fdi:010095960 fre