Biological invasions have become a defining feature of marine Mediterranean ecosystems with significant impacts on biodiversity, ecosystem services, and human health. We systematically reviewed the current knowledge on the impacts of marine biological invasions in the Mediterranean Sea. We screened relevant literature and applied a standardised framework that classifies mechanisms and magnitude of impacts and type of evidence. Overall, 103 alien and cryptogenic species were analysed, 59 of which were associated with both negative and positive impacts, 17 to only negative, and 13 to only positive; no impacts were found for 14 species. Evidence for most reported impacts (52%) was of medium strength, but for 32% of impact reports evidence was weak, based solely on expert judgement. Only 16% of the reported impacts were based on experimental studies. Our assessment allowed us to create an inventory of 88 alien and cryptogenic species from 16 different phyla with reported moderate to high impacts. The ten worst invasive species in terms of reported negative impacts on biodiversity include six algae, two fishes, and two molluscs, with the green alga Caulerpa cylindracea ranking first. Negative impacts on biodiversity prevailed over positive ones. Competition for resources, the creation of novel habitat through ecosystem engineering, and predation were the primary reported mechanisms of negative effects. Most cases of combined negative and positive impacts on biodiversity referred to community-level modifications. Overall, more positive than negative impacts were reported on ecosystem services, but this varied depending on the service. For human health, only negative impacts were recorded. Substantial variation was found among Mediterranean ecoregions in terms of mechanisms of impact and the taxonomic identity of impacting species. There was no evidence that the magnitude of impact increases with residence time. Holistic approaches and experimental research constitute the way forward to better understanding and managing biological invasions.

In order to assist the implementation of the European policies on Invasive Alien Species (IAS Regulation) and mitigate the impact of alien species that threaten the marine ecosystem services and biodiversity, accurate lists of alien species per European Member State are required. Although inventories of marine alien species and relevant information about them have been reported for the Black Sea marine region of Romania and Bulgaria, a validated list of alien species for the two countries is still lacking. In this paper we (i) propose a validated list of the marine alien species occurring in the Romanian and Bulgarian Black Sea regions, and (ii) address errors, gaps and uncertainties associated with listing alien species from the marine waters of the two countries. The list of alien species includes 37 species for Romania and 26 for Bulgaria, with the highest number of alien species belonging to the phylum Arthopoda for both countries. The majority of alien species have been classified as successfully established in the marine waters of both countries. Thirteen alien species were classified as invasive in Romania whilst eight alien species were classed as invasive in Bulgaria. The historical overview indicates that the number of alien species has steadily increased over the last 50 years both in Romania and Bulgaria. The results of our study provide the baseline list and information for addressing marine alien species at a national level and a starting point for selecting the invasive alien species for risk assessment towards an effective implementation of the IAS Regulation. In conclusion, this study provides supporting information for implementing measures to tackle the introduction of alien species in the western region of the Black Sea. These measures are needed to build an early warning, prevention, and control of invasive alien species in the Black Sea, a unique and fragile ecosystem.

New Chinese mitten crab sequence data were obtained from eight European localities spanning France, The Netherlands, England and Wales. The Netherlands and Wales samples were taken from two areas where mitten crab populations had never before been sequenced. DNA results uncovered the presence of a second Eriocheir species coexisting sympatrically with E. sinensis H. Milne Edwards, 1853 in the Dee Estuary, Wales and at two sites sampled in The Netherlands. Molecular results and comparative morphology analyses suggest that the second species in European waters is likely to be an undescribed cryptic species. Additional evidence for the presence of a second European invasive mitten crab species is provided by confocal laser scanning microscopy of the male first gonopod. The origins of the Welsh introduction may date back to the early 2000s, when several Dutch dredging operators were active in the Dee Estuary. Despite several localities being sampled, the Eriocheir cryptic species was not found in England, which suggests that English and Welsh mitten crab populations originate from independent introductions. These results radically challenge current assessments of Eriocheir invasion in Europe and should be considered by stakeholders as a new baseline to implement efficient management strategies against invasive mitten crabs.

Melanoides tuberculata (Müller, 1774) is a widely distributed aquatic gastropod, primarily found in tropical regions, which has also invaded geothermally heated waters in temperate regions. The spread of M. tuberculata globally has primarily been in association with the aquarium trade and aquatic plants. In New Zealand, M. tuberculata is known from both the aquarium trade and from one non-indigenous population in a geothermal stream. To date, the origins of M. tuberculata in the aquarium trade in New Zealand—or elsewhere—has not been systematically examined genetically, and it has not been confirmed whether New Zealand’s non-indigenous population is genetically similar to, or originated from, individuals sold in aquarium stores. We examined the origins of New Zealand M. tuberculata, in the aquarium trade, and in the established non-indigenous population using COI marker sequences. Representatives of M. tuberculata in New Zealand have not originated from a single source; two distinct genetic clades were identified for individuals, with the origins of each inferred to be from Asia and Africa. The non-indigenous population and individuals sourced from one private aquarium owner were found to originate in Asia, and are genetically similar to those collected from Singapore, one of the most significant countries globally for the culturing and distribution of aquarium species. The remaining collections, from a display aquarium and three online suppliers in various New Zealand locations, were most similar genetically to populations native to Lake Malawi and Tanganyika, and non-indigenous populations from Australia. Overall, as M. tuberculata is an obligate parthenogen, each independently introduced clone may be considered a distinct non-indigenous taxon. We recommend further systematic genetic analyses of individuals in the aquarium trade elsewhere, to assess more fully the diversity and origins of Melanoides being transported in the aquarium trade.

