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This Collective Article presents new information about the occurrence of 23 marine taxa that belong to five Phyla: two Chlorophyta, one Annelida, six Mollusca, three Arthropoda, eleven Chordata (one Ascidiacea, one Elasmobranchii and nine Teleostei) and extending from the Western Mediterranean to the Levantine Sea. All these records were reported from nine countries from the western to the eastern Mediterranean Sea, with a broad biogeographical coverage as follows: Spain: first records of the sacoglossans Cyerce graeca and Placida tardyi for the Alboran Sea and first records of the nudibranch Marionia gemmii for the Mediterranean waters of Spain; first records of nudibranch Algarvia alba in the Mediterranean Sea. Italy: First report of the long-legged crab Paragalene longicrura, a further new Mediterranean record of the rare offshore rockfish Pontinus kuhlii, first documented record of the spiny butterfly ray Gymnura altavela in Sardinia (Tyrrhenian Sea), new record of the red‐spotted wrasse Lappanella fasciata from the Messina Strait, first documented record of the rarely observed brown moray Gymnothorax unicolor in the Ionian coast, first record of the colonial ascidian Botrylloides israeliensis, first record of the Morocco dentex Dentex maroccanus, first record of mottled shore crab Pachygrapsus maurus and of an adult specimen of barracudina Lestidiops sphyrenoides in the Adriatic Sea, and further new Mediterranean records of Ross worm Sabellaria spinulosa along the same coast. Tunisia: second record of smalleye squaretail Tetragonurus cuvieri from the African Mediterranean coast. Slovenia: first records of the sea slug Trapania graeffei and Melanochlamys wildpretii, with the last one also representing the northernmost finding of this species. Croatia: northernmost record of the endangered twaite shad Alosa fallax in the Adriatic coast. Albania: first records of the Mediterranean spearfish Tetrapturus belone. Türkiye: first record from the Aegean coasts for Türkiye of the green macroalgae Cladophora battersii and Valonia aegagropila. Israel: first record of the skeleton shrimp Caprella acanthifera in the Southeastern Mediterranean Sea. Syria: first record of the Smooth grenadier Nezumia aequalis from Syrian marine waters.

To enrich spatio-temporal information on the distribution of alien, cryptogenic, and neonative species in the Mediterranean and the Black Sea, a collective effort by 173 marine scientists was made to provide unpublished records and make them open access to the scientific community. Through this effort, we collected and harmonized a dataset of 12,649 records. It includes 247 taxa, of which 217 are Animalia, 25 Plantae and 5 Chromista, from 23 countries surrounding the Mediterranean and the Black Sea. Chordata was the most abundant taxonomic group, followed by Arthropoda, Mollusca, and Annelida. In terms of species records, Siganus luridus, Siganus rivulatus, Saurida lessepsianus, Pterois miles, Upeneus moluccensis, Charybdis (Archias) longicollis, and Caulerpa cylindracea were the most numerous. The temporal distribution of the records ranges from 1973 to 2022, with 44% of the records in 2020–2021. Lethrinus borbonicus is reported for the first time in the Mediterranean Sea, while Pomatoschistus quagga, Caulerpa cylindracea, Grateloupia turuturu, and Misophria pallida are first records for the Black Sea; Kapraunia schneideri is recorded for the second time in the Mediterranean and for the first time in Israel; Prionospio depauperata and Pseudonereis anomala are reported for the first time from the Sea of Marmara. Many first country records are also included, namely: Amathia verticillata (Montenegro), Ampithoe valida (Italy), Antithamnion amphigeneum (Greece), Clavelina oblonga (Tunisia and Slovenia), Dendostrea cf. folium (Syria), Epinephelus fasciatus (Tunisia), Ganonema farinosum (Montenegro), Macrorhynchia philippina (Tunisia), Marenzelleria neglecta (Romania), Paratapes textilis (Tunisia), and Botrylloides diegensis (Tunisia)., peer-reviewed

