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The Eastern Mediterranean Sea is an exceptional habitat. Its relative isolation and distinct characteristics create a unique ecosystem recognized as a marine biodiversity hot spot, where one-fifth of the species are endemic. Yet, native Mediterranean biodiversity is under increasing threat, mainly due to massive alien species invasions of Indo-Pacific origin. To date, more than 800 non-indigenous species have been reported in the Eastern Mediterranean Sea, justifying its reputation as one of the most severely affected habitats in the world in terms of marine biological invasions. Here we summarized the Mediterranean invasion dynamics of the long-spined echinoid Diadema setosum (Leske, 1778), one of the most ubiquitous Indo-Pacific sea urchin species. We show an alarming exponential population growth of D. setosum throughout the Eastern Mediterranean since 2018, following more than a decade of ‘invasion lag’ since its first detection in 2006. Molecular analyses illustrate the presence of a single genetic D. setosum clade in the Mediterranean Sea – corresponding to the Arabian Peninsula clade of this species, reinforcing the notion of a Red Sea origin. Our data support the current working hypothesis that the initial introduction of D. setosum occurred in the Northern Levantine Basin from which it gradually expanded in both north-west and south-east trajectories – in contrast to a stepping-stone hypothesis of gradual advancement from the opening of the Suez Canal. Demographic data of D. setosum along the Israeli Mediterranean coastline reveals a well-established population of broad size distributions, from juveniles to adult individuals of remarkably large size. Additionally, we provide evidence of the reproductive capacity of D. setosum in its new environment. Due to the magnitude of Diadema’s ecological footprint, it poses a severe threat to the entire Eastern Mediterranean Sea, including the Levantine Basin and South Aegean Sea, calling for rapid and coordinated action at both national and regional scales.

In this Collective Article on alien and cryptogenic diversity in the Mediterranean Sea we report a total of 19 species belonging to nine Phyla and coming from nine countries. Several of these records concern fish species, and of particular interest are the first records of: Terapon puta for Italian waters; Pteragopus trispilus from Malta; Plotosus lineatus from Cyprus; and the northernmost Mediterranean record of Lagocephalus sceleratus. The northernmost Mediterranean record was also reported for the sea urchin Diadema setosum. The portunid crab Thalamita poissonii was recorded for the first time in Libya. The copepod Pseudodiaptomus marinus was recorded for the first time in the Marmara Sea. The polychaete Branchiomma luctuosum was recorded for the first time from the Mediterranean coast of France. The alien anemone Diadumene lineata was recorded for the first time from Slovenia. The macroalgae Sargassum furcatum was recorded for the first time from Italy. The new Mediterranean records here reported help tracing abundance and distribution of alien and cryptic species in the Mediterranean Sea., European Union [730984]; ASSEMBLE Plus project; Greek Ministry of Environment and Energy [OPS 5010856], Francesco Tiralongo and Aylin Ulman are indebted to Francesco Balbini who posted the observation of the species in the Facebook group of the AlienFish project (Fauna Marina Mediterranea) and provided us data ad-ditional on the records. Tuba Terbiyik Kurt, Ximena Ve-lasquez and Tamar Guy-Haim declare that the sampling was performed in the framework of Integrated Marine Pollution Monitoring 2020-2022 Programme carried out by Ministry of Environment and Urbanization/General Directorate of EiA, Permit and Inspection/Department of Laboratory, Measurement and coordinated by TUBI-TAK-MRC ECPI. They also thank Hakan Atabay and Dr. Ibrahim Tan for supplying the zooplankton sample and WGEUROBUS of the International Council for the Ex-ploration of the Sea (ICES)for facilitating this research. Mehmet Goekoglu thanks Hakan Erdoen for the video shoot and Adnan Buyuk for the photo shoot. Joseph A Borg and Julian Evans are grateful to Edward Vella for providing photographs and details on his sighting. The study of Montesanto and Mastrototaro was supported by the European Union's Horizon 2020 research and innovation program under grant agreement N degrees 730984, ASSEMBLE Plus project. The authors wish to thank G. Chatzigeorgiou and P. Kasapidis for the assistance provided. Sara A.A. Al Mabruk and Fabio Crocetta are grateful to Ashour Al-Slaedie (Libya) for sharing pictures and data of Thalamita poissonii. Jakov Dulcic and Branko Dragicevic are thankful to Mr. Juraj Juricev for provid-ing us data and the photo of the specimen. Daniele Grech was supported by the PO-FEAMP 2014-2020 project ECOGESTOCK Approccio ECOsistemico per la tute-la e la GEStione delle risorse biologiche e STOCK ittici nelle acque interne Priorita 1, Misura 1.44, par. 6 Pesca nelle acque interne e fauna e flora nelle acque interne. The same author would like to thank the colleagues Gianni Brundu, Barbara Loi and Anuta Chindris for field and lab support (IMC, International Marine Centre). The work of Constantinos G. Georgiadis and Dimitra-Lida Rammou has been funded by the Greek Ministry of Environment and Energy within the framework of Marine Strategy monitoring Project (2018-2023; OPS 5010856). The authors would like to thank Dr. Sotiris Orfanidis. The work of Georgiadis and Evangelopoulos has been funded by the Greek Ministry of Environment and Energy within the Marine Strategy Framework Directive. The authors would like to thank Dr. Sotiris Orfanidis. Damla Beton and Mehmet Fatih Huseyinoglu thank the fisher Hasan Sari for providing information and specimens of Plotosus lineatus from Cyprus.

The invasive alien lionfish, Pterois miles is a ray-finned fish from the Scorpaenidae family, which has twelve accepted species from the genus Pterois. It was recorded in May 2014 at the Pina Reef, 2.28 miles off Kas harbour (southwestern coasts of Turkey) for the first time, and showed a very rapid establishment in the rocky habitats throughout Kas-Kekova Special Environmental Protected Area (SEPA). During the summer of 2014 only three individuals were recorded at two different locations, however as of 2017, regular observations could be made at each diving spot, which is indicative of an increasing trend. The temporal perspective of this study increases the understanding on the progression of expected range expansion of P. miles within a relatively small region, covering the period of 2014–2019. Fish count observations from underwater visual census surveys in addition to records from citizen science approach were evaluated based on spatio-temporal analysis. Resulting visualizations were prepared to present annual dynamics of spatio-temporal distribution within and outside the SEPA. Rapid expansion, up to 22 individuals per 500 m linear transects of an estimated area of 15.000 m2 each, and the increases in the numbers of fish are quantified in this study, which can be used as a baseline to develop conservation strategy for native species. Efforts to cull and create consumption demand to decrease the population trend of the lionfish have already started in the region.