One of the most dominant concepts in invasion ecology is the stage-based invasion model, consisting of transport, introduction, establishment and spread. Many species fail at one of the stages, with propagule pressure (i.e. number of introduced individuals) identified as a principal factor affecting establishment success. Population characteristics such as phenotypic plasticity and beneficial life history traits may facilitate successful transition of species through different stages of the process; however, studies on the latter are not so common and most of those studies focus on terrestrial taxa. In this study, we hypothesized seven life history traits that may be beneficial for invasion success of aquatic species, and determined those traits for established non-indigenous species (NIS) in the North and Baltic Seas (i.e. marine environment) and Great Lakes-St. Lawrence River regions (i.e. freshwater environment). This is the first study that examined certain life history traits of all NIS established in particular regions, as well as compared those traits between marine and freshwater habitats. Our study determined some differences in life history traits between NIS in the marine and freshwater habitats. Those differences were connected to different taxonomic groups that were dominant NIS in these two types of habitats. Furthermore, species originating from different donor regions had also different life history traits. The majority of NIS in both regions were r-strategists. There was a significantly higher number of NIS that were able to reproduce both asexually and sexually and to produce dormant stages in the freshwater than in marine habitat. Finally, as r-strategy, asexual reproduction and dormancy were dominant traits of NIS in the freshwater habitat, freshwater ecosystems may be under greater invasion risk than marine ones, as those traits reduce both demographic and environmental stochasticity during the invasion process.

Human activities are contributing to the homogenization of the world’s biota by transporting thousands of species to areas outside their native range. Some of these species can lead to profound changes in the structure and function of natural ecosystems in areas where they are introduced, with dramatic economic and ecological impacts. The red alga Asparagopsis armata is considered an invasive species in Europe, but can be particularly abundant in the Azores (NE Atlantic), where there is virtually no information on its ecology. Here, we investigate the patterns of spatial distribution and impacts of A. armata in the Azores, as a first step to understand its potential spread. Adopting a hierarchical design, we surveyed the distribution of A. armata, and associated assemblages, at a range of spatial scales (from islands (10s of km apart), locations (kms apart), sites (10s of m apart) to quadrats (m apart)). Results showed that the abundance of A. armata varied significantly among locations and that there was substantial variability in its abundance among islands, locations and quadrats and little variability among sites. At the scale of locations, the abundance of A. armata did not correlate with any of our predictor variables (sea urchin density, Latitude, Longitude and sea temperature). However, at the quadrat scale, there was a significant and negative correlation between the abundance of A. armata and diversity of associated macrophytes, as well as, with the abundance of the conspecific A. taxiformis and the abundance of most of the remainder macroalgal groups. The potential role of biotic and abiotic factors in generating the observed patterns is discussed. This study further highlights the suitability of the analytical tools used here to examine patterns of distribution over a range of spatial scales and its applicability in the field of aquatic invasions.

A mechanistic understanding of the factors that influence establishment and secondary spread of introduced species is critical for predicting the spatial extent and magnitude of negative impacts of species invasions. In the northwest Atlantic, an ecologically significant invasive bryozoan (Membranipora membranacea) has expanded its range northwards over the last 30 years. Warm ocean temperature has been linked to population outbreaks of M. membranacea within its established invasive range in southwestern Nova Scotia; however, rates of spread and the physical and biological factors affecting establishment of founding populations have not been explicitly quantified. Here, we use unique baseline data on presence/absence and abundance of this bryozoan near its current northern range limit to quantify rates of spread and identify factors influencing its establishment in three regions in the Gulf of St. Lawrence. Except for one potential incident of human-facilitated long-distance dispersal, the rate of spread of M. membranacea in the northwest Atlantic was comparable to that of other invasive marine invertebrates. Contrary to our predictions based on long-established populations in Nova Scotia, the abundance of M. membranacea was not strongly related to regional differences in ocean temperature at its northern range limit. Our results suggest that colder temperatures are not limiting the northward spread of this bryozoan in the northwest Atlantic. Instead, relationships between temperature and abundance of M. membranacea in the Gulf of St. Lawrence may be moderated by a lack of available kelp substrate. Our findings, therefore, have important implications regarding the potential for M. membranacea to impact vulnerable Arctic ecosystems where laminarian kelps are widespread.

