Management of biological invasions strongly depends on early and accurate detection of non-native species, yet species identification is often complicated for various reasons. One prominent example relates to the controversy about the genetic specimen assignment of Asian and Oriental weatherfish species introduced into Europe. Weatherfishes, comprising the genera Misgurnus and Paramisgurnus (Cobitidae) are small benthic freshwater fishes with a wide range of habitats in the temperate to subtropical regions of Eurasia. Many of the eleven described species have been introduced outside their native ranges, mainly through ornamental trade and as food. Due to their poorly known life cycles, unclear morphology, overlapping meristic features and frequent hybridisation, the challenges associated with accurate species identification in this group comprise cryptic species and cryptic invasions, unresolved classical and molecular taxonomy, haplotype sharing and incomplete molecular genetic reference databases. Based on our newly generated molecular phylogeny comprising 289 published weatherfish COI barcodes, the existence of distinct phylogenetic clusters is evident. Except for the endangered Central European species, Misgurnus fossilis, and an unnamed cluster from Vietnam, all clusters were polyphyletic. Haplotype sharing was frequently observed, as well as specimens only labelled to genus or higher taxonomic levels. We conclude that genetic analysis of type specimens or type regions to resolve the underlying taxonomy and complete the reference databases would be necessary as prerequisite for accurate species identification in the weatherfish group. Such information is crucial in assessing their worldwide species distribution patterns, ecosystem impacts and invasive potential. As molecular genetic databases are constantly growing, new taxa are being proposed, and taxonomies are being changed in light of new data, it is obligatory to consider past publications in light of the dynamics of species names and taxonomic phylogenies. We still recommend early sharing of exotic species records since such knowledge is particularly crucial when it comes to management of invasive species.

Senecio brasiliensis (Spreng.) Less., native to South America (Brazil, Paraguay, Uruguay, Bolivia and Argentina), is reported for the first time as a naturalized alien species from Europe (near Porto; Portugal). The species’ taxonomy and nomenclature were analysed based on literature data. Its ecology, habitat, biology and possible vector of introduction are discussed. As a necessary step towards a risk assessment, the suitability of European climates for the species’ settlement was analysed. Fourteen countries with suitable climatic conditions appeared in all our models: Albania, Bosnia-Herzegovina, Bulgaria, Croatia, Cyprus, France, Greece, Italy, Macedonia, Montenegro, Portugal, Slovenia, Spain and the United Kingdom (England). Given the ecological characteristics of the species and the climatic features of the areas analysed, it is concluded that S. brasiliensis exhibits a great potential to become an invasive alien in Europe.

Invasive aquatic plants can spread rapidly causing many negative effects on both the structure and functioning of aquatic ecosystems. They can also hinder navigation, fishing, water tourism, and recreation. Early detection and rapid response to invasive aquatic plants can prevent their further spread and negative impacts. In this study, we present the first record of two invasive alien aquatic plants Limnobium laevigatum and Pontederia crassipes in Poland. The plants were found in an artificial pond in the eastern part of the city of Kraków, southern Poland, on 16 September 2020. The pond was also occupied by Pistia stratiotes, another invasive alien aquatic plant that has been recently recorded in Poland. The locality is mapped using the ATPOL cartogram method, photographic documentation of the habitat and species is provided and the status of L. laevigatum and P. crassipes in the Polish flora is discussed.

Orientus ishidae (Matsumura, 1902) (Hemiptera: Cicadellidae), known as the mosaic leafhopper, is an invasive alien species native to the Eastern Palaearctic, but also widespread and well established on the eastern territory of North America. Since its first detection in Europe in 1998, this polyphagous species has expanded its distribution area and rapidly spread through central and western European countries, inhabiting a wide range of broadleaf trees and shrubs (e.g. Gleditsia triacanthos, Salix spp., Corylus spp., Acer spp., Betula spp., Populus spp. and Carpinus spp.), which are usually present in the surrounding of vineyards. Over the last decade, O. ishidae was frequently found within European vineyards and was proven to be able to complete its life cycle on grapevine. Moreover, this leafhopper was shown to be naturally infected with the Flavescence dorée phytoplasma (FDp), a devastating disease which causes significant economic losses in the major vine-producing countries. This study provides data on the occurrence of O. ishidae in Serbia, clearly confirming that it has expanded its distribution range to the Balkan Peninsula which could lead to establishment of new FDp epidemiological cycles inside the local vineyard regions. As a consequence, there could be an increased negative impact on phytosanitary situation with a continual spread of this vector species and FDp epidemic outbreaks in Southeastern Europe.

