Non-indigenous species (NIS) represent a serious problem worldwide, where ascidians are one of the most important taxa. However, little has been done to document the non-indigenous ascidians in Iceland, and over the past decade only two species had been recorded prior to the present study, Ciona intestinalis in 2007 and Botryllus schlosseri in 2011. To increase the knowledge of this taxon, extensive sampling was carried out in shallow waters around Iceland, during the summer 2018, in ports and on ropes of a long-line mussel aquaculture. In total, eleven species were identified, four native and seven NIS, of which Diplosoma listerianum, Ascidiella aspersa, Botrylloides violaceus, Molgula manhattensis and Ciona cf. robusta, are now reported for the first time in Iceland. The highest abundance of non-indigenous ascidians appeared among the ports in southwestern Iceland (Sandgerði, Hafnarfjörður). As pointed out for other regions, the most likely vector is maritime traffic (hull fouling and ballast water), although other vectors cannot be ruled out. The future expansion of these non-indigenous ascidians around Iceland must be monitored, where local maritime traffic could play an important role. Furthermore, global warming may facilitate the access and establishment of these species in colder areas with arctic influence (north and east of Iceland), which are likely still free of these species.

Metridium senile is a circumboreally distributed sea anemone (Cnidaria: Anthozoa: Actiniaria) native to the northern hemisphere, and has been presumed as introduced to several locations in the southern hemisphere. Although the sea anemone fauna of the Falkland Islands is not well known, to date no historical records of Metridium senile exist. In 2019, we conducted biodiversity surveys to gather information about sea anemones present in the islands. During these 2019 surveys, we detected M. senile from three locations: Stanley (eastern Falklands), the Bird Island (south-west Falklands), and in the northern Jason Islands (north-west Falklands). The species was well established at each location, with evidence for asexual reproduction occurring in the western sites. We confirmed the identification by morphology and DNA analysis. Stanley populations are distinct in color variation from the southwestern and northwestern populations, which may indicate separate introduction waves or pathways and the potential for additional, undetected locations of M. senile.

The introduced ctenophore Mnemiopsis leidyi A. Agassiz, 1865 was observed in the Vaccarès Lagoon (River Rhône delta, southeast France) for the first time in 2016. Large blooms had been observed during the summer in 2016, with monthly zooplankton community monitoring carried out from November 2016 to October 2017. This was done to better understand the dynamics of this ctenophore within this brackish Lagoon. In parallel, we report the first record of Mnemiopsis leidyi in another Mediterranean lagoon near Camargue delta (Estomac Lagoon) in 2019.

A single specimen of a pelagiid jellyfish collected next to Ashdod port, Israel, is referred to the genus Chrysaora Péron and Lesueur, 1810 based on molecular examinations. Despite the inability to check morphological features of diagnostic value, molecular analyses based on the mitochondrial barcoding gene cytochrome oxidase sub unit I (COI), 16S and 28S ribosomal DNA reveal marked dissimilarities from both the Northeast Atlantic-Mediterranean native Chrysaora hysoscella (Linnaeus, 1767) and the closest GeneBank/BoLD available congener, the West African C. africana (Vanhöffen, 1902). It is suggested that the species is new to science and non-native to the Mediterranean Sea, possibly the sixth introduced scyphozoan species reported in the Levant Sea.

The white-spotted jellyfish Phyllorhiza punctata von Lendenfeld, 1884, native to the Indo-Pacific, is a well-established alien species in the Mediterranean Sea, with records from several countries in the eastern and the western part of the basin. We hereby first record this taxon from Libya based on postings made on the Facebook® social media website by a recreational spear-fisherman who took images of a specimen at Misratah, to the west of the country. The finding date reported here is almost in sync with records held in the neighbouring Mediterranean countries, solidifying the importance of social media as an effective retrospective tool by providing information that were otherwise lost. There are no certainties regarding possible pathway(s) of arrival of P. punctata in Libya. However, shipping or natural dispersal from nearby populations in the Mediterranean Sea constitute the most probable ones.

The Red Sea intertidal gastropod Nerita sanguinolenta is reported here for the first time from the Northern Mediterranean coast of Israel, where it was found at the landward face of a vermetid reef on 11 November 2019, and identified morphologically and by using molecular barcoding. Possible vectors for its introduction primarily include ship ballast or larval transport via the Suez Canal in multiple migration events. Investigation of the thermal affinities of N. sanguinolenta is required for understanding the potential for its proliferation in the Mediterranean Sea.