Non-native marine species thrive on artificial habitat. Expanding coastal infrastructure has led to concerns that increasing artificial habitat will facilitate the spread of non-native species overall and to natural ecosystems. In the Santa Barbara Channel (SBC), California, USA, the non-native bryozoan Watersipora subatra (Ortmann, 1890) has previously been reported only in harbors and on offshore oil platforms. To assess the distribution and potential for spread of W. subatra between coastal infrastructure and natural habitat in the SBC, we surveyed 61 open coastal sites, including natural and artificial habitat, and evaluated the potential dispersal connectivity of W. subatra larvae from harbors and oil platforms to natural reefs using survey results and three-dimensional biophysical modeling. We discovered that this bryozoan has invaded natural reefs in the region. W. subatra was present on approximately 50% of the oil platforms and mainland reef sites, but only 17% of offshore island reef sites. Modeling indicated high potential connectivity from one harbor to the closest reef with W. subatra, 4–5 km distant from the harbor mouth, but dispersal to the more distant sites would likely require intermediate stepping stone sites or anthropogenic transport of spawning adults. Populations on offshore platforms, in contrast, were virtually unconnected to reef sites through ocean circulation. The survey data indicated potential opportunities to manage current and future invasions through the control of larval sources. In particular, manual removal of small isolated populations on reefs at the northern Channel Islands would reduce this source of larvae to uninvaded areas. Consideration of potential connectivity in the placement of mariculture infrastructure along the coast with respect to harbors and other sources of non-native propagules could also help to reduce the potential for stepwise spread of non-natives to natural reef habitat.

The Manila clam, Ruditapes philippinarum (Adams and Reeve, 1850), was introduced in many estuaries along the Atlantic and Mediterranean coasts for fisheries and aquaculture, being one of the top five most commercially valuable bivalve species worldwide. In Portugal, the colonization of the Tagus estuary by this species coincided with a significant decrease in abundance of the native R. decussatus (Linnaeus, 1758). This study aimed at identifying the main food sources supporting populations of the non-native bivalve in the Tagus estuary, using carbon and nitrogen stable isotopes, and evaluate the potential for food competition with the native bivalves R. decussatus and Cerastoderma glaucum (Bruguière, 1789). Results showed that these species relied on the same food sources, and that the trophic niche of R. philippinarum overlapped with the trophic niche of R. decussatus by 40% and with C. glaucum by 23%. The most likely food sources included particulate organic matter (POM), microphytobenthos (MPB), and sediment organic matter (SOM). The Bayesian stable isotope mixing model indicated that POM was the food source with the highest proportional contribution (up to 92%), followed by MPB (up to 32%), and SOM (up to 23%). Although the majority of the food sources identified were filtered from the water column, reliance on SOM and MPB suggests they may also feed on resuspended organic matter. Because these bivalve species feed on the same sources, there is some potential for food competition in this ecosystem. However, further studies are needed to analyze the long-term consequences of these trophic interactions to verify if the co-existence between the native and the invasive species will generate competition for food resources when those are limited in quantity and/or quality.

Reproductive success is critical for the establishment and spread of non-native species. The apple snail, Pomacea canaliculata, is a dioecious and internally fertilizing freshwater gastropod that causes serious economic losses in its non-native ranges annually. Previous studies on P. canaliculata using laboratory-controlled experiments indicated that mating with a second male displaces sperm from the first one, suggesting that multiple paternity occurs in this snail. To investigate the occurrence and frequency of multiple paternity in natural populations, we provide results from two polymorphic microsatellite loci in eight egg clutches collected from four localities in China. By genotyping the hatchlings from each population, we found at least one to three siring fathers per clutch. The paternal contributions of the multiply sired clutches were strongly skewed in P. canaliculata. The total population genetic diversity in the hatchlings exhibited higher observed heterozygosity than that in the adult snails. Increased offspring heterozygosity through multiple paternity is a potential mechanism that may aid P. canaliculata in overcoming genetic bottlenecks, and enhance the ability to adapt to new habitats as an invasive species.

Chemical communication plays a significant role in the aquatic environment, enabling the mutual recognition of mates, predators and food items. Invasive species facing new communities are subjected to a huge variety of new scents. Their ability to recognize and interpret them adequately could be the reason of their successful establishment. We studied reciprocal chemical recognition among invasive freshwater Ponto-Caspian gammarids (Dikerogammarus villosus, Dikerogammarus haemobaphes and Pontogammarus robustoides) that often co-occur in native and invaded areas, to test their ability to avoid via chemical signals possible competitor and/or top intraguild-predator. We used a Y-maze with inflow of water to the Y-arms from two source tanks containing the signal (scent of living animals or the alarm cues released by crushed individuals) or a control (dechlorinated tap water), respectively. Time spent in the Y-maze zones, number of visits to the Y-arms and animal activity were analysed. Only D. villosus was attracted to conspecific and heterospecific alarm cues as well as to living heterospecifics. The other two species avoided the scent of D. villosus. We conclude that D. villosus, as the strongest competitor and intra-guild predator (including cannibalism) among the tested species, perceived various scents as potential food cues and posed a threat to other gammarids.

