Recent spatial distribution of alien species visually inspected along the Turkish Aegean coast and environment

Biodiversity in the Mediterranean is undergoing rapid restructuring due to climate change and the spread of non-indigenous species (NIS). This study presents the first integrated assessment of alien species distribution along the Turkish Aegean coast, utilizing SCUBA surveys, acoustic seabed mapping, and in situ environmental measurements collected between June and August 2024. Data from 321 dives (5–40 m) and grab samples (40–50 m) were analyzed to characterize NIS assemblages and their habitat associations. A total of 20 alien species were recorded, including eight macroalgae, one seagrass ( Halophila stipulacea ), seven fishes, and four invertebrates. Three macrophytes— Caulerpa cylindracea , Caulerpa taxifolia , and Stypopodium schimperi —were dominant, with C. cylindracea increasing toward the north while others remained confined to southern sectors. Invasive fishes, such as Pterois miles and Torquigener flavimaculosus , showed strong associations with seagrass patches and mixed substrates between 10 and 30 m. Significant regional differences in species composition were observed (PerMANOVA, p < 0.05), driven primarily by distinct water masses. Near-bottom temperature and salinity were the strongest environmental drivers, explaining 23.6% and 18.7% of the variance, respectively. Thermophilic and halophilic species were most abundant in southern sites influenced by Levantine water intrusions. These findings provide essential baseline data for monitoring, risk assessment, and the development of targeted management strategies in this highly dynamic marine region.