The Asian pond mussels (Sinanodonta spp.) are invasive species rapidly spreading throughout the world. In Russia, non-native populations of Sinanodonta woodiana and S. lauta were firstly discovered in an artificially heated section of the Yenisei River, Eastern Siberia. Here, we report that these mussels successfully colonized the downstream of the Volga River, where they established rather abundant populations (3–36% of the total samples of freshwater mussels). Furthermore, these species were recorded from the Belovo Reservoir, Ob River, Western Siberia. Based on our molecular data, we propose that the recent invasion of Sinanodonta woodiana and S. lauta in Russia was associated with fish stocks imported from Kazakhstan. The rapid expansion of these mussels throughout Russia was caused by a human-mediated dispersal of infested fishes from a site(s) of initial invasion to other freshwater systems, i.e. the Volga (at least since 2002), Yenisei (2004), and Ob (2007) rivers. Recent establishments of Sinanodonta woodiana and S. lauta in native environments of the Volga River appear to have major ecological consequences, as it is the largest river in European Russia and the entire Europe harboring species-rich native mussel and fish assemblages.

The distribution and density of the zebra mussel, Dreissena polymorpha (Pallas, 1771), were studied from 2012–2017 in the Dobra River Basin, Croatia, where a new reservoir of Hydroelectric Power Plant Lešće was created in June 2010. The first zebra mussels were found in July 2012 in low density (38 ind.m^-2) at a site 500 m downstream from the Lešće reservoir. In 2017 the density at this site had increased to 137 ind.m^-2. The Lešće reservoir was colonized by zebra mussel from two upstream and connected reservoirs constructed in 1958. Maximum recorded density in the Lešće reservoir was 12,496 ind.m^-2, with dominance of the smallest size categories. This population was the main source of propagules by which D. polymorpha colonized almost the whole remaining course of Gojačka Dobra through 2015. Most sites downstream from the dam showed increased densities from 2016 to 2017. The Lešće reservoir population should be monitored and managed as it represents the main “hotspot” of zebra mussels in the Dinaric karst area in Croatia from which other karst rivers could be invaded.

Dispersal of parasites into novel host populations is often initiated by human activities at global scales, but largely determined by parasite and host life history traits at local scales. In summer 2017, we discovered the parasitic bopyrid isopod, Orthione griffenis Markham, 2004, in a population of mud shrimp, Upogebia pugettensis (Dana, 1852), in Central British Columbia, Canada. This parasite has infested most host populations along the Pacific coast of North America to the south. We estimated that 25% of susceptible hosts on Calvert Island were parasitized during summer 2018 and winter 2019. Steady infestation of mud shrimp on Calvert Island confirmed that the invasive bopyrid has successfully established itself in a novel host population 300 km further north than previously recorded, but with lower parasite prevalence than in the known invasive range, where heavy infestations have been implicated in host population declines. Prevalence rates increased with host size as previously documented, with a minimum infestation size of 10.6 mm carapace length. The parasite has a planktonic larval phase, suggesting that its spread along the coast is not limited to transport via boat traffic. The prognosis for the persistence of Upogebia in Canada and further north is unclear, but population declines further south in the presence of O. griffenis provide a troubling outlook. As biodiversity monitoring efforts continue in this region, tracking the spread of parasites like O. griffenis and their impacts on host populations will yield new insights into the traits that make invasive species successful and the ecosystem consequences of invasions in changing environments.

The success of biological invasions relies, among other factors, on the abiotic environment. The presence or absence of an invasive alien species in its non-native range may therefore be helpful in understanding the factors facilitating or hampering its invasion. The aim of our study was to determine whether the Carpathians are free from the spiny cheek crayfish Faxonius limosus (Rafinesque, 1817), a successful freshwater invader in Europe and locally in Africa. We hypothesised that local factors, mostly climate (i.e. water temperature), have an impact on this species, limiting its spread. However, atypical local habitats with suitable abiotic conditions—in particular artificial reservoirs—might be hubs for F. limosus. The study was conducted in 2018 in Carpathian waters. We found that despite being present in plain areas adjacent to the Carpathians for decades, F. limosus does not ingress deeper into this mountain range. The exceptions are two dam reservoirs and the lower reaches of some of their tributaries, close to their inflows. The Carpathians are climatically distinct from the surrounding areas, which is also reflected in mean water temperature – ca. 1.5 °C lower than in adjacent lowland areas. Thus, we suggest that relatively severe climate may be the main reason behind the limited range of F. limosus in mountains. However, other factors, like current velocity, substrate type, or slope, may contribute to the absence of the species in the studied area. Dam reservoirs providing warmer aquatic habitats and lentic environment may facilitate its invasion related to climate warming.