To learn whether the recent increase in the number of Central European spider species reflects a still-incomplete state of faunistic research or real temporal changes in the Central European fauna, we evaluated the records of 47 new species observed in 2008–2020 in Czechia, one of the faunistically best researched regions in Europe. Because of the intensified transportation of materials, enabling the introduction of alien species, and perhaps also because of climatic changes that allow thermophilic species to expand northward, the spider fauna of this region is dynamic. Our analysis showed that only 15 spider species newly recorded in Czechia likely belong to the indigenous fauna. The remaining two-thirds likely appeared in this region recently. Half of these species are likely thermophilic species that expanded their distribution to the north, possibly due to global warming, and the second half are subtropical or tropical species introduced to heated buildings. Only three species were introduced to natural habitats, and only two of them, Mermessus trilobatus and Erigone autumnalis (Linyphiidae), can be considered true invasive species.

The red imported fire ant, Solenopsis invicta Buren, is one of the most damaging pest species in the world. A native to the floodplains of subtropical South America, it has been spread to North America and more recently to Australia and Asia. In order to effectively control, manage, and mitigate its impact and spread, it is critical to understand its current distribution. While S. invicta has long been known from the smaller Caribbean islands, it has not been documented from the two largest islands (Hispaniola and Cuba). We present the first record of S. invicta from Hispaniola and discuss its presence in relation to its native congener, S. geminata.

Invasive alien species pose a threat to ecosystems and the native biodiversity they contain. Accordingly, potential or confirmed invasions require that non-native species are accurately identified and reported. Spionidae is one of the most diverse families of annelid worms, and a dominant group in terms of the number of species that have been introduced to non-native areas. Pseudopolydora is a genus of spionids for which most species are known from the Western Pacific. Collections around Fort Pierce, Florida resulted in identification of four Pseudopolydora species, including P. floridensis, P. achaeta, P. paucibranchiata, and P. rosebelae, with the last three representing new records for the Atlantic coast of North America. Specimens were identified based on morphological descriptions for this group, and the standard COI barcode region was sequenced. Introductions and subsequent invasions of marine worms such as spionids often go unrecognized and thus their ecological impacts understudied. Here, we call attention to non-native Pseudopolydora species so that their dispersal potentials and ecological consequences can be investigated.

The freshwater polychaete Hypania invalida is indigenous to the Ponto-Caspian region. The transfer of the species from the Black Sea to most of Europe was mainly via the Danube River, and its distribution increased rapidly after the opening of the Rhine-Main-Danube Canal in 1992. The presence of this species may influence the relationships of other species within the benthic community and thus has significant implications for the food webs of aquatic ecosystems. Our aim was to provide detailed records of the distribution of this potentially invasive species in Croatia, with evidence of upstream invasion and a description of environmental conditions at the sites, together with species co-occurrence, as a basis for assessing the future risk of upstream invasion and for monitoring purposes. Hypania invalida individuals were recently found at five different sites in Croatia in the Drava and Sava River systems and at one additional site in the Danube. Most of the new records support the idea of upstream invasion predominantly via large rivers. We found that the species inhabits both semi-natural river sections, and smaller canals, contradicting the idea that it inhabits only hydromorphologically heavily impacted main river channels.

