Numerous freshwater crayfish species are known to become successful invaders when introduced to new territories. One of the most invasive species in this group is the red swamp crayfish, Procambarus clarkii (Girard, 1852). In addition to other characteristics facilitating its invasiveness, it is also a vector of crayfish plague, a disease mostly lethal to crayfish of non‐North American origin. Procambarus clarkii, at present the most widespread crayfish species globally, is popular in many countries both for human consumption and as an ornamental animal. An established population of this species was documented for the first time within Indonesia, representing the first record for maritime Southeast Asia. The species is also common in the local ornamental pet trade. Infection by the crayfish plague pathogen Aphanomyces astaci Schikora was confirmed both in the outdoor population of P. clarkii and in one of the surveyed pet shops. Furthermore, A. astaci was detected in specimens of freshwater crab and shrimp species coexisting with P. clarkii. Local climatic conditions and the species temperature requirements suggest a high probability of the establishment of P. clarkii in Indonesia. Its further spread could irreversibly damage populations of many native endemic crustaceans in the country (as well as elsewhere in the region), and the thriving local aquacultures of the redclaw crayfish Cherax quadricarinatus (von Martens, 1868) may also be threatened. The total ban of P. clarkii and other crayfish species of North American origin in Indonesia is strongly recommended, especially considering that aquaculture and trade with Australasian species is a viable alternative option.

Crayfish of North American origin are amongst the most prominent high-impact invasive invertebrates in European freshwaters. They contribute to the decline of European native crayfish species by spreading the pathogen causing crayfish plague, the oomycete Aphanomyces astaci. In this study we validated the specificity of four quantitative PCR (qPCR) assays, either published or newly developed, usable for environmental DNA (eDNA) screening for widely distributed native and non-native crayfish present in Central Europe: Astacus astacus, Pacifastacus leniusculus, Faxonius limosus and Procambarus virginalis. We then conducted an eDNA monitoring survey of these crayfish as well as the crayfish plague pathogen in a wide variety of habitat types representative for Central and Western Europe. The specificity of qPCR assays was validated against an extensive collection of crayfish DNA isolates, containing most crayfish species documented from European waters. The three assays developed in this study were sufficiently species-specific, but the published assay for F. limosus displayed a weak cross-reaction with multiple other crayfish species of the family Cambaridae. In the field study, we infrequently detected eDNA of A. astaci together with the three non-native crayfish species under examination. We never detected eDNA from A. astaci together with native crayfish, but in a few locations eDNA from both native and non-native crayfish was captured, due either to passive transport of eDNA from upstream populations or co-existence in the absence of infected crayfish carriers of A. astaci. In the study, we evaluated a robust, easy-to-use and low-cost version of the eDNA sampling equipment, based mostly on items readily available in garden stores and hobby markets, for filtering relatively large (~5 l) water samples. It performed just as well as the far more expensive equipment industrially designed for eDNA water sampling, thus opening the possibility of collecting suitable eDNA samples to a wide range of stakeholders. Overall, our study confirms that eDNA-based screening for crayfish and their associated pathogen is a feasible alternative to traditional monitoring.

Aphanomyces astaci is the causative agent of crayfish plague, a disease responsible for numerous mass mortalities of native crayfish across Europe. In this study, we aim to extend knowledge about the A. astaci distribution in Eastern Europe, with specific focus on the River Dnieper (Ukraine), and summarize presently available information about the distribution of genotypes of this pathogen across the Western Palaearctic. We compiled published records about genotype groups of A. astaci, assembled them to a comprehensive map, and added the newly obtained results from Ukraine. The native narrow-clawed crayfish Pontastacus leptodactylus was sampled from the river Dnieper in Svydivok and Kiev, ca 170 km apart, and screened for the pathogen presence in soft cuticles by quantitative PCR. We confirmed infections by A. astaci at both sites, with prevalence exceeding 30% and low to medium agent levels in infected crayfish. Pathogen genotyping confirmed the presence of the A. astaci haplogroup B, associated with the signal crayfish Pacifastacus leniusculus but also known from some chronically infected narrowclawed crayfish from Turkey and Moldova. Our results support the notion that latent A. astaci infections among narrow-clawed crayfish populations may be widespread in Eastern Europe and the Middle East.

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-Established populations of the non-indigenous parthenogenetically reproducing marbled crayfish Procambarus fallax f. virginalis have been recently reported from various European countries. The colonised sites are usually lentic and relatively isolated from major watercourses and in such cases the immediate threat of the spread of this taxon is limited. Here we report on a marbled crayfish population that is likely to become a seed for colonisation of the Danube in Slovakia. It is located in a channel within the Slovak capital Bratislava in the immediate vicinity of a pumping station that occasionally releases significant amounts of water into the side arm of the Danube. The population is well established with a high growth potential: numerous adult marbled crayfish individuals were observed at the site in September and October 2016 and the progeny (eggs or first two developmental stages) of 27 berried females exceeded 11 000 individuals. The maximum observed fecundity per female reached 647 juveniles in the second developmental stage. The Danube side arm downstream of the pumping station harbours a population of spiny-cheek crayfish Orconectes limosus infected with the crayfish plague pathogen Aphanomyces astaci. We presume that marbled crayfish is already present below the pumping station and it is just a matter of effort and time until it is discovered. The investigated specimens of marbled crayfish were found free of A. astaci, but horizontal transmission from infected spiny-cheek crayfish may be expected, as well as further spread of marbled crayfish in the Danube.Keywords: pet trade / aquatic invasion / fecundity / asexual reproduction / Slovakia Résumé -Une portion slovaque du Danube est un site de reproduction bien établie d'écrevisse marbrée Procambarus fallax f.virginalis. Des populations établies d'écrevisses marbrées non-indigènes à reproduction parthénogénétique Procambarus fallax f. virginalis ont récemment été signalées dans différents pays européens. Les sites colonisés sont généralement lentiques et relativement isolés des grands cours d'eau et, dans de tels cas, la menace immédiate de propagation de ce taxon est limitée. Nous rapportons ici sur une population d'écrevisses marbrées qui risque de devenir une source pour la colonisation du Danube en Slovaquie. Elle est localisée dans un canal situé dans la capitale slovaque Bratislava, à proximité immédiate d'une station de pompage qui libère occasionnellement d'importantes quantités d'eau dans le bras latéral du Danube. La population est bien établie avec un fort potentiel de croissance: de nombreux adultes d'écrevisse marbrée ont été observés sur le site en septembre et octobre 2016 et la progéniture (oeufs ou deux premiers stades de développement) de 27 femelles grainées dépasse 11 000 individus. La fécondité maximale observée par femelle a atteint 647 juvéniles au deuxième stade de développement. Le bras latéral du Danube en aval de la station de pompage abrite une population d'écrevisses américaines Orconectes limosus infectées par l...