This collaborative effort by many specialists across the Mediterranean presents an updated annotated list of alien marine species in the Mediterranean Sea. Alien species have been grouped into six broad categories namely established, casual, questionable, cryptogenic, excluded and invasive, and presented in lists of major ecofunctional/taxonomic groups. The establishment success within each group is provided while the questionable and excluded records are commented in brief. A total of 963 alien species have been reported from the Mediterranean until December 2005, 218 of which have been classified as excluded (23%) leaving 745 of the recorded species as valid aliens. Of these 385 (52%) are already well established, 262 (35%) are casual records, while 98 species (13%) remain “questionable” records. The species cited in this work belong mostly to zoobenthos and in particular to Mollusca and Crustacea, while Fish and Phytobenthos are the next two groups which prevail among alien biota in the Mediterranean. The available information depends greatly on the taxonomic group examined. Thus, besides the three groups explicitly addressed in the CIESM atlas series (Fish, Decapoda/Crustacea and Mollusca), which are however updated in the present work, Polychaeta, Phytobenthos, Phytoplankton and Zooplankton are also addressed in this study. Among other zoobenthic taxa sufficiently covered in this study are Echinodermata, Sipuncula, Bryozoa and Ascidiacea. On the contrary, taxa such as Foraminifera, Amphipoda and Isopoda, that are not well studied in the Mediterranean, are insufficiently covered. A gap of knowledge is also noticed in Parasites, which, although ubiquitous and pervasive in marine systems, have been relatively unexplored as to their role in marine invasions. Conclusively the lack of funding purely systematic studies in the region has led to underestimation of the number of aliens in the Mediterranean. Emphasis is put on those species that are current or potential threats to the marine ecosystems, namely the Worst Invasive Alien Species providing their record across major groups.

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

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