For risk assessment and prevention of further invasion of the alien apple snail, Pomacea canaliculata, into new habitats in Japan, we estimated the snail’s potential overwintering areas on the basis of climate data and a field census. Presence or absence was examined at 38 sites in six regions of Kyushu Island, where the snail has invaded most areas in which it can overwinter. The effects of cumulative low temperature during winter on the presence/absence data were examined by using a logistic regression analysis. As a result, a model using cumulative low temperature below 10 °C (CLT10) was adopted as the best determinant of snail overwintering success, with the risk evaluation formula y = 1 / [1 + exp(–4.7931474 + 0.007574x)], where x is CLT10 (5-year average) and y is the probability of overwintering success. To verify the model, y was calculated for 1348 sites on Honshu, Shikoku, and Kyushu islands where P. canaliculata was confirmed to be present by recent field censuses. Of these sites, 1344 (> 99.7%) had y values exceeding 0.4; the remaining four had y values ranging from > 0.2 to 0.4, with a minimum of 0.377. These results indicate that the model can be used to predict the overwintering success and subsequent localization of P. canaliculata. A potential invasion map that we drew by using the model will help to prevent further expansion of this invasive snail into new areas.

Non-indigenous species may drive declines in global freshwater biodiversity. Japanese mystery snails, Heterogen japonica (previously Cipangopaludina / Bellamya / Viviparus japonica), have invaded numerous freshwater systems in North America. To resolve questions about its population demography and genetics, we surveyed ponds and rivers from six Mid-Atlantic USA locations (Richmond, Virginia to Washington, D.C.) in 2018 and 2019 for mystery snails and co-occurring indigenous snails. A random subset of each snail species (max N = 80) per location was assessed for population demographics (size, sex), and brooding embryos were counted in mystery snails. Because morphological identification can be difficult to discern from its congener, the Chinese mystery snail (Cipangopaludina chinensis), we used a mitochondrial barcoding gene (COI) to confirm identity of H. japonica and also serve as a population genetics marker. Our barcoding confirmed that the mystery snails detected in our surveys were H. japonica, and that, compared to the indigenous range, its populations have low genetic diversity and limited genetic structure. Even so, H. japonica had the highest overall catch-per-unit-effort among all snails and sites. In demographic analyses, H. japonica populations skewed towards females, and females brooding live young were the largest across all sites. The number of live young ranged from 14 to 101/female (average: 52 live young/female). Further, a linear relationship existed between brooding female shell length and the number of live young for all sites, except one. Possible explanations could include site-level differences in abiotic or biotic parameters, but this requires further research. Altogether, the snail’s reproductive capacity documented here suggests that H. japonica has the potential to undergo additional population growth, especially if large females spread to new locations. Moreover, it highlights another example of an invasive species with high population abundance, demographic performance, and distributional range even with depauperate genetic diversity.

We examined the association between habitat parameters and gastropod communities across a number of freshwater ecosystems, whose biodiversity has been affected by increased mining activities over decades. Reservoirs inhabited by the New Zealand mud snail (Potamopyrgus antipodarum (Gray, 1843)) exhibited different abiotic conditions than waterbodies that were not, such as higher salinity indicators (max. conductivity 13400 μS/cm, TDS 7700 mg/l and chlorides 7200 mg/l), and degraded water quality. In such water bodies P. antipodarum density was high (max. 23686/m²). Further, invaded waters were inhabited by less diverse gastropod communities than reservoirs without P. antipodarum. The invader has established reproducing populations and were the dominant species (D 87.53%), or formed mono-specific assemblages, in the reservoirs. Our results indicate that P. antipodarum occurred in habitats whose conditions are unfavorable or less preferred by other species, such as those with high conductivity, and high concentrations of chlorides and calcium. Since this snail is known to inhabit water bodies with degraded water quality, we assumed that along with the progressing development of industries worldwide, this species will continue to spread to industrial areas and will inhabit increasing numbers of anthropogenically affected water environments. Our study contributes to the global understanding of the mechanisms involved in successful establishment in waters by P. antipodarum, and is especially novel in addressing its occurrence in anthropogenically created ecosystems. Our results showed that human impact is the cause of the trend in P. antipodarum populations, and compared habitats from the perspective of both abiotic parameters and community structure. Human activities can have a significant impact on water quality that can result in the permanent establishment of P. antipodarum populations, especially when the environment is not favorable for the native species.