Diving into the lower mesophotic coral ecosystems (65-93 m depth) of Reunion Island (Southwestern Indian Ocean) sheds light on the hidden diversity of hydroids (Cnidaria, Hydrozoa)

This study explored the unknown biodiversity of mesophotic coral ecosystems (MCEs) in Reunion Island, located in the Southwestern Indian Ocean. MCEs are still largely unexplored whereas they are assumed to shelter a rich and unique biodiversity. As they might likely serve as a source for recolonization after disturbances in shallower reefs, their study is urgently needed. This work focused on an understudied large group of structural and functional ecological importance in reef benthic communities, the hydroids. Performing only 8 deep dives with closed-circuit rebreathers, we recorded 74 species between 65 and 93 m among which 11 species and 3 genera are likely new to science. Furthermore, the explorations made in contrasted geographical zones around the island suggest particular eco-physiological traits for some species: specialized vs. widely tolerant. This preliminary report only shows the tip of the iceberg and opens an avenue for exploration and understanding of such extraordinary biodiversity at such depths.