A Mediterranean melting pot: native and non-indigenous sea slugs (Gastropoda, Heterobranchia) from Lebanese waters

Abstract

The Lebanese coastal area (Eastern Mediterranean Sea), despite being only 220 km long, is a sentinel zone for detecting and monitoring thermophilic newcomers entering the Mediterranean Sea from the Red Sea and the Indo-Pacific province. This is due to its strategic geographical position near the Suez Canal and the high sea surface temperature. In the framework of the Blue Tyre Project – Local Partnership for Sustainable Marine and Coastal Development (AID 012314/01/6), the coastal area of the Tyre Municipality was investigated to update the existing list of the sea slug species (Mollusca, Heterobranchia) known from this area. A total of 24 species were observed and studied through snorkelling, scuba diving and fishing by-catch census during three scientific expeditions in June and November 2022, and June 2023, at depths ranging from 0.5 to 65 meters. Results allowed to increase from 35 to 55 the number of known Lebanese species of the orders Nudibranchia, Cephalaspidea, Pleurobranchida, Sacoglossa and Umbraculida. The Nudibranchia order had the highest increase, rising from 19 previously documented species to a total of 35 species now reported for Lebanon. Among the newly reported sea slugs, five were not identifiable as already known species and will require additional analysis, being potentially new species to science. Four species were new records for the entire Mediterranean fauna, and 42% of the total recorded taxa were non-indigenous species (NIS). These results highlight the area great potential as a hot spot for heterobranch diversity, particularly regarding cryptic and/or unknown diversity. They also confirm the paramount role of Lebanese waters in providing early warning of NIS reaching the Mediterranean basin.