This Collective Article presents new information about the occurrence of 20 taxa that belong to six Phyla: one Cnidaria, one Ctenophora, two Annelida, four Mollusca, two Arthropoda, and ten Chordata. These records were reported from ten countries from the western to the eastern Mediterranean Sea as follows: Spain: early colonization signs of the Mar Menor lagoon by the cigar jellyfish Olindias muelleri; France: second record of the sea chub of the genus Kyphosus in French Mediterranean waters; Italy: first record of the marbled crab Pachygrapsus maurus in Sardinian waters; first records of the polychaetes Malmgrenia polypapillata and Levinsenia tribranchiata in the Tyrrhenian Sea; new record of the deep-sea squid Ancistrocheirus lesueurii in the Tyrrhenian Sea; first record of the pignosed arrowtooth eel Dysomma brevirostre in the Adriatic Sea; Tunisia: first documented record of the blue butterfish Stromateus fiatola and new record of the iconic great white shark Carcharodon carcharias in the Gulf of Gabes; Slovenia: first records of the sea slug Diaphorodoris alba and the sharpnose sevengill shark Heptranchias perlo; Montenegro: new record of the rare tope shark Galeorhinus galeus; Greece: new records of the rabbitfish Chimaera monstrosa and the electric ray Tetronarce nobiliana; first published record of the nuribranch Discodoris rosi; Turkey: first record of the ctenophore Hormiphora plumosa at country level; first records of the anomuran decapod Munida speciosa and the Mediterranean tripodfish, Bathypterois mediterraneus from the Levantine Sea; Cyprus: first documented record of the nuribranch Scyllaea pelagica; Lebanon: first record of the killer whale Orcinus orca from the Levantine Sea.

Marine Non-Indigenous Species (NIS) are animals and plants introduced accidently or deliberately into the European seas, originating from other seas of the globe. About 800 marine non-indigenous species (NIS)currently occur in the European Union national marine waters, several of which have negative impacts on marine ecosystem services and biodiversity. Under the Marine Strategy Framework Directive (MSFD) Descriptor 2 (D2), EU Member States (MSs) need to consider NIS in their marine management strategies. The Descriptor D2 includes one primary criterion (D2C1: new NIS introductions), and two secondary criteria (D2C2 and D2C3). The D2 implementation is characterized by a number of issues and uncertainties which can be applicable to the Descriptor level (e.g. geographical unit of assessment, assessment period, phytoplanktonic, parasitic, oligohaline NIS, etc.), to the primary criterion D2C1 level (e.g. threshold values, cryptogenic, questionable species, etc), and to the secondary criteria D2C2 and D2C3. The current report tackles these issues and provides practical recommendations aiming at a smoother and more efficient implementation of D2 and its criteria at EU level. They constitute a solid operational output which can result in more comparable D2 assessments among MSsand MSFD regions/subregions. When it comes to the policy-side, the current report calls for a number of different categories of NIS to be reported in D2 assessments, pointing the need for the species to be labelled/categorised appropriately in the MSFD reporting by the MSs. These suggestions are proposed to be communicated to the MSFD Working Group of Good Environmental Status (GES) and subsequently to the Marine Strategy Coordination Group (MSCG) of MSFD. Moreover, they can serve as an input for revising the Art. 8 Guidelines.

Marine Non-Indigenous Species (NIS)are animals and plants introduced accidently or deliberately into the European seas, originating from other seas of the globe. About 800 marine non-indigenous species (NIS)currently occurin the European Union national marine waters, several of which have negative impacts on marine ecosystem services and biodiversity. Under the Marine Strategy Framework Directive (MSFD) Descriptor 2 (D2), EU Member States(MSs)need toconsider NIS in their marine management strategies. The Descriptor D2 includes one primary criterion (D2C1: new NIS introductions),and two secondary criteria (D2C2and D2C3). The D2 implementation is characterized by a number of issues and uncertaintieswhich can beapplicable to the Descriptor level (e.g. geographical unit of assessment, assessment period, phytoplanktonic, parasitic, oligohaline NIS, etc.), to the primary criterion D2C1 level (e.g. threshold values, cryptogenic, questionable species, etc), and to the secondary criteria D2C2 and D2C3. The current report tackles these issues and provides practical recommendations aiming at a smoother and more efficient implementation of D2 and its criteria at EU level. They constitute a solid operational output whichcan result in more comparable D2 assessments among MSsand MSFD regions/subregions. When it comes to the policy-side, the current report callsfor a number of different categories of NIS to be reported in D2 assessments, pointing the need for the species to be labelled/categorised appropriately in the MSFD reporting by the MSs.These suggestions are proposed to be communicated to the MSFD WorkingGroup of Good Environmental Status (GES)and subsequently to the Marine Strategy Coordination Group (MSCG) of MSFD. Moreover, they can serve as an input for revising the Art. 8 Guidelines.