We report the occurrence of non-indigenous Branchiodrilus hortensis (Stephenson, 1910) in Serbia and its rapid spread in Southeast Europe. These new findings were observed at locations about 1000 Rkm downstream from the nearest known record in the Danube (in July 2008) in the middle part of the river section in the Slovak-Hungarian part. Branchiodrilus hortensis was registered for the first time in Serbia in September 2018 during routine monitoring of the Danube River in the lower part of the river section. Since then, we have continuously found this species in the Serbian part of the Danube, and it has spread to the main tributaries and canals of the Danube basin. These new records are a valuable contribution to the knowledge of this species’ distribution. Recent findings and the native distribution of this worm suggest that its introduction in the Danube was probably human-mediated, and it has successfully established its niche in the new environment. At this phase we cannot predict the possible effects of its presence on aquatic ecosystems and therefore further monitoring of its distribution and population dynamics is necessary.

The Vermetidae is a family of sessile marine gastropods whose members are difficult to identify accurately, but one species, Eualetes tulipa is widely documented as an introduced species globally. In Asia, there is only one known record of this species from India to date. We report the presence of E. tulipa in Singapore based on DNA barcoding. This is the second known record from Asia and the first confirmation of a non-native vermetid for Southeast Asia.

Epitonium (Parviscala) vaillanti (Jousseaume, 1912), a species previously known only from the Red Sea and Gulf of Aden, is recorded for the first time in the Mediterranean Sea. This new Erythrean alien was found in the course of examination of unidentified specimens from Mediterranean coast of Israel, showed morphological features unknown to any previously recorded Mediterranean epitonid species. To confirm identification, morphological comparison with the syntypic serie of Avalitiscala vaillanti Jousseaume, 1912, stored in the Museum d’Histoire Naturelle de Paris (MNHN) was performed and some remarks of the taxonomy of the species are discussed.

The River Danube has played a pivotal role in the range expansion of Ponto-Caspian faunal elements in recent decades; therefore, the monitoring of its biota is of high scientific and conservation importance. In this publication, the records on peracarid crustaceans yielded by the macrozoobenthos samples for the 4th Joint Danube Survey (2019) are presented. Altogether 21 species (16 Amphipoda, 2 Isopoda, 3 Mysida) were recorded at 44 sites in the Danube (between river km 2581 and 18) and its major tributaries. Invasive Ponto-Caspian species showed the most common occurrence, some of which (Chelicorophium robustum, Chelicorophium sowinskyi, and Paramysis lacustris) have been observed beyond their previously known distribution. Nevertheless, the records of Gammarus spp. in the German Danube section might potentially indicate an improvement in the status of these native species in the region. The survey also confirmed the intensifying colonization of the River Tisza by Ponto-Caspian species, showing the first record of C. robustum in the river as well as additional occurrences of the previous invader, Pontogammarus robustoides. Besides these notable records, the dataset will also serve as a useful reference for potential further range expansions.

We report the presence of the North American Crangonyx floridanus co-occurring with C. pseudogracilis in two Irish rivers, following surveys in three rivers, eleven lake sites and an ornamental pond in 2020. This is the first record of C. floridanus in Ireland which was previously recorded in Britain in 2017. We predict that this species is likely to expand its range through the river and canal connections in Ireland as well as from inadvertent anthropogenic transmission. The taxonomic criteria are cryptic and for this reason, we used genetic methods to distinguish the two species in this study. The results highlight the need to investigate the distribution and impact of these two species in freshwater habitats in Ireland.

The brown shrimp Penaeus aztecus Ives, 1891, a well-established alien species in the Mediterranean Sea, is here recorded from Croatia (central-northern Adriatic Sea) on the basis of 18 specimens found since November 2016. Such numbers triple the number of confirmed specimens known from the basin so far and testify its northern spreading in the Adriatic Sea. There are no certainties regarding possible pathway(s) of arrival of this species in the area. However, unaided dispersal from neighbouring countries constitutes the most probable one, although introduction through ballast water or deliberate introduction or escape from aquaculture facilities cannot be ruled out. All the specimens were captured while trawling on muddy substrates at depths ranging from 50 to 200 m, and were communicated to the authors of the present note by local fishermen. This confirms the importance of fishermen knowledge to detect the occurrence of non-indigenous megafauna in the Adriatic Sea as a complementary tool to proper scientific surveys.

