Unveiling the Microeukaryotic Landscape of the Red Coral <scp> <i>Corallium rubrum</i> </scp> Across the Northwestern Mediterranean Sea

Octocorals such as Corallium rubrum are key components of temperate Marine Animal Forests (MAFs), providing three-dimensional habitats that support diverse marine life. However, C. rubrum faces growing threats from overexploitation and climate stressors such as ocean warming. While the coral's bacterial microbiome is well-documented and stable across spatial and temporal scales, the associated microeukaryotes, collectively referred to as the coral eukaryome, remain poorly characterised. In this study, we used 18S rRNA gene metabarcoding to explore the eukaryome of 46 C. rubrum colonies collected from five sites (~44,670 km²) in the northwestern Mediterranean. We identified a limited set of core microeukaryotic families, including Licnophoridae and Dino-Group I Clade 1, which were present at all sampling locations. Despite sharing core taxa, eukaryome composition showed high variability between sampling sites, seasons and years. This suggests the red coral eukaryome is strongly influenced by local environmental factors. Given the increasing frequency of marine heatwaves and habitat degradation, further research is needed to understand the ecological roles of key eukaryotic taxa and their contribution to coral holobiont resilience. Clarifying the function of the eukaryome is essential for predicting how C. rubrum and other habitat-forming octocorals will respond to future climate scenarios.