This Collective Article includes records of 29 alien and cryptogenic species in the Mediterranean Sea, belonging to eight Phyla (Rhodophyta, Ochrophyta, Cnidaria, Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata) and coming from 11 countries. Notes published here can be divided into three different categories: occupancy estimation for wide areas, new records for the Mediterranean Sea, and new records of species expanding within the Mediterranean Sea. The first category includes a visual survey held along the coastline of Peloponnese (Greece), which yielded records of 15 species. The second category includes the first Mediterranean records of the Coho salmon Oncorhynchus kisutch (Greece) and of the Arabian monocle bream Scolopsis ghanam (Tunisia). The third category includes new records for countries (Ganonema farinosum in Malta, Cassiopea andromeda in Libya, Cingulina isseli in Greece, Okenia picoensis in Italy, Callinectes sapidus in Slovenia, Charybdis cf. hellerii in Malta, Urocaridella pulchella in Cyprus, Ablennes hians and Aluterus monoceros in Lebanon, and Fistularia petimba in Greece and Lebanon), new records for MSFD areas or regional seas (Septifer cumingii in the Greek Ionian Sea and F. petimba in the Marmara Sea), and confirmation of old, doubtful, or spurious records/statements (Branchiomma luctuosum in Tunisia, Thalamita poissonii in the Saronikos Gulf, and Pterois miles in Albania). Noteworthy, the three new records of F. petimba suggest that it may soon spread further in the Mediterranean Sea, as already happened for its congeneric Fistularia commersonii. Distributional data reported here will help tracing colonization routes of alien species in the basin and may facilitate the development of mitigation measures., peer-reviewed

Marine Non-Indigenous Species (NIS) are animals and plants introduced accidently or deliberately into the European seas, originating from other seas of the globe. About 800 marine non-indigenous species (NIS) currently occur in the European Union national marine waters, several of which have negative impacts on marine ecosystem services and biodiversity. Under the Marine Strategy Framework Directive (MSFD) Descriptor 2 (D2), EU Member States (MSs) need to consider NIS in their marine management strategies. The Descriptor D2 includes one primary criterion (D2C1: new NIS introductions), and two secondary criteria (D2C2 and D2C3). The D2 implementation is characterized by a number of issues and uncertainties which can be applicable to the Descriptor level (e.g. geographical unit of assessment, assessment period, phytoplanktonic, parasitic, oligohaline NIS, etc.), to the primary criterion D2C1 level (e.g. threshold values, cryptogenic, questionable species, etc), and to the secondary criteria D2C2 and D2C3. The current report tackles these issues and provides practical recommendations aiming at a smoother and more efficient implementation of D2 and its criteria at EU level. They constitute a solid operational output which can result in more comparable D2 assessments among MSs and MSFD regions/subregions. When it comes to the policy- side, the current report calls for a number of different categories of NIS to be reported in D2 assessments, pointing the need for the species to be labelled/categorised appropriately in the MSFD reporting by the MSs. These suggestions are proposed to be communicated to the MSFD Working Group of Good Environmental Status (GES) and subsequently to the Marine Strategy Coordination Group (MSCG) of MSFD. Moreover, they can serve as an input for revising the Art. 8 Guidelines.

Two major biogenic formations, composed mainly by dead corallites of the Mediterranean stony coral (Cladocora caespitosa), have been recently studied in Slovenian marine waters. The paper presents new data about the presence of coralline algae on the biogenic formation situated off Cape Ronek and off Cape Debeli rtič. Coralline algae are very important for the creation, development and maintenance of calcareous bio-concretions that offer new niches for many invertebrates and other algae. They are listed as important builders of the coralligenous biocoenosis in the “Draft Lists of coralligenous/maërl populations and of main species to be considered by the inventory and monitoring” of the RAC-SPA, and should be further deeply studied and appropriately protected.

One specimen belonging to the nudibranch Marionia blainvillea (Risso, 1818) was found crawling on the gorgonian Leptogorgia sarmentosa (Esper, 1789) at Sv. Nedjelja, Boka Kotorska Bay, Montenegro. This is the first record of the species for Montenegrin waters and the third for the Adriatic Sea., Jedan primjerak puža golaća Marionia blainvillea (Risso, 1818) nađen je da puzi na gorgoniji Leptogorgia sarmentosa (Esper, 1789) na lokaciji Sv. Nedjelja, Bokokotorski zaliv, Crna Gora. Ovo je prvi nalaz te vrste za Crnu Goru i treći za Jadransko more.

