The hidden diversity of Saudi Arabian Red Sea octocorals revealed through a morpho-molecular assessment across bathymetric and latitudinal gradients

Octocorals, a globally distributed class of Cnidaria, inhabit a wide range of environments, from cold to tropical waters and from shallow to deep-sea ecosystems. In the Red Sea, studies on octocoral diversity have mostly been focused on the Gulf of Aqaba and selected families or genera. While these studies have revealed a remarkable richness and diversity of shallow-water species, mesophotic and deep-sea octocoral research remains limited in the region, in particular along the Saudi Arabian coast. Here, we provide a first comprehensive assessment of this group's genetic diversity across the basin's bathymetric and latitudinal gradients. Following six Red Sea oceanographic expeditions and various biodiversity surveys conducted between 2020 to 2023, we analysed a collection of 728 octocoral specimens sampled along 13 degrees of latitude in the Saudi Arabian Red Sea, from shallow-water reefs to deep-sea habitats. We combined morphological identification and sequencing of mitochondrial barcode markers (mtMutS and COI) to delimit lineages. Our integrated results revealed the occurrence of 26 families and 56 genera in the basin from 3 to 859 m of depth. While the description of new species was beyond the scope of this work, here we provide a reference dataset for octocoral diversity from a biodiversity hotspot, as well as essential insights to inform biodiversity management and planning of conservation measures, particularly relevant for the rapidly developing Saudi Arabian Red Sea coast.