Assessing the resistance of fouling communities to anthropogenic disturbances is an important goal for the development of effective management and control strategies. In this context, we conducted a manipulative experiment on natural and artificial habitats to examine fouling communities that developed outside and inside a marina on Madeira Island (NE Atlantic Ocean) following the application of two types of stressors frequently observed in coastal habitats, namely chemical and physical disturbances. The tested fouling communities, dominated by native and non-indigenous species respectively, were in general strongly affected by the chemical but not by the physical disturbance applied, and a higher resistance to disturbance was observed in the communities outside the marina. This suggests higher capacities for communities richer in native species to tolerate anthropogenic disturbances, while non-indigenous species did not play a key role. Further research can assess the resilience of natural and artificial fouling communities when exposed to disturbances.

We assess the trends and influences of non-indigenous and cryptogenic species (hereafter simply referred to as “NIS”) on Danish marine community compositions using three decades of quantitative monitoring data. Since the initiation of the Danish marine monitoring programmes in the 1980s, the number of marine NIS recorded in Denmark increased from 30 to 77 in 2014. Thus, of the total 77 marine NIS known from Denmark, 56 (73%) were captured in the standardized monitoring program, while the remaining 21 species (27%) were not detected (in particular gelatinous zooplankton, shallow water fish, parasitic invertebrates, and littoral angiosperms) because of limited spatial-temporal sampling efforts as well as methodological limitations. Significant exponential increases in records of non-native phytoplankton, benthic invertebrates and macroalgae (only in one region) relative to the total species records in the database were observed. Multivariate analyses of presence-absence data, indicated that the contribution of NIS to total community similarity increased over time, highlighting that NIS is becoming an increasingly important component of Danish marine communities. While the presence of NIS generally explained less than 10% of long-term changes in the community similarity across large regions, NIS presence showed local dominance. A correlation analysis indicated that changes in overall species composition within functional groups (phytoplankton, zooplankton, macroalgae and benthic fauna) were related to changes in NIS contribution but more strongly influenced by salinity, highlighting a well-described general positive relationship between salinity and species richness in Danish waters. The process described within this study could form the basis for the analysis of impact of NIS in marine water as required by the Marine Strategy Framework Directive (MSFD).

Alien macroalgae, mostly invasive species, are a constant concern for coastal areas, especially in the northern Adriatic Sea where several taxa have colonized the main transitional environments. A revision of the alien macroalgae in the Venice Lagoon shows that, currently, the number of valid non-indigenous species (NIS) is 29, and this number is growing steadily. On the basis of numerous surveys carried out in the last decade the total alien standing crop (SC) was estimated to be ca. 146,534 tonnes fresh weight (fw), i.e. 32% of the total species SC (ca. 456,000 tonnes fw) measured in May–June 2014 in the whole lagoon. The most abundant species were the invasive Agarophyton vermiculophyllum (approx. 66,383 tonnes fw), Agardhiella subulata and Hypnea cervicornis (approx. 36,714 and 28,305 tonnes fw, respectively). These species grow mainly free-floating and usually colonize the soft substrata of the lagoon. Two other invasive species, Sargassum muticum and Undaria pinnatifida, grow mainly on the docks of Venice historical centre, therefore their distribution is significantly more limited. Recent studies have shown that A. vermiculophyllum in protected, eutrophic areas produced a biomass of approx. 24 kg fw m^-2 y^-1. This species can replace Ulvaceae with positive effects on the environment and the presence of macrofaunal taxa. Indeed, historical data analysis shows that, despite the high presence of NIS, biodiversity is increasing. In addition, A. vermiculophyllum, S. muticum, U. pinnatifida, A. subulata and Solieria filiformis may be promising species for phycocolloid and antioxidant or cosmetic productions.

Evidence is presented in this paper for the first time that reproduction without cross-fertilisation can occur in the arhynchobdellid leech Barbronia weberi (Blanchard, 1897). The discovery of “virgin birth” in this invasive Asian species was serendipitous in that a single unmated hatchling reared in isolation to maturity unexpectedly laid viable cocoons. In order to confirm unequivocally that this reproduction was indeed non-sexual the experiment was repeated rigorously for five successive generations. The formal possibility that cross-fertilisation may have occurred in the cocoon prior to hatching was specifically investigated by isolating embryos at the earliest stages of viability, well before reproductive structures developed. The cocoons and eggs of each successive generation were carefully quantified. No evidence of diminution of fertility was found in spite of severe inbreeding by the fifth generation. In fact fecundity of the inbred laboratory leeches was comparable to that of the original wild population. A total of 1,147 cocoons containing 4,455 eggs were laid during this year-long study. This was undoubtedly an underestimate of reproductive potential in that a number of healthy individuals had to be culled for pragmatic reasons. That a single individual is capable of producing thousands of progeny in one year, without mating, is sufficient to explain the success of B. weberi as an invasive species. In recent years this alien leech has become established on every inhabited continent. The mechanism of reproduction without cross-fertilisation in B. weberi is unknown but is probably attributable to self-fertilisation linked to its hermaphroditism. Taxonomic and phylogenetic implications of self-fertilisation in B. weberi are discussed.