We present the first Belgian records of potentially invasive alien crayfish of the Procambarus acutus species complex, including the first confirmed record of P. acutus acutus. The species complex was observed at four different sites in three provinces in the north of the country. Only at one site the presence of a form I male specimen made identification to species level possible, based on gonopod morphology. The other three observations are considered as belonging to the P. acutus species complex. Procambarus acutus acutus is the fifth alien crayfish species known to Belgium. In Europe, it was previously only known as an established alien species from the Netherlands and Great Britain.

On November 2019, a single individual of Acanthopagrus bifasciatus was captured along the coast of Barcelona. Its external characteristics were described and morphologically analysed. This record represents the second confirmed observation of this species in the Mediterranean Sea and the first for the western basin. Its detection in a commercial harbour supports the hypothesis of a ship-borne introduction.

The common lionfish Pterois miles and the blue-barred parrotfish Scarus ghobban are Indo-Pacific marine fishes that have invaded several regions in the Mediterranean Sea. Two P. miles individuals and a single S. ghobban were caught off the coast of Egypt in 2018. These are first reports on the occurrences of both species in Egyptian waters, resulting in an update of their distribution in the Mediterranean Sea.

The sailfin molly, Poecilia latipinna (Lesueur, 1821), is a popular aquarium fish, native to the eastern coastline of North America. It has been introduced to many countries worldwide as biological control agent and through releases from aquarium hobbyists. The aim of this paper is to report the presence of sailfin molly in a brackish lake in northern Egypt (Lake Manzala). Twenty-five individuals (8 males and 17 females) of sailfin molly were incidentally collected while fishing for tilapia fish in the lake. All individuals were identified as P. latipinna based on morphological features and this is possibly the first documented record of sailfin molly in Africa.

The blood parrot cichlid, an artificial fish hybrid, was recorded for the first time in a rice agroecosystem based on specimen collected in Seberang Perai Tengah, Penang, Peninsular Malaysia. A potential parental species of this hybrid fish, the Midas cichlid (Amphilophus citrinellus) was also recorded from this agroecosystem. There was no evidence of self-sustaining populations of this hybrid fish in the agroecosystem. Nevertheless, their presence increased the risk of food and space competition with native species (e.g., Pristolepis fasciata), as well as the spread of disease such as pneumocystectomy.

Alien species can strongly impact local environments and compete against native species, which can lead to their extinction. Marsh frogs of the Pelophylax ridibundus complex are one of the most invasive amphibians in Northern Eurasia. It was previously thought that three water frog species of the genus Pelophylax (the marsh frog, P. ridibundus, the pool frog, P. lessonae and their hemiclonal hybrid, the edible frog, P. esculentus) inhabited Kaliningradskaya Oblast’ along the Russian Baltic coast. However, based on our study of the intron-1 of the nuclear serum albumin gene, two other marsh frog species were detected (the Balkan marsh frog, P. kurtmuelleri, and the Anatolian marsh frog, P. cf. bedriagae) as well as putative hybrids between P. ridibundus and P. cf. bedriagae. The majority of individuals of P. ridibundus and hybrids between P. ridibundus and P. cf. bedriagae had mitochondrial (mt) DNA of P. lessonae, while all others featured the P. kurtmuelleri mtDNA. The prevalence of P. lessonae mtDNA haplotypes in populations of P. ridibundus from the Baltic Coast of Russia suggests that local individuals of the latter species originated from crosses between P. esculentus individuals. Two hypotheses could explain the records of P. kurtmuelleri and P. cf. bedriagae in the region. The establishment of local populations of the first species could have occurred via postglacial dispersal from the Balkan refugium. The origin of local P. cf. bedriagae could be an occasional introduction of individuals from the Ponto-Caspian region. Since our study is preliminary (19 individuals), in the future it would be important to continue the study of water frogs in Kaliningradskaya Oblast’ and neighboring countries by applying multiple genetic markers. Additional genetic markers will enable researchers to study routes of dispersal and introductions of marsh frogs, to clarify peculiarities of their hybridization and distribution, and to evaluate the impact of P. kurtmuelleri and P. cf. bedriagae on the reproduction success of hybridogenous populations and abundance of local amphibians.

