A Baseline for the Conservation of the Native and Protected Centrostephanus longispinus (Philippi, 1845) and the Management of the Invasive Diadema setosum (Leske, 1778) (Echinoidea: Diadematidae) in the Mediterranean Sea

Centrostephanus longispinus (Echinodermata: Echinoidea) is an endemic and protected sea urchin species of the Mediterranean Sea. Another long-spined sea urchin, Diadema setosum from the Red Sea, entered the Mediterranean Sea in 2006 and rapidly spread throughout the eastern basin. As a keystone competitive grazer, D. setosum has the potential to cause significant alterations in the native benthic community assemblages. The main objectives of this work were (i) to provide a synthesis of main morphological and ecological traits of Diadematidae, (ii) to determine the spatiotemporal distribution patterns of the endemic and the invasive sea urchin species, (iii) to identify critical areas where conservation efforts for C. longispinus and management strategies for D. setosum should be prioritized and (iv) to implement a simple protocol for training researchers and citizen scientists in species identification. The largest dataset on spatial and temporal distribution of the two species at Mediterranean scale was obtained from different sources, while Generalized Additive Models were implemented to forecast probability of species presence across the basin. The north-western Mediterranean emerged as a hotspot for C. longispinus, highlighting it as a key area for conserving this endemic species and studying its population dynamics. In contrast, data collection needs to be intensified in the eastern Adriatic, Levantine and central-eastern Mediterranean regions, where current information is limited or lacking. The Aegean and Ionian seas emerged as priority regions for assessing invasion risk posed by D. setosum, where applying the proposed identification protocol will be essential to prevent misidentification with the native species.