Introduction rates of alien freshwater organisms are increasing worldwide. In Israel, freshwater habitats suffer from heavy anthropogenic stress resulting in numerous records of introduction to date. During 2014 a non-native caridean shrimp was found in several locations in the Jordan River system and in the Yarqon Stream. These systems represent different basins, hydrologically disconnected. Using both DNA analysis (mitochondrial genes COI and 16S) and morphological tools the species was identified as Neocaridina denticulata, an ornamental species originating from South-East Asia. Phylogenetic analyses of 56 Israeli and 18 Japanese specimens revealed no geographical differentiation among Israeli specimens, affiliated to a single haplotype, in contrast to the division of two groups within Japanese specimens. It is therefore possible that this species had been released by aquarium hobbyists. Field and laboratory observations revealed a highly reproductive and environmentally tolerant populations, reaching a reproductive stage within few months and releasing more than 70 larvae per ovigerous female. In addition, N. denticulata was found to be significantly larger in comparison to the native Atyidae shrimp Atyaephyra orientalis. We suggest that N. denticulata may expand its geographical distribution into similar freshwater systems in Europe and other Mediterranean countries, where suitable environmental conditions exist. We further provide tools for taxonomic identification and field observations for effective detection and inspection protocols.

Chinese mitten crabs are one of the largest species of invasive crustaceans in the Baltic Sea catchment basin. Due to their catadromous migration, they can serve as a vector for small organisms, transporting them over distances greater than 500 km. Research on various crab species has shown their notable role in the spread of microorganisms to new aquatic environments. The aim of this study was to determine the diversity and abundance of microorganisms migrating with the Chinese mitten crab on the densely-distributed setae found on their claws. Our analysis also considered the potential differences in the number and genera of microorganisms between male and female Chinese mitten crabs. The study consisted of forty-eight (48) crabs (sex ratio 1:1) fished with a fyke net during their intensive downstream migration in Lake Dąbie (which is the southern part of the Oder River estuary) between November 8 and 11 2017. In these Chinese mitten crabs, a rich microbiological flora was found, which can be divided into three basic groups: heterotrophic bacteria, yeasts, and fungi. Microorganism genus biodiversity on claw setae was greater in males than in females, although the total aerobic microbial count (TAMC), total halophile count (THC), and total combined yeasts and molds count (TYMC) showed no statistically significant differences between sexes. Phylogenetic analysis of PCR products and genetic taxonomic analysis of dominating strains in samples from crab claw setae showed the dominance of Bacillus tequilensis bacterium. The presence of these microbiologically antagonistic bacteria, capable of adapting perfectly to different environmental conditions, indicates that the crabs are capable of introducing potentially dangerous microorganisms into new ecosystems.

Randall’s threadfin bream (Nemipterus randalli) is a Red Sea species that has successfully invaded the Mediterranean via the Suez Canal (Lessepsian migrant). It has established a large population in its new region, shortly after arriving to the Mediterranean. In the present study, the mitochondrial DNA D-loop (control region) of Red Sea specimens of N. randalli was compared to that of specimens from the Mediterranean. A pronounced was compared to that of specimens from the Mediterranean. A pronounced decrease in genetic variability was observed in the Mediterranean fish, indicating a bottleneck effect. Ten different haplotypes were found among 14 Red Sea specimens, while only six haplotypes were found among 42 Mediterranean specimens. The Simpson’s Index of Diversity was significantly larger for the Red Sea than for the Mediterranean fish. In addition, the average genetic distance between any pair of individuals was significantly lower in the Mediterranean samples (4.526 × 10^-3) than that of the Red Sea samples (6.239 × 10^-3). It can be concluded that the decrease in genetic variability did not hinder the success of the Lessepsian migrant Nemipterus randallitr settlement in the eastern Mediterranean.

Explaining variation in life-history traits of invasive species is a key goal in invasion ecology. The goldfish, Carassius auratus, is one of the most successful invaders in freshwater systems and has successfully invaded the Yarlung Zangbo River located in the Qinghai-Tibetan Plateau, China. Although many studies on life-history trait variations in goldfish have been carried out worldwide, few have focused on plateau areas, and factors driving this variation have received little attention. In this study, six life-history traits of goldfish and twenty-five environmental variables, including climate, habitat and human disturbance, were selected to examine their impacts on life-history traits. Multiple regression and hierarchical partitioning analyses were performed to determine the relationships between life-history traits and environmental variables. Maximal body length (MaxL), corrected total fecundity (TFecC) and egg diameter (EggD) were significantly explained by environmental variables. MaxL was positively related to elevation (ELE), population density (POP) and precipitation of the wettest month (BIO13) but negatively related to isothermality (BIO3). TFecC was positively related to ELE, POP and BIO13 but negatively related to BIO3, whereas EggD was negatively related to ELE and BIO13. Body size, fecundity and egg size were significantly affected by the environment, which suggested that a trade-off between growth and reproduction was the key to goldfish adaptation to plateau environments. Climate, habitat and human disturbance are closely related to the life history, suggesting that goldfish will expand their distribution in response to future global environmental changes. Therefore, we advocate that current efforts should focus on taking targeted action and preventing future introductions of goldfish to other areas. Education to promote public awareness of the threats caused by goldfish and other non-native species should be the first priority.

