Spatial distribution and density of the invasive sea urchin Diadema setosum in Turkey (eastern Mediterranean)

The sea urchin, Diadema setosum, is a poisonous species that originates in the Indo-Pacific and the Red Sea. Although this species has been recorded in several areas in the Mediterranean Sea, its habitat selection, density, distribution and ecological impacts have not been comprehensively documented to date. This study combined the diving observations (n = 53 sites) and local ecological knowledge (LEK) of SCUBA divers (n = 100) to provide information on the distribution and density of this invasive sea urchin on the Aegean and Levantine coasts of Turkey between September and October 2020. The results indicated that D. setosum was more prevalent along the southern coast of Turkey compared with the western coast, where it is colder. The highest densities of D. setosum were observed in rocky habitats at a depth range of 0–5 m. Moreover, the densities increased with increasing bottom temperatures (up to 150 ind./100 m²). Recent reports from the Levantine and south Aegean coasts highlighted that the densities of D. setosum increased dramatically during the past year. The results of the present study suggested that D. setosum provides shelter for a total of four fish species, including Chromis chromis, Gobius sp., Cheilodipterus novemstriatus and Thalassoma pavo. This species of urchin poses a threat due to its potential grazing and bioerosion effects in the Mediterranean. Thus, future studies should focus on the feeding ecology of D. setosum and the competition with native species that share the same habitat to obtain an improved understanding of the ecological impacts of this invasive species. Although the present study provides the first comprehensive dataset on the density and distribution of this invasive sea urchin for Turkey in the eastern Mediterranean region, we suggest that its population should be monitored for long periods using direct observations, LEK and citizen science.