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Bythotrephes longimanus (spiny waterflea) and Cercopagis pengoi (fishhook waterflea) are large invasive predatory cladocerans that alter the composition, density, and behavior of native zooplankton communities. Lake Champlain was invaded by Bythotrephes and Cercopagis in 2014 and 2018, respectively. This study was conducted to determine the changes in crustaceous zooplankton diel vertical migration (DVM) associated with the presence of these two invasive species. Daytime and nighttime zooplankton samples were collected from vertical net tows at 5 m intervals using 153 µm and 250 µm closing plankton nets at a 50 m deep site in Lake Champlain during the month of August (2013–2016, 2019, 2023, and 2024). Sampling dates encompassed years before and after each invader entered the lake. The results show increased DVM activity in several native zooplankton taxa associated with invasion years, including Daphnia retrocurva, Bosmina longirostris, and Diacyclops thomasi. Zooplankton in Lake Champlain appear to occupy deeper depths during the daytime after Bythotrephes and Cercopagis invaded than in previous years. Alterations associated with Bythotrephes were more extreme, whereas changes associated with Cercopagis were longer lasting. These shifts in DVM behavior have potential implications for trophic dynamics in Lake Champlain by altering competitive interactions and foraging behavior of zooplankton and their predators.

We present a comprehensive survey of the scientific literature pertaining to non-indigenous and invasive zooplankton published across the first decades of the twenty-first century (i.e., 2000–2018). We provide a concise summary of the manner in which the scientific community has allocated its efforts to this issue in recent decades, and to illuminate trends that emerge from the literature. Our search yielded 620 publications encompassing 139 invasive zooplankton species, with invasive zooplankton reported from every region of the planet—including the Arctic and Antarctic. Most taxa were reported in a small number of publications, with the majority being mentioned in only a single paper. In contrast, approximately half of the surveyed publications concerned just four species: Bythotrephes longimanus, Mnemioposis leidyi, Cercopagis pengoi, and Daphnia lumholtzi. Our survey reveals strong geographic patterns among the literature, with most publications arising from economically developed western nations. We found that the majority of publications pertained to holoplanktonic organisms from freshwater habitats, especially from the North American Great Lakes. Based on these results, we present several recommendations for future research topics that may hold considerable opportunity for growth in our understanding of the invasion process.

Non-indigenous species (NIS) are on a rise globally. They can pose strong impacts on ecosystems in their non-native range and can therefore be a serious threat to biodiversity. Here, we compile the existing information available regarding the extent to which commercially and recreationally used fish stocks (exploited fish species) are affected by NIS. To do so, we conducted a literature review to summarize the known and presumed impacts of four case study NIS with already known strong effects on the Baltic Sea ecosystem: round goby, Neogobius melanostomus , sea walnut, Mnemiopsis leidyi , mud crab, Rhithropanopeus harrisii , and fishhook water flea, Cercopagis pengoi. We found that round goby, mud crab and fishhook water flea are documented to serve as a new food source for native fish species, while sea walnut and fishhook water flea are supposedly affecting planktivorous fish through resource competition. Round goby is very likely a strong competitor for the benthivore fish community. There are also indications that it feeds on juvenile fish and fish eggs. Generally, our results show that large knowledge gaps exist, while the published impacts on exploited fish species are often solely based on correlations (e.g. decreasing abundances of native species with cooccurring increasing abundances of NIS), regionally restricted studies or expert judgements. In addition, many studies are older and the current population size of the NIS, which is obviously associated with their impact, is unknown. Thus, the majority of described impacts of NIS on commercially and recreationally used fish stocks seems to stem from assumptions. Therefore, more field observations and experimental studies are needed to be able to scientifically evaluate the impact of NIS. Nevertheless, in this review, the available information was summarized, even if they are speculative, and specific knowledge gaps were identified. Moreover, we outline further investigations that are needed to advance our mechanistic understanding of the interactions between NIS and exploited fish species in the Baltic Sea. This knowledge is essential for the sustainable management of aquatic resources and management of NIS of the Baltic Sea.

