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1. The disproportionately low presence of marine species in the list of invasive alien species (IAS) of Union concern of the European Union (EU) Regulation 1143/2014 does not fully acknowledge the threat they pose to the EU marine environment.2. In this study, the first EU-scale Horizon Scanning (HS) focusing on marine alien species was performed, aiming to deliver a ranked list of species that should be of high priority for risk assessment (Article 5 of the EU IAS Regulation).3. Species absent from or with a limited distribution in EU marine waters were targeted. In total, 363 alien species were initially screened for HS by a panel of experts, including a broad range of taxonomic groups. Species were scored for their likelihood of arrival, establishment, spread, and impact in EU waters. 4.A consensus workshop ranked 267 species, including a subset of 26 prioritized species. These species are considered to be mainly introduced by shipping (fouling and ballast water), via the Suez Canal, and aquaculture activities. The 26 priority species were also scrutinized in terms of feasibility of their management; 18 of them were suggested for performing risk assessments on the basis of the EU IAS Regulation.5. Since biological invasions are dynamic and connected with accelerated globalization and diversified human activities, we recommend HS to be repeated periodically to review the species already listed and assess new ones.

The widespread sea hare species Aplysia parvula includes four genetically distinct lineages, containing a total of ten different species. While the four lineages can be differentiated by their external characteristics, species in each clade are often morphologically indistinguishable. A review of literature and type material revealed that several available names exist for species recognized herein: Aplysia parvula is retained for a species from the north-eastern Atlantic; A. atromarginata, A. elongata, A. nigrocincta and A. japonica are resurrected for species from the western Pacific Ocean, the Hawaiian Islands, the Indian and western Pacific Oceans, and Japan and Korea, respectively. Two new species names are introduced for animals from the eastern Pacific, and the Atlantic and Indian Oceans. Mitochondrial sequences from Mediterranean and eastern Atlantic specimens identified as A. parvula, resulted to be A. punctata. However, two specimens were heterozygotes of histone H3 alleles of A. punctata and of a new Atlantic species described herein, suggesting they could be hybrids. These results contradict the hypothesis that the Mediterranean was colonized by A. parvula. If an invasion occurred, it was a limited introgression of nDNA from an Atlantic species into native A. punctata populations.

This Collective Article presents information on 22 species belonging to 5 Phyla, arranged geographically from from west to east. The new records were found in 8 countries as follows: Spain: first record of the two scarcely known nemerteans Baseodiscus delineatus and Notospermus geniculatus in Formentera; Malta: second record of the alien fish Lagocephalus sceleratus; Italy: the alien polychaete Syllis pectinans and the isopod Paranthura japonica, as well as the cryptogenic opisthobranch Anteaeolidiella lurana, were found in the fouling assemblages along the docks of the port of Livorno. New decapod records are reported from Sicily (the alien Callinectes sapidus and the native Pachygrapsus maurus) and Apulia (Percnon gibbosus and Procambarus clarkii); the lesser amberjack Seriola fasciata extended its geographical range to the Egadi Isands and Siganus luridus was documented for the first time along the Ionian coasts of Apulia and Calabria. Slovenia: the first record of the alien bivalve Brachidontes pharaonis is reported, together with a survey of up to date Adriatic records. Greece: the first record of the gastropod Rhinoclavis kochi is reported from Gavdos island. In addition, two records of endangered and rare cartilaginous fish were reported, namely, the shark Hexanchus griseus and the ray Leucoraja fullonica, as well as additional records of Siganus luridus for Lesvos and Malleus regula and Fulvia fragilis from Astypalaia. Turkey: the black wing flyingfish Hirundichthys rondeletii is reported for the very first time from the Black Sea. Egypt: the Indo-Pacific crab Halimede ochtodes is reported as established in Port Said. In addition, biometric parameters and meristic counts are reported for Anthias anthias in Damietta. Cyprus: the alien opisthobranch gastropod Bursatella leachii is reported for first time.

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The census of natural populations provides relevant information for both conservation purposes and environmental monitoring. In the present work, natural populations of the killifish Aphanius fasciatus (Teleostei: Cyprinodontidae) were censused in the Italian territory. The historical distribution of the species was reconstructed with the aid of bibliographic material and museal samples, in order to evaluate the extent of changes in its distribution. Three periods (pre-1990s, 1990s, post-1990s) and five habitat types (lagoons/coastal ponds, saltworks, canals, river mouths, inland water bodies) were considered. Overall, we recorded the presence of A. fasciatus in 82 localities, the majority of which are lagoons/ponds. The species showed a widespread decline and sometimes local population extinction, due to habitat degradation and competition with the introduced poeciliid Gambusia holbrooki. With a few exceptions, A. fasciatus is currently extinct in inland waters, being mainly present in coastal brackish-water habitats. The occurrence of stable populations along the central Tyrrhenian, northern Adriatic, Sardinian and Apulian coastal biotopes is confirmed. The populations of Sicilian inland waters, considered extinct, showed recovery capabilities. In northern Tuscany, where the species was also considered extinct, there are also recent records. From a temporal perspective, the present work shows that A. fasciatus experienced population reductions throughout its range in the 1990s, when the species appeared locally extinct, or strongly rarefied, in several localities. Since then, the species recovered in the majority of the native range. However, careful monitoring is needed in order to control the effects of habitat alterations and competition with G. holbrooki on natural populations of A. fasciatus. Given the importance of brackish-water habitats as both biodiversity hotspots and buffer areas against rainfalls and floods, A. fasciatus may be raised to the status of "umbrella species" with regard to these habitats.

Non-indigenous species (NIS) represent one of the most relevant threats to biodiversity, ecosystem functioning and human activities, and their occurrence and spread have been the subject of numerous works and revisions. However, the information available is rather confused for several taxa, including polychaetes, which are characterised by having a high number of cryptogenic and questionable species. This work aims at providing a revised checklist of alien polychaetes occurring along the coasts of Italy, based on the examination of newly collected and deposited material and on the critical analysis of published and gray literature, and whenever possible on the re-examination of historical material. Of the 86 polychaete species reported as NIS in Italian waters, 25 are confirmed as alien species, while 3 are cryptogenic, and 40 should be considered questionable. Finally, 18 species were excluded from non-indigenous species checklists, either because they are native, or because they represent misidentifications of other species. The high number of cryptogenic and questionable species points at the need of molecular studies and taxonomic revisions for the majority of polychaete taxa reported as NIS, in order to clarify their taxonomy, origin, introduction pathways and spreading patterns.