The invasive American Harris mud crab is reported for the first time from the Thames catchment in proximity to East India Dock basin and the confluence with the River Lea (Bow Creek), East London. Rhithropanopeus harrisii (Gould, 1841) has only once previously been recorded in British waters from Roath Docks, Cardiff, South Wales. Although the current status of R. harrisii in Roath Dock and the Thames at Bow Creek requires clarification, the Harris mud crab has been established in North European estuaries since 1874 and has spread rapidly elsewhere.

Numerous native European freshwater crayfish populations are in decline due to the lethal disease crayfish plague caused by the pathogen Aphanomyces astaci (Oomycetes). Presence of this pathogen has already been recorded in the Plitvice Lakes National Park (NP), Croatia, where two protected native crayfish species, the noble crayfish Astacus astacus and the stone crayfish Austropotamobius torrentium occur. Data presented in this manuscript are part of a two-year monitoring project of the pathogen in the NP. Previously, we have reported the overall prevalence of A. astaci positive individuals in A. astacus and A. torrentium populations (14% and 2% of tested crayfish, respectively) within the NP. Here, we report the detailed distribution of the pathogen within the NP and differences in its prevalence in populations of both crayfish species collected at different locations within the NP. Moreover, we have identified the A. astaci genotype present within the NP and discuss our findings in the context of crayfish plague related management activities in the NP. The majority of pathogen records were from the A. astacus populations which were all in close proximity, and this presumably facilitated the pathogen spread among them. Prevalence of A. astaci positive A. astacus varied in different populations from 10% to 18%. In comparison, only one out of three tested A. torrentium populations, in the proximity of the A. astacus populations, was A. astaci positive (prevalence 17%), while in the other two geographically distant populations pathogen was not recorded. The microsatellite genotyping identified the presence of A. astaci haplogroup A (As genotype). Although known for its low virulence, A. astaci of haplogroup A could still cause mass mortalities, especially if combined with other stressors that can impair crayfish health. Obtained results were used as a baseline for the development of monitoring protocol for A. astaci and were incorporated in the management plan for protection of vulnerable native species in the NP. We present the proposed management activities for prevention of unintentional spread of the crayfish plague pathogen to adjacent streams that are inhabited by pathogen free native crayfish populations.

Mediterranean biomes have been recognized as having high value due to their diversity; they are under threat due to anthropic pressures. Both freshwater fish and amphibians show high endemism and vulnerability to several threats; among the latter is the introduction of fishes. In Chile there is scarce documentation of the presence of the non-native Australoheros facetus (chameleon cichlid), where it cohabits with other non-native and native fish and anurans. In this study we first sampled for the presence/absence of A. facetus in 69 localities in the Chilean Mediterranean region. Secondly, to estimate their potential impact, we evaluated the predatory capacity of A. facetus and other two sympatric fish, Gambusia holbrooki (mosquitofish; non-native) and Cheirodon pisciculus (pocha; native), on larvae of four species of native anurans and on the eggs of a non-native anuran. We report that A. facetus has expanded in the entire Chilean Mediterranean region; its presence is related to low altitude areas. We confirmed that lentic environments and irrigation systems of streams increase the probability of successful establishment. The results for predatory capacity showed that A. facetus regularly predated larvae of all the tested anurans, being significantly more successful in predating larvae, but also significantly less successful in predating eggs, compared to other fish species. The control of non-native fish is a priority to avoid the invasion of new localities in these ecosystems.

We report the presence of the alien freshwater teleost Gymnocephalus cernua in the volcanic lake of Bracciano in central Italy. The contribution of local citizens was essential for this finding, particularly the interviews with local professional fishermen made it possible to report the presence of this species. To confirm this datum, some specimens were collected and identified at the species level by morphological characteristics. Biological parameters were collected (standard length, mass, sex, mass of gonads and of digestive tracts) from each specimen. From the observation of gonads, we confirm the presence of mature individuals in Lake Bracciano during autumn 2020. This report represents the southernmost data for this species in the European distribution range. For this reason, this population may be further studied to evaluate the adaptability and invasiveness potential of this species in southern habitats, particularly in volcanic lakes.

