Range Expansion and Environmental Determinants of the Invasive Freshwater Leech Helobdella europaea (Hirudinea: Glossiphoniidae) in Morocco

Invasive species represent a major threat to global biodiversity, highlighting the need for continuous monitoring and research to better understand their ecology, distribution, and invasion dynamics. Among freshwater leeches (Hirudinea), Helobdella europaea is considered one of the most successful and widespread invasive taxa. This study provides an updated assessment of the distribution of H. europaea in Morocco and identifies the main environmental factors influencing its occurrence and spread across the country. Field surveys were conducted between 2014 and 2024 in northern Morocco, with particular focus on protected wetlands, including Ramsar sites, and major biogeographical barriers such as the Middle Atlas Mountains and the Sebou and Moulouya river basins. Specimens were collected using Surber samplers (20 × 25 cm, 400 μm mesh), while physicochemical water parameters and other abiotic variables were simultaneously recorded. Since its first detection in Morocco in 2014, H. europaea has significantly expanded its geographic range. The species exhibited broad ecological tolerance, occurring in a wide variety of habitats, including environmentally degraded sites, and enduring substantial variation in water conditions. Among the environmental variables examined, water velocity and salinity were identified as the primary factors constraining the species’ distribution and abundance. Nevertheless, additional studies are required to better predict the future expansion dynamics of H. europaea, particularly under ongoing hydrological alterations and climate change scenarios.