Biological invasions are increasingly recognized ecological and economic threats to biodiversity and are projected to increase in the future. Introduced freshwater crayfish in particular are protruding invaders, exerting tremendous impacts on native biodiversity and ecosystem functioning, as exemplified by the North American spiny-cheek, signal and red swamp crayfish as well as the Australian common yabby. The marbled crayfish is among the most outstanding freshwater crayfish invaders due to its parthenogenetic reproduction combined with early maturation and high fecundity. As their introduced ranges expand, their sympatric populations become more frequent. The question of which species and under what circumstances will dominate in their introduced communities is of great interest to biodiversity conservation as it can offer valuable insights for understanding and prioritization of management efforts. In order to examine which of the aforementioned species may be more successful as an invader, we conducted a set of independent trials evaluating survival, growth, claw injury, and reproduction using single-species stocks (intraspecific interactions) and mixed stocks (interspecific interactions) of marbled crayfish vs. other crayfish invaders since the onset of exogenous feeding. In both single and mixed stocks, red swamp crayfish and yabby grew faster than marbled crayfish, while marbled crayfish were superior to both spiny-cheek and signal crayfish in terms of growth. With the exception of signal crayfish, the faster-growing species consistently reached a higher survival rate. The faster-growing species tended to negatively impair smaller counterparts by greater claw injury, delayed maturation, and reduced fecundity. Only marbled crayfish laid eggs as early as 14 weeks in this study, which is earlier than previously reported in the literature. Thus, the success of marbled crayfish among invasive crayfish is significantly driven by relatively fast growth as well as an early and frequent reproduction. These results shed light on how interactions between invasive populations can unfold when their expansion ranges overlap in the wild, thereby contributing to the knowledge base on the complex population dynamics between existing and emerging invasive species.

For management strategies aimed at biological invasions, a detailed survey of the general public’s knowledge is crucial. For this purpose, our present study aimed to (i) investigate the level of comprehension regarding biological invasions, (ii) assess the ability of responders to discriminate between invasive and native species, and (iii) evaluate the conservation support. Increased awareness of biological invasions on the part of the respondents positively influences the willingness to protect native fish and crayfish species and not their invasive counterparts. Higher identification scores of species by participants significantly decreased the conservation support of invasive species. Female respondents were more willing to protect a species, including invasive ones. Respondents could discriminate between native and invasive fish species to a significantly better extent than between native and invasive crayfish. Without public awareness and citizen-aimed education, we will be unable to conduct sustainable management and prevent further species’ introductions and translocations.

The marbled crayfish, Procambarus fallax f. virginalis, is a taxon widely available in the aquarium pet trade, which has been introduced to open waters in several European countries and in Madagascar. Recent studies confirmed this parthenogenetically reproducing crayfish as a high-risk invasive species, and vector of the crayfish plague pathogen, Aphanomyces astaci. It has been first discovered in Slovakia in 2010, but the status of the local population was not studied since then. Due to enlarged sampling area around the first report and one locality, where we presupposed the crayfish occurrence, we identified new marbled crayfish populations. Here, we report presence of three newly established marbled crayfish populations in Slovakia. Two populations are located critically close to the Váh River, a major tributary of the Danube River; one of them being directly connected to the Váh River via a side channel during occasional floods. The third established marbled crayfish population was found at the mouth of a thermal stream flowing into the Nitra River, a tributary of the Váh River. In this stream, crayfish coexist with other exotic fish and gastropod species of aquarium origin. We presume that the reported localities may serve as a source for further expansion of the marbled crayfish in the mid-part of the Danube catchment. Floods, active dispersal (including overland), passive dispersal by zoochory or anthropogenic translocations are among the major drivers facilitating the marbled crayfish colonization. We have not detected the crayfish plague pathogen in any of the studied populations. However, if spreading further, the marbled crayfish will encounter established populations of crayfish plague carriers in the Danube River, in which case they may acquire the pathogen by horizontal transmission and contribute to spread of this disease to indigenous European crayfish species.

The release of ornamental pets and associated pathogens outside their native range might directly or indirectly impact the recipient community. In temperate regions, e.g., central Europe, feral freshwater species of tropical and sub-tropical origins are mainly constrained to thermally polluted waters and thermal springs. However, species with high environmental plasticity and reproduction rates, such as the shrimp Neocaridina davidi, may adapt to colder water regimes over time. A widening thermal niche may eventually overcome thermal barriers, further expanding the range and enhancing transmission opportunities for host generalist parasites. This study assesses the observed (field observations) and theoretical (species distribution models) range expansion of N. davidi and associated parasites in Europe. We report three newly established N. davidi populations from thermally polluted waters in central Europe (Germany, Hungary, and Slovakia) and provide further evidence of its range expansion into colder environments. Species distribution models predict thermally suitable habitats in the Mediterranean and a foreseeable expansion into Western Europe and the Balkans by 2050. We confirm the presence of the microsporidian parasite Ecytonucleospora hepatopenaei in feral N. davidi populations across Europe and expand the list of microsporidians found in this host from two to four. Furthermore, we provide the first evidence of parasite spillover from/to the invasive crayfish Procambarus clarkii, suggesting that parasite exchange with native biota might be possible. Such possibility, coupled with an ongoing range expansion of N. davidi bolstered by human-mediated introductions and climate change, will likely exacerbate the impact on native biota.

The ornamental pet trade has emerged as one of the most important pathways for the introduction of non‐native aquatic species. In the last two decades, freshwater crayfish have become a popular ornamental group, with numerous non‐native populations established through this way, including some populations of the North American red swamp crayfish Procambarus clarkii (Girard, 1852). Whilst the introduction routes of aquaculture‐derived populations have been identified through molecular methods and historical documents, obtaining traceability in the pet trade market is often more challenging. We analysed 283 pet‐traded and feral P. clarkii individuals, originating from Europe and Southeast Asia, for variation of a fragment of the mitochondrial gene for cytochrome c oxidase subunit I (COI) to determine the genetic variation amongst 11 pet shops in various countries, outdoor aquaculture in Indonesia and 27 established feral populations in Europe likely originating from releases of ornamental animals. We found 13 haplotypes, four of which had not been detected in P. clarkii before. Overall, higher haplotype diversity was found in established feral populations (Hd = 0.555) and outdoor aquaculture (Hd = 0.667) compared to pet shops (Hd = 0.141). We observed up to three haplotypes shared between the Asian pet trade and Europe. Whilst one of them appeared in a pet shop in Indonesia and open waters in Hungary, another was observed in the pet trade in Cambodia, outdoor aquaculture in Indonesia and in the stock of European wholesalers. The third was the most common haplotype and provided little information regarding the species' introduction routes. Additionally, one haplotype present in the European pet trade was commonly found in European waters. Particularly noteworthy was the high haplotype variability of Hungarian populations collected in the metropolitan area of Budapest, which is considered a European hotspot of ornamental crayfish introductions. The findings of our study support the low haplotype diversity in pet shops, likely stemming from a limited number of founders of most P. clarkii ornamental stocks. In contrast, higher genetic variability in feral populations may be due to recurrent introduction events from different sources. Furthermore, the results support the hypothesis that the crayfish occurring in the European pet trade originate from the Asian ornamental trade rather than from the North American native range of P. clarkii. Legally binding regulations on listing non‐native species do not seem to curb the importation of ornamental non‐native species. However, the implementation of molecular methods conducted at border controls (by veterinarians) and post‐border (by wholesalers and retailers) in cooperation with research institutions can efficiently serve as preventive measures to halt species importations. We emphasise using molecular analyses for both species identification and tracking introduction routes of biological invasions.