There are reasonable concerns that climate change could reduce marine biodiversity in the Mediterranean Sea. Warmer sea temperatures are also associated with the spread of invasive species and marine diseases. Due to the increased frequency of above-average temperatures and the occurrence of warm-water organisms, the Mediterranean Sea is under a process known as tropicalization. Mass mortality of marine invertebrates is becoming more common in temperate seas. Several episodes of mass mortalities, affecting populations of corals and other sessile invertebrates, have been recorded over the past 20 years in the Mediterranean Sea. Symbiotic coral species suffered the most extensive damages during the mass mortality events. Current hypotheses about the causes of coral mass mortality events mostly focus on their relationship with the occurrence of distinctive climatic anomalies during the late summer and early fall (exceptionally high and constant temperatures for over one month) usually related to the local hydrological conditions. Mortality events of the scleractinian corals Cladocora caespitosa (Linnaeus, 1767), Madracis pharensis (Heller, 1868) and Balanophyllia europaea (Risso, 1826) were recorded in the Mediterranean Sea. Coral mortality resulted from polyp bleaching (massive zooxanthellae loss) and polyp tissue necrosis, leaving the calyx rim deprived of tissue coverage. The highest mortality rates were registered after the exceptionally hot summers of 2003, 2007, 2008, 2009, 2011 and 2012, when up to 30% specimens of C. caespitosa and M. pharensis colonies were affected and more than 40% of the B. europaea specimens died, all caused by bleaching events and polyp tissue necrosis. In most cases bleaching has been attributed to elevated temperature. However, some populations could recover after bleaching events. These possible coral recoveries provide clear evidence as to how corals can adapt to stressors and thus to potentially future climate change. Unfortunately, nothing is still known of the inherent variability of key environmental factors that regulate coral fitness (like sea temperature, light, symbionts, food) or how this variability drives sensitivity to bleaching-induced mortality. Taking into account the global warming context in the Mediterranean Sea, monitoring programs of physical-chemical parameters and vulnerable coral populations should be rapidly set up.

The tetrasporophyte of the non-indigenous red alga Asparagopsis armata (the Falkenbergia stage) is considered to be established in Slovenian coastal waters. However, until 2016, it was found only in low coverage and in few localities with hard substrata. The paper reports a recent bloom of these tetrasporophytes in the mediolittoral belt of the Bay of Piran, where thalli of this red alga overgrew the articularted coralline alga Corallina officinalis. The sites affected by this large expansion of tetrasporophytes of A. armata should be regularly monitored in the future, in order to point out which environmental factors are responsible for such phenomenon, and to formulate proper conclusions on the status of this non-indigenous alga in the coastal area concerned., Tetrasporofite ne-autohtone crvene alge Asparagopsis armata (Falkenbergia faza) smatra se prisutnom u slovenskim obalnim vodama. Međutim, do 2016. godine, ustanovljena je isključivo mala pokrivenost i to na nekoliko lokaliteta s tvrdom podlogom (sediment). U ovom radu se navodi nedavni procvat tetrasoprofita u mezolitoralnom pojasu Piranskog zaljeva gdje je crvena alga A. armata nadjačala koraljnu algu Corallina officinalis. U budućnosti će se redovito nadzirati mjesta na koja utječe ova velika ekspanzija tetrasporofita, kako bi se istaknulo koji su ekološki čimbenici odgovorni za takav fenomen, te da bi se mogli formulirati odgovarajući zaključci o statusu ove ne-autohtone alge u tim obalnim područjima.

Cladocora caespitosa is an endemic coral of the Mediterranean Sea and an important carbonate bioconstructor that adds 3D complexity to the habitat, thus increasing marine biodiversity. Despite its important role in the ecosystem, the real status of the population along the majority of the Mediterranean coastline is still poorly investigated while very little is known about the resilience of the species. Using non-destructive visual surveys, colonies of C. caespitosa were investigated by SCUBA- diving in 2013 at 7 sites along the southern part of the Gulf of Trieste. Besides, one site was also sampled in July 2015. Data about colony size, index of sphericity and corallite diameter along each transect were collected. Almost all biometrical parameters significantly differed among sampling sites showing low occurrence of the larger size classes, which were relatively rare, compared to the abundance of small-sized colonies. This pattern of distribution is typical of long lived organisms. The positively skewed colony size distribution could be attributed both to a high mortality rate of small colonies unable to reach larger size class and to high fragmentation rate of colonies due to high hyrodynamism force. The historical comparison of our data of Bernardin site with the Schiller (1993) studies (1984-1989) has shown a change in the colonies size distribution, with a decrement of the small class and an increase of the medium class. In view of these findings we think that further assessment is required in order to evaluate the trend of the northernmost C. caespitosa population of the Mediterranean Sea.

