Cryptic Diversity and Genetic Differentiation of Mesophotic Hydroids in the Southwestern Indian Ocean.

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