Co-Occurrence of Shallow Scleractinians Cladocora caespitosa (Linnaeus, 1767) and Oculina patagonica De Angelis D’Ossat, 1908 in the Ligurian Sea

Cladocora caespitosa is an endemic hermatypic scleractinian in the Mediterranean Sea, currently threatened by both environmental and anthropogenic pressures, whereas Oculina patagonica is a cryptogenic hermatypic scleractinian that is expanding across the basin. This study provides the first assessment of co-occurring natural populations of these shallow-water taxa, examining their population structures, habitat preferences, and responses to environmental stressors based on SCUBA surveys conducted in the summers and autumns of 2022 and 2023. Both species were dominated by medium- to large-sized colonies, indicating relatively stable population structures, with C. caespitosa exhibiting significantly higher densities than O. patagonica. Both scleractinians showed a preference for sub-vertical and vertical rocky substrates, although O. patagonica appeared more tolerant of horizontal surfaces. Disease events were more frequent and severe in C. caespitosa, particularly affecting larger colonies during autumn 2022, whereas O. patagonica showed lower incidence and greater resilience overall. Temporal comparisons suggest that O. patagonica may act as a strong competitor to C. caespitosa; however, both species demonstrate a considerable capacity for recovery, likely due to adaptation to high-stress environments. These findings highlight key differences in ecological strategies and vulnerability to environmental stressors, emphasizing the need for targeted conservation approaches to preserve Mediterranean shallow-water coral communities under accelerating climate change.