In this study we present the results of a long-term monitoring project aimed at documenting the spatial and temporal dynamics of Halophila stipulacea , an alien seagrass species that was introduced to the Mediterranean Sea from the Red Sea over 120 years ago. The study was conducted in a coastal area off the southern Tyrrhenian Sea, specifically in the harbour of Palinuro, a highly touristic place in Southern Italy. The species was first recorded in September 2006 and monitored until September 2025. During this 19-year period, three local disappearances were documented in 2010, 2020, and 2022 due to different external causes: the construction of a submerged barrier close to the largest settlement in 2008, a series of strong winter storms in 2020, and local erosion of mooring structures in 2022. These local disappearances were followed by three reoccurrence events in 2017 (six years after the first disappearance), 2021, and 2025, when the species reappeared the area, often in the same or very close to the previously established spots. The morphometric parameters of the plant remained quite constant throughout the years and were consistent with the survey period (generally May–June in each year). The occurrence of male flowers was recorded during every reappearance event, from June to early August. These data represent a rare example of multiple introductions of this alien species in the same area, and long-term monitoring demonstrating the high spatial and temporal dynamics of this seagrass and the high ecological suitability of the Palinuro harbour for its continuous colonization.
Gambi et al. (Mon,) studied this question.