Resistance to the crayfish plague, Aphanomyces astaci (Oomycota) in the endangered freshwater crayfish species, Austropotamobius pallipes

Expression of C/EBP and Kr-h1 transcription factors under immune