The Yangtze River basin in China has one of the most highest rates of biological invasion of any freshwater ecosystem. As it is also an important agriculture, aquaculture and biodiversity hotspot, it is critical to implement early detection and rapid response, monitoring and control of new invasive species as they are discovered. This is the first report of the presence of the invasive aquatic vascular plant Sagittaria platyphylla in the middle and lower reaches of the Yangtze River. We propose that more research should focus on control of the aquarium trade and wetland restorations

Invasive species continue to spread and alter ecological function and structure in natural systems. Invasive alien plant species can be particularly ecologically damaging and costly to control, yet their success might be influenced by key habitat characteristics which can be empirically measured. The present study employs field surveys and laboratory analyses to examine whether the abundance and key characteristics (i.e. height and number of stems) of the non-native balloon milkweed Gomphocarpus physocarpus E. Mey are influenced by wetland zonation (i.e. permanent, temporary and seasonal zones) and soil characteristics, within a designated Ramsar wetland. Significant site- and zonation-specific differences were observed in a range of soil parameters (e.g. pH, conductivity, Na, Ca, Mg, Cu, Zn, Mn, Fe). Overall, milkweed numbers did not differ significantly according to wetland zonation, but differed significantly among sites. However, in turn, both plant heights and stem numbers related significantly to habitat zonation and sampling sites. Whilst unrelated to most soil properties, milkweed variables were found to relate significantly positively to Mn (abundance), negatively to Cu and positively to P (stem numbers). Furthermore, principal components analyses concerning milkweed abundances indicated clear patterning across sites, finding strong associations with soil variables (i.e. soluble S, Mn, pH, Na, SOM and conductivity). The present study illustrates distributions of a non-native plant species and assesses how its characteristics relate to environmental properties in an internationally recognised protected area. This information could be employed to help predict future distributions and better target management efforts towards sites at high-risk of invasion.

Amaranthus tuberculatus, commonly known as the rough-fruited water-hemp, is a dioecious annual weed species originating from North America. It has been introduced into Europe and Asia, where it is mostly considered a naturalized or casual alien occupying riparian areas, agricultural fields and disturbed habitats, especially in commercial port areas. Here we report the first occurrence of the species in Croatia, the second in Southeastern Europe. Flowering male and female individuals were found in 19 localities in the Pannonian part of Croatia. Female individuals from all localities had flowers with only one tepal and a dehiscent fruit and were therefore identified as A. tuberculatus var. rudis. In the studied area, the species was most frequently recorded along the banks of the Sava River and eutrophicated drainage channels passing through arable land. Here, it was present within vegetation of the alliance Nanocyperion Koch 1926 (class Isoëto-Nanojuncetea Br.-Bl. et Tx. in Br.-Bl. et al. 1952) and the alliance Bidention tripartitae (class Bidentetea Tx. et al. ex von Rochow 1951), as well as on the edges of alluvial willow and poplar forests (Salicion albae Soó 1951, class Salicetea purpureae Moor 1951). Furthermore, it was recorded on river sandbars, concrete embankments and other disturbed habitats (along the railroad, a road, on a construction site). Although the species is a well known agricultural weed and the majority of our study area is composed of arable land, A. tuberculatus was not detected in this habitat type. However, we believe that it is highly probable that it will spread into new areas and suitable habitats, and strongly recommend the occupied area to be monitored.

Cardamine occulta is an annual or biennial plant native to Eastern Asia. It was introduced to Europe in the second half of the 20th century. So far, it was recorded in 16 European countries, usually in anthropogenic habitats of urban areas. Abundant populations of C. occulta have been found in the surroundings of Lake Constance in Germany, Austria, and Switzerland. In this study, I present the first record of C. occulta in Poland. It was found in two stands in the city of Kraków, southern Poland, in November 2019, growing in a flower bed and flower ponds. The distribution of the species is mapped using the ATPOL cartogram grid and its status in Poland is briefly discussed.