Canals provide wide-ranging economic benefits, while also serving as corridors for the introduction and spread of aquatic alien species, potentially leading to negative ecological and economic impacts. However, to date, no comprehensive quantifications of the reported economic costs of these species have been done. Here, we used the InvaCost database on the monetary impact of invasive alien species to identify the costs of those facilitated by three major canal systems: the European Inland Canals, Suez Canal, and Panama Canal. While we identified a staggering number of species having spread via these systems, monetary costs have been reported only for a few. A total of$33.6 million in costs have been reported from species linked to European Inland Canals (the fishhook waterflea Cercopagis pengoi and the zebra mussel Dreissena polymorpha ) and $ 8.6 million linked to the Suez Canal (the silver-cheeked toadfish Lagocephalus sceleratus , the lionfish Pterois miles , and the nomad jellyfish Rhopilema nomadica ), but no recorded costs were found for species facilitated by the Panama Canal. We thus identified a pervasive lack of information on the monetary costs of invasions facilitated by canals and highlighted the uneven distribution of costs.

Predicting the spread of invasive species and understanding the role of niche dynamics in niche transferability are critical challenges in the management of biological invasions, both theoretically and practically. We used complementary species distribution modelling approaches, such as multivariate niche analysis and reciprocal distribution models, to test the niche conservatism hypothesis and to predict the potential distribution of the fishhook waterflea, Cercopagis pengoi. Our analysis indicated a significant similarity between its native and invasive ranges, suggesting that a subset of the Ponto-Caspian propagules may have been the founders of European populations. However, our results contradict the niche conservatism hypothesis, showing that C. pengoi has not fully occupied the available niche within its current invasive ranges. Moreover, we observed a notable niche expansion, reflecting a significant shift in niche following its intercontinental introduction in North America. Given the suitability of new environments for the expansion of C. pengoi and its tendency to evade detection prior to population surges, we recommend a focus on early detection through monitoring of both water columns and bottom sediments. This should be complemented by strict enforcement of ballast water regulations to curtail its spread in North America, Europe, and other suitable non-native regions globally.

The zooplankton community of Lake Champlain has been altered over the past decade due to the introduction of two predatory zooplankton species. Bythotrephes longimanus Leydig, 1860 was first detected in Lake Champlain in August 2014, and Cercopagis pengoi (Ostroumov, 1891) was detected in August 2018. Monitoring for both invasive species at 15 lake sites using whole water tow sampling has been ongoing since 2010 with no detection of either species until 2014. Utilizing data from the Lake Champlain long-term monitoring program, we assessed pre- and post-invasion population dynamics of both invasive species on the native zooplankton community. Our results showed shifts in community structure following invasion, including a reduction in Diacyclops thomasi (Forbes, 1882) populations by both invaders and a reduction in Daphnia retrocurva Forbes, 1882 following the introduction of B. longimanus. Other cyclopoids, bosminids, and rotifers were also reduced, corresponding to both introduced species. The native large-bodied plankton predator Leptodora kindtii (Focke, 1844) appears to be unable to coexist with either invasive predatory cladoceran, displaying seasonal partitioning between L. kindtii and the Cercopagidae. These findings suggest that the invasion of both large predators in Lake Champlain have impacted pelagic zooplankton community structure.

Detecting the presence of rare species has interested ecologists and conservation biologists for many years. A particularly daunting application of this problem pertains to the detection of non-indigenous species (NIS) as they colonize new ecosystems. Ethical issues prevent experimental additions of NIS to most natural systems to explore the relationship between sampling intensity and the detection probability of a colonizing NIS. Here we examine this question using a recently introduced water flea, Cercopagis pengoi, in Lake Ontario. The species has biphasic population development, with sexually-produced 'spring morphs' developing prior to parthenogenetically-produced 'typical' morphs. Thus, this biphasic morphology allows distinction between new colonists (spring morphs) from subsequent generations. We repeatedly sampled Hamilton Harbour, Lake Ontario for the presence of both spring and typical morphs. Probability of detection was positively related to both the number of samples taken and animal density in the lake; however, even highly intensive sampling (100 samples) failed to detect the species in early spring when densities were very low. Spatial variation was greatest when densities of Cercopagis were intermediate to low. Sub-sampling, which increased space between adjacent samples, significantly decreased the number of samples required to reach greater, calculated detection probabilities on these dates. Typical sampling protocols for zooplankton have a low probability (< 0.2) of detecting the species unless population density is high. Results of this study suggest that early detection of colonizing, aquatic NIS may be optimized through use of a risk-based sampling design, combined with high sampling intensity in areas deemed most vulnerable to invasion, rather than less intensive sampling at a wider array of sites.

