Fine-scale variability in habitat selection and niche differentiation between sponges and cold-water corals on vertical walls of the Charlie-Gibbs Fracture Zone

Deep-sea vertical walls are characterized by enhanced hydrodynamics and hard substrates, making them particularly suitable habitats for suspension feeders, including vulnerable marine ecosystems (VMEs) indicator taxa, like cold-water corals (CWC) and sponges. These species enhance the complexity of the abiotic background and the retention of trophic resources increasing habitat availability for other organisms. While some areas of vertical walls present a high density of VMEs, others are mostly bare rock. However, the habitat characteristics that favour the presence of CWC and sponges within vertical walls at a fine-scale (under 1 m) are still poorly understood despite their ecological importance. Owing to technological limitations, fine-scale terrain variables (e.g., orientation, curvature, verticality, roughness, slope) have seldom been quantified in deep-sea habitats. These terrain variables can represent proxies of community structuring factors (e.g., hydrodynamics conveying food, sedimentation rates) important to understand habitat selection of VME indicator species. In this study, we investigate the fine-scale habitat selection and niche differentiation of CWC and sponges on vertical walls of the Charlie-Gibbs Fracture Zone. Structure-from-motion photogrammetry was employed to create high-resolution 3D models of three vertical walls based on remotely operated vehicle (ROV) videos (at 1060-1901 m depth). The 3D models were used to derive terrain variables at fine-scale and geotag corals and sponges. Using ecological niche factor analysis, we reveal that corals and sponges selected habitat features that significantly differ from the average habitat available. The corals and sponges studied showed large or complete niche partitioning. Solenosmilia sp. had total niche separation with Geodia spp. and globular sponges and partial separation with Hertwigia spp., while Scleralcyonacea showed partial niche separation with Geodia spp. and globular sponges. Conversely, the niches of closely related sponges were more similar. This research advances our understanding of the processes enabling species co-existence among these organisms and the factors influencing habitat preferences of VME indicator species on steep underwater landscapes.