Improving the Knowledge on the Distribution and Ecology of the Protected Echinoid Centrostephanus longispinus (Philippi, 1845) in the Alboran Sea

Centrostephanus longispinus (Philippi, 1845) is a sea urchin widely distributed across the tropical and temperate Atlantic Ocean (including the Caribbean) and Mediterranean Sea. Although it is present along the Alboran Sea coastline (Western Mediterranean), it is generally considered rare and is listed under conservation and protection lists and conventions due to fragmented populations threatened by seabed degradation. This study provides the first density and size distribution data for this echinoid in the circalittoral and bathyal bottoms of the Alboran Sea, aiming to relate its presence to seabed features, environmental variables, and human pressures. A series of 142 underwater image transects were collected during CIRCAESAL expeditions (2021, 2023, 2024) using a ROV and a photogrammetric sledge from infralittoral to bathyal bottoms (17–856 m depth). Images were processed with OFOP software to quantify and classify individuals by size classes, depth, substrate, seafloor roughness, micro-habitat, and coverage of key benthic structuring species. A total of 524 individuals of C. longispinus were detected in 13 transects, with the highest densities recorded at 48–100 m depths in rough, rocky substrates with crevices and a moderate to low coverage of key benthic structuring species. Differences in habitat use were also observed across depth strata: individuals in shallower zones tend to remain hidden within crevices and structurally complex substrates, displaying a more cryptic behaviour, whereas those in deeper strata rely less on refuge and occupy less complex habitats. The largest aggregations occurred near the Guadiaro Canyon, outside the “Estrecho Oriental” Special Area of Conservation (SAC), suggesting this area may serve as a population reservoir deserving conservation. Despite these findings, ecological knowledge of C. longispinus remains limited, and future studies should improve the knowledge gaps, particularly in the eastern and southern Alboran Sea.