Despite a well-developed theoretical basis for the role of genetic diversity in the colonization process, contemporary investigations of genetic diversity in biological invasions have downplayed its importance. Observed reductions in genetic diversity have been argued to have a limited effect on the success of establishment and impact based on empirical studies; however, those studies rarely include assessment of failed or comparatively less-successful biological invasions. We address this gap by comparing genetic diversity at microsatellite loci for taxonomically and geographically paired aquatic invasive species. Our four species pairs contain one highly successful and one less-successful invasive species (Gobies: Neogobius melanostomus , Proterorhinus semilunaris ; waterfleas: Bythotrephes longimanus , Cercopagis pengoi ; oysters: Crassostrea gigas , Crassostrea virginica ; tunicates: Bortylloides violaceous , Botryllus schlosseri ). We genotyped 2717 individuals across all species from multiple locations in multiple years and explicitly test whether genetic diversity is lower for less-successful biological invaders within each species pair. We demonstrate that, for gobies and tunicates, reduced allelic diversity is associated with lower success of invasion. We also found that less-successful invasive species tend to have greater divergence among populations. This suggests that intraspecific hybridization may be acting to convert among-population variation to within-population variation for highly successful invasive species and buffering any loss of diversity. While our findings highlight the species-specific nature of the effects of genetic diversity on invasion success, they do support the use of genetic diversity information in the management of current species invasions and in the risk assessment of potential future invaders.

We describe, explain, and \"predict\" dispersal and ecosystem impacts of six Ponto-Caspian endemic species that recently invaded the Great Lakes via ballast water. The zebra mussel, Dreissena polymorpha, and quagga mussel, Dreissena bugensis, continue to colonize hard and soft substrates of the Great Lakes and are changing ecosystem function through mechanisms of ecosystem engineering (increased water clarity and reef building), fouling native mussels, high particle filtration rate with selective rejection of colonial cyanobacteria in pseudofeces, alteration of nutrient ratios, and facilitation of the rapid spread of their Ponto-Caspian associates, the benthic amphipod Echinogammarus ischnus and the round goby, Neogobius melanostomus, which feeds on zebra mussels. The tubenose goby,Proterorhinus marmoratus, which does not feed on zebra mussels, has not spread rapidly. Impacts of these benthic invaders vary with site: in some shallow areas, habitat changes and the Dreissena round goby piscivore food chain have improved conditions for certain native game fishes and waterfowl; in offshore waters, Dreissena is competing for settling algae with the native amphipod Diporeia spp., which are disappearing to the detriment of the native deep-water fish community. The predatory cladoceran Cercopagis pengoi may compete with small fishes for zooplankton and increase food-chain length.

In August 2023, a study of the plankton community of the sluice chambers of the Lenin Volga–Don shipping Canal (VDSC) and adjacent territories (the Volga River near Volgograd, between the locks and in the Tsimlyansk reservoir near Kalach-na-Donu) was conducted. Alien species from various biogeographic complexes have been recorded in all its components (phyto-, proto-, and metazooplankton). As part of the phytoplankton, Skeletonema subsalsum (A. Cleve) Bethge, 1928 is from the Ponto-Caspian complex. The marine species Leprotintinnus pellucides (Cleve, 1899) is included as part of the ciliates. Nine species from various biogeographic complexes were found in zooplankton, namely, Heterocope caspia Sars G.O., 1897; Eurytemora caspica Sukhikh & Alekseev, 2013; Cercopagis (Cercopagis) pengoi (Ostroumov, 1891) from the Ponto-Caspian; Cornigerius maeoticus (Pengo, 1879) from the Ponto-Azov; Calanipeda aquaedulcis Krichagin, 1873 from the Mediterranean; Thermocyclops taihokuensis , Harada, 1931 from the East Asian; and the American Acanthocyclops americanus (Marsh, 1893) and Moina cf. micrura Kurz, 1875 with uncertain status. It has been established that, despite the intensive mixing of water in the sluice chambers, alien species are actively developing and entering the rank of dominant ones. The proportion of dead Copepoda individuals in sluice chambers is lower than that of Cladocera. However, the abundance of Cladocera is so large that, even with high mortality, they are part of the dominant complex of species, while some zooplankter individuals were in an active reproductive state. Significant abiotic factors for alien species in the VDSC are electrical conductivity (for Heterocope caspia , Moina cf. micrura , and Acanthocyclops americanus ) and temperature (for Skeletonema subsalsum ). For crustaceans consuming detritus, bacteria, algae, ciliates, and other zooplankters, reliable correlations with the listed food resources are noted.