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Background: In the Mediterranean sea, about 150 species of invasive macroalgae are recorded until now these alien species have produced serious effect due to their capability to modify the physical and chemical property of the invaded habitats and to compete with native assemblages so as threatening the biodiversity and the ecosystem functioning. Results: Based on morphological characters, the invasive green alga Caulerpa taxifolia var. distichophylla, has been recognised for the first time in the Strait of Messina area, off the northeastern coast of Sicily (Tyrrhenian Sea, western Mediterranean). This record confirms the rapid spread of this taxon from the Levantine area towards the western basins, and suggests a human-mediated dispersion. The new invader occurs in different habitats than C. taxifolia (Vahl) C. Agardh, and the respective habitats do not overlap in the Strait of Messina. A previously undescribed association of this species with tropical-subtropical phanerogams and green algae, is a further example of the global change-mediated reorganisation of Mediterranean benthic assemblages. Conclusions: This record expands the known distribution range of this invasive green algae in Mediterranean Sea. [ABSTRACT FROM AUTHOR]

The current amendments to the Mediterranean marine Non-Indigenous Species (NIS) inventory for the period 2017-2019 are the result of a continuous literature search and update of the Hellenic Centre for Marine Research (HCMR) offline database. They take into account recent findings, previously missed records, back-dated records based on the re-examination of existing material or phylogenetic studies and changes in nomenclature. During the period 2017-2019, 70 new species were added to the inventory of established species, 25 that had escaped our attention in the past and 23 newly introduced, which have already established self-sustaining populations. Meanwhile, 22 species previously known only with casual records have established viable populations and a total of 36 species have expanded their distribution into new Marine Strategy Framework Directive regions, primarily the Central Mediterranean and the Adriatic Sea. Intensified research efforts, prompted by the reporting obligations created by recent legislation, complemented by ever expanding networks and initiatives involving citizen scientists have certainly contributed to higher rates of discovery of alien species presences. However, the expansion of tropical and sub-tropical species into the cooler waters of the Aegean, the Adriatic and the western Mediterranean indicates that the warming of Mediterranean waters due to climate change is also facilitating the geographic expansion of NIS in the region. The rate of new introductions in this 3-year period is 8 species per year for the whole Mediterranean, without taking into account casual records or species with reporting lags. Only 4 species per year enter through the Suez Canal, while a considerable number of species are introduced through shipping vectors and the aquarium trade. Acknowledging the dynamic nature of invasions and the uncertainty inherent in compiling check lists, we consider the present work as the most accurate and up-to-date NIS list to inform policy, management and decision-making. [ABSTRACT FROM AUTHOR]