The aorid amphipod Grandidierella japonica Stephensen, 1938, originally described from northern Japan, is reported for the first time from the Gulf of Maine and Long Island Sound in the Northwest Atlantic Ocean. It was discovered among grab samples of eelgrass (Zostera marina Linnaeus, 1753) beds taken in Casco Bay, Maine in summer 2018 and has been found intertidally in Long Island Sound since 2013 along Connecticut shores. It occurs in habitats like its native range but also in rocky areas and tidepools in bedrock. The presence of adults of both sexes, ovigerous females, and immature stages in all collections indicates the species is established. Among males, some morphological characteristics of gnathopod 1 vary with increasing total body length: the number of accessory carpal teeth increases, the carpus shape (length/width ratio) changes, and numbers of stridulating ridges increase. Maine and Connecticut specimens are distinguished from each other by the shape of male gnathopod 1 basis. Morphological variation, temporal differences in discovery, and separation by the biogeographic barrier Cape Cod suggests Maine and Connecticut populations originate from separate introductions.

The shore crab Hemigrapsus takanoi Asakura and Watanabe, 2005, native to the Northwest Pacific, was recorded in European waters about 25 years ago and it was first found in the Baltic Sea in 2014. Information on population structure of invaders and their new niche is needed in order to understand their biological impact. Over one year, we assessed temporal changes in relative abundance, size-class and sex ratio, as well as breeding season of H. takanoi in the Kiel Fjord (Western Baltic Sea). In addition, prey size preference and consumption rates on mussels (Mytilus edulis Linnaeus, 1758) were experimentally assessed in spring, summer and autumn. A total of 596 individuals were collected with highest and lowest abundances in June and February, respectively. Females were dominant over males (sex ratio 1.4:1), but males grew to larger sizes. H. takanoi reproduced between June and August with ovigerous females representing 30% of the entire female abundance registered over the entire year. Males were able to open larger mussels (due to larger claws) and consumed twice as many mussels when compared to females of similar size. Consumption rates for males were 6 and 2 times higher in summer (seawater temperature of 19 °C) compared to spring (8 °C) and autumn (13 °C), respectively. Females consumed 3 times more mussels in autumn than in spring. H. takanoi is an active predator, capable of reproduction in stressful brackish water conditions. Due to large abundances and high feeding pressure, this recently introduced species could play a key role in structuring post-settlement population dynamics of the dominant habitat builder M. edulis.

The corixid Trichocorixa verticalis originates from North America and is the only aquatic hemipteran alien to Europe. It is spreading in south-west Iberia where it is the dominant corixid in permanent, saline wetlands, where it may have excluded halotolerant native corixid species. In the Doñana wetland complex in south-west Spain, it is abundant in temporary brackish ponds but almost absent from temporary fresh ponds where the native S. lateralis is the dominant corixid. We conducted an outdoor experiment to test whether the alien is intolerant of low salinities, or if it can be competitively excluded by the native species S. lateralis. We established single species tanks for each species in semi-natural conditions, as well as mixed species tanks and controls lacking corixids. Trichocorixa verticalis was able to complete its life-cycle at a low salinity of 2 g/l. Compared to S. lateralis, it showed higher daily adult mortality and nymph production, but a shorter generation time. When mixed together, both species had lower adult mortality than in the alone treatment, suggesting they can coexist. Adult abundance at the end of the experiment was highest for T. verticalis, which performed better in mixed tanks whereas S. lateralis performed better alone. These results confirm that T. verticalis can tolerate freshwaters and suggest that its rarity in these ponds in nature is due to other ecological factors such as dispersal limitation, poor adaptation to fluctuating water levels, or the effects of ectoparasitic water mites known to prefer the alien as a host.

Unusual tolerance of ecological conditions and an ability of bisexual and unisexual reproduction allows Carassius gibelio (Bloch, 1782) to colonize various habitats from northern Europe to Asia. This non-native species of negligible economic importance has a negative impact on native species and the environment, and is undesirable in any part of Europe. As the reproductive mode is indirectly indicated by sex and ploidy ratios, these two features together with the karyotypes as well as sex- and ploidy-related length-weight relationships were studied in C. gibelio population inhabiting the Siemianówka Reservoir (northern-east Poland). Moreover, we tested the hypothesis that the frequency of triploid or diploid C. gibelio individuals in European populations will be greater than that in East Asia where it is native. The studied population is bisexual, clearly indicated by the presence of females (the sex ratio 3.1:1), represented by triploids and diploids in a 1:1 ratio. The parity of the sex ratio among diploids was noticeable in that most males (78.8%) were diploids but, surprisingly, the rest were triploids. The sex and ploidy ratios indicated both unisexual (via gynogenesis) and bisexual reproduction of C. gibelio. The weight-length relationships were related to sex but not to the ploidy. Compared to native populations, our findings reveal a higher incidence of sexually-reproducing diploids, which may have facilitated their success in invading Europe.