The round goby Neogobius melanostomus is a small invasive benthic fish that has caused multiple and variable impacts in freshwaters. One of the most recent round goby introductions occurred in the upper Elbe in 2015. In this case, the new population was isolated from other populations both by land and by at least 600 river km. Monitoring of this population has provided a unique opportunity to describe the characteristics of round gobies naturally spreading from a single introduction point and to examine its effects on native fish. Passive downstream dispersal of early life stages has played a central role in population spread, with the proportion of small fish increasing downstream and decreasing with time. The sex ratio of the novel population was strongly female-biased and with no apparent spatial structure. Round goby introduction appears to have had a negative impact on native 0+ fish abundance and species richness. Relationships with ≥ 1+ fish were more diverse, ranging from a significant decrease in chub Squalius cephalus abundance to a significant increase in European eel Anguilla anguilla abundance as goby abundance increased. As no impacts on these species have been reported before, the study appears to provide further evidence for case-specificity in round goby impact.

The poorly-regulated pet trade is a major source of potential invasive species, with deliberate release often resulting in introductions of non-native species without invasion histories. Predicting the potential impacts of species with no invasion history is particularly difficult. Functional responses (FRs; resource use as a function of resource density) have proven useful in the quantification of ecological impacts of invasive species, and may be used to screen likely impacts of species which lack invasion histories. Here, we used laboratory experiments to quantify the FRs of four freshwater turtles commonly traded as household pets: Trachemys scripta scripta, the yellow-bellied slider; T. s. troostii, the Cumberland slider; Sternotherus odoratus, the common musk turtle; and Kinosternon subrubrum, the Eastern mud turtle, towards representative chironomid and gammarid prey under eight densities in the presence and absence of substrate. All turtles exhibited potentially population destabilising Type II (hyperbolic) FRs towards each prey type, irrespective of the presence of substrate, characterised by high predation rates at low prey densities. Magnitudes of FRs were generally higher for T. s. scripta and T. s. troostii compared to S. odoratus and K. subrubrum. The presence of substrate reduced the magnitude of FRs towards both prey types overall, however, these effects were modest and most pronounced for the two. T. scripta subspecies. We demonstrate marked and sustained predatory impacts of non-native turtles on native prey, irrespective of benthic habitat contexts. We conclude that commonly traded turtles that lack invasion histories could precipitate substantial ecological impact, particularly in freshwater ecosystems where there are no native analogues.

We provide a history of swamp eel (family Synbranchidae) introductions around the globe and report the first confirmed nonindigenous records of Amphipnous cuchia in the wild. The species, native to Asia, is documented from five sites in the USA: the Passaic River, New Jersey (2007), Lake Needwood, Maryland (2014), a stream in Pennsylvania (2015), the Tittabawassee River, Michigan (2017), and Meadow Lake, New York (2017). The international live-food trade constitutes the major introduction pathway, a conclusion based on: (1) United States Fish and Wildlife Service’s Law Enforcement Management Information System (LEMIS) database records revealing regular swamp eel imports from Asia since at least the mid-1990s; (2) surveys (2001–2018) documenting widespread distribution of live A. cuchia among ethnic food markets in the USA and Canada; (3) indications that food markets are the only source of live A. cuchia in North America; and (4) presence of live A. cuchia in markets close to introduction sites. Prayer release appears to be an important pathway component, whereby religious practitioners purchase live A. cuchia from markets and set them free. Prevalence of A. cuchia in US markets since 2001 indicates the species is the principal swamp eel imported, largely replacing members of the Asian complex Monopterus albus/javanensis. LEMIS records (July 1996–January 2017) document 972 shipments containing an estimated 832,897 live swamp eels entering the USA, although these data underestimate actual numbers due to undeclared and false reporting. LEMIS data reveal most imports originate in Bangladesh, Vietnam, and China. However, LEMIS wrongly identifies many imported swamp eels as “Monopterus albus”; none are identified as A. cuchia although specimens from Bangladesh and India are almost certainly this species. Some imported A. cuchia are erroneously declared on import forms as Anguilla bengalensis. To date, there is no evidence of A. cuchia reproduction in open waters of North America, presumably because it is a tropical-subtropical species and all introductions thus far have been in latitudes where winter water temperatures regularly fall near or below freezing.