Two major biogenic formations, composed mainly by dead corallites of the Mediterranean stony coral (Cladocora caespitosa), have been recently studied in Slovenian marine waters. The paper presents new data about the presence of coralline algae on the biogenic formation situated off Cape Ronek and off Cape Debeli rtič. Coralline algae are very important for the creation, development and maintenance of calcareous bio-concretions that offer new niches for many invertebrates and other algae. They are listed as important builders of the coralligenous biocoenosis in the "Draft Lists of coralligenous/maërl populations and of main species to be considered by the inventory and monitoring" of the RAC-SPA, and should be further deeply studied and appropriately protected.

A juvenile male of porbeagle (Lamna nasus) was caught in waters off Piran (Slovenia, northern Adriatic) on December 22, 2015. The specimen was accurately measured and weighed. Cephalopods and fish remains were found in its stomach contents. This is the first record of a porbeagle in the waters of Slovenia and in the Gulf of Trieste, and one of the few records up to date reported in the northern Adriatic Sea. The presence of this juvenile specimen arises a question whether the Adriatic Sea is a reproductive ground of this species. According to an older record from 1910, a female porbeagle with four embryos was caught on Ugljan Island, confirming the fact that porbeagles reproduced at least in the past in the Adriatic Sea., Nedorasla jedinka mužjaka kučine (Lamna nasus) uhvaćena je u vodama Piranskog zaljeva (Slovenija, sjeverni Jadran) 22. prosinca 2015. Primjerak je izmjeren i izvagan. U sadržaju želuca pronađeni su glavonošci i ostaci riba. Ovo je prvi zapis o kučini u tršćanskom zaljevu, i jedan od rijetkih zapisa prijavljenih u sjevernom Jadranskom moru. Prisutnost ovog nedoraslog mužjaka povlači pitanje da li je područje mrijesta ove vrste Jadransko more. Prema starijem zapisu iz 1910. kod otoka Ugljana je uhvaćena ženka psine kučine s četiri embrija. Time se potvrdila činjenica da se ova vrsta barem u prošlosti razmnožavala u Jadranskom moru.

The Lesser Neptune grass, Cymodocea nodosa, is the most common seagrass species in shallow sheltered to semi-exposed sites along the Mediterranean soft bottom. The MediSkew index was recently developed as an improvement of the CymSkew index, in order to assess the status of C. nodosa meadows correctly in view of the implementation of three European Directives. The index takes into account the length of the photosynthetic part of C. nodosa leaves, which increases from the less degraded meadow to the most degraded meadow. To adequately assess temporal and spatial trends in the status of C. nodosa meadows, including estimates of the effects of natural disturbances within marine protected areas, the MediSkew index was applied to new samples collected at 3 m and at 6 m of depth, during two months (in July and in September). The analyses are discussed in view of monitoring and conservation of C. nodosa meadows. According to the results of this study, the monitoring programme in the northern Adriatic Sea should be conducted in July, with the collection of samples at 3 m of depth. The presented sampling and assessing methodology proved to be time- and cost-effective for the evaluation of the status of C. nodosa meadows and of human-induced pressures., Čvorasta morska resa, Cymodocea nodosa, najučestalija je vrsta morske cvjetnice u plitkim zaklonjenim i poluzaklonjenim uvalama duž mediteranskog mekog dna. MediSkew indeks je nedavno razvijen kao poboljšanje indeksa CymSkew u cilju ispravne procjene stanja livada vrste C. nodosa u skladu s provedbom triju europskih direktiva. Indeks u obzir uzima duljinu foto sintetskog dijela lišća C. nodosa, koji se povećava od manje do najviše degradirane livade. Kako bi se na odgovarajući način procijenili vremenski i prostorni trendovi stanja livada vrste C. nodosa, uključujući procjene učinaka prirodnih poremećaja unutar zaštićenih morskih područja, MediSkew indeks primijenjen je na nove uzorke prikupljene na 3m dubine i na 6m dubine, tijekom srpnja i rujna. Analize su rađene s obzirom na praćenje i očuvanje livada vrsteC. nodosa. Prema rezultatima ovog istraživanja, program praćenja u sjevernom Jadranu trebao bi biti proveden u srpnju, a uzorci bi se prikupljali na 3m dubine. Prikazana metodologija uzorkovanja i procjenjivanja pokazala se vremenski i troškovno učinkovita za procjenu stanja livada vrste C. nodosa i ljudskog utjecaja na njih.