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We present a contribution to the knowledge of marine and brackish water alien species (infraspecific taxa included) recorded along the Italian coasts. The Italian Peninsula, with over 7,000 km of coastline, is located in the center of the Mediterranean Sea, splitting the Western and the Eastern basins. Data were collected from published material, mostly authored by the experts of different marine taxa participating in the "Allochthonous Species Group" of the Italian Society of Marine Biology (SIBM). The data have been reviewed according to the taxonomic expertise of the authors and are organized in a referenced database containing information on each species about: distribution along Italian coasts, the native range, most probable vectors of introduction, population status and impact. The total number of marine alien species recorded along Italian coasts during the selected time period 1945-2009 is relatively high: 165 species, in many cases native from tropical regions of the world. Most of them were introduced in the 1980s and 1990s, whereas in the last few years the number of new records has decreased. The highest number of alien species has been observed in the northern Adriatic Sea, particularly in the Lagoon of Venice, which is the main hotspot of introduction. Of the total number of species, 46% was unable to establish resident populations in the Italian seas; 15% (24 species) rapidly increased their populations and extended their geographical range, consequently they may be considered invasive species

Assessment of the ecological and economic/societal impacts of the introduction of non-indigenous species (NIS) is one of the primary focus areas of bioinvasion science in terrestrial and aquatic environments, and is considered essential to management. A classification system of NIS, based on the magnitude of their environmental impacts, was recently proposed to assist management. Here, we consider the potential application of this classification scheme to the marine environment, and offer a complementary framework focussing on value sets in order to explicitly address marine management concerns. Since existing data on marine NIS impacts are scarce and successful marine removals are rare, we propose that management of marine NIS adopt a precautionary approach, which not only would emphasise preventing new incursions through pre-border and at-border controls but also should influence the categorisation of impacts. The study of marine invasion impacts requires urgent attention and significant investment, since we lack the luxury of waiting for the knowledge base to be acquired before the window of opportunity closes for feasible management.