These are the first records of the spotted raphael catfish Agamyxis pectinifrons in a Mexican basin. Three adult specimens, two female and one undetermined, were collected in the floodplain of the Grijalva river near the Metropolitan Zone of Villahermosa. The presence of catfish is likely due to releases by aquarists. Whether this non-native catfish is able to settle and spread in the floodplain of the Grijalva basin is uncertain. If established, this detection will indicate another step in continuous increase in non-native catfish species in Mesoamerican basins dominated originally by native cichlid and poeciliid species.

We report the first confirmed record of the invasive alien species, eastern tubenose goby Proterorhinus nasalis (De Filippi, 1863) in the Baltic Sea as a first verified record of this species outside Azov and Caspian Sea basins and the Volga River system. Several tubenose gobies with inconclusive morphometrical characteristics were caught from the Narva Bay, eastern part of Gulf of Finland in September 2020. Phylogenetic analysis of caught individuals confirmed their taxonomic classification as Proterorhinus nasalis. Relatively high abundance of tubenose gobies hints that P. nasalis may have established a naturalized population in the Narva Bay.

An individual of the non-indigenous fish Terapon jarbua and one of Acanthopagrus bifasciatus were fished from the coastal waters off Alexandria, Egypt in November 2020. The former was collected with shore jigging and the latter with a trammel net. The record of T. jarbua constitutes the first one for the Mediterranean Egyptian waters and the second one for the entire Mediterranean basin. The record of A. bifasciatus is the first one for the Mediterranean Egyptian waters and the fourth for the Mediterranean. The morphometric and meristic characteristics of the recorded individuals are hereby presented, along with different hypotheses explaining the entry within the Mediterranean of these non-indigenous species.

Alien species are considered one of the main threats to biodiversity, and amongst them turtles are the most traded taxon worldwide. We are reporting observations on the Chinese striped-necked turtle (Mauremys sinensis Gray, 1834) and particularly the first evidence of reproduction in a European wetland. During an eradication project of non-native turtles, underway since 2019, we observed a hatchling of Mauremys sinensis in the Torre Flavia wetland (Tyrrhenyan coast of Central Italy). This finding suggests the naturalization of the local population, requiring increased attention since the disturbance by M. sinensis adds to that of the invasive population of Trachemys scripta in the study area. In the absence of control and management on introduced nuclei, this situation can lead to a widespread naturalization of M. sinensis, with detrimental and irreversible impacts on endangered native taxa such as Emys orbicularis.

Spatial baseline distribution information on the presence of the freshwater invasive alien species pond slider Trachemys scripta and the red swamp crayfish Procambarus clarkii, in Cyprus, is comprehensively reported for the first time. These species have established populations in 44 inland lentic and lotic water bodies, both natural and man-made. The species occur in syntopy in eight locations. Trachemys scripta is found in 15 locations, of which only 3 locations do not have established populations. Procambarus clarkii is being reported in 30 sites with established populations. This information is a stepping-stone for promoting management measures, for the successful control and/or local eradication of these species in Cyprus.

We report on an established population of the beauty rat snake, Elaphe taeniura Cope, 1861, a large, oviparous colubrid native to Southeastern Asia, in Belgium. The snakes have invaded a railroad system next to a city in the northeast of the country. Our report is based on validated citizen science observations, supplemented with directed surveys. The species has been recorded in the wild since 2006, most probably following an introduction linked to the pet trade. Genetic identification, based on the COI gene, confirms that the sampled individuals belong to E. taeniura. In addition, the snakes recorded in Belgium phenotypically match E. t. taeniura, a Chinese subspecies. Exact date of introduction, invasion extent and population size are currently unknown, but the number of observations has increased in recent years. Sightings exist from an area of 208 km², yet the core distribution is estimated to be no more than 2 km². Based on what is currently known on its ecology and distribution, we estimate that the species represents medium environmental risk. However, the species’ distribution and invasive potential in Belgium remain largely unknown. As management of more widely established snake populations is notoriously difficult, we advocate a rapid response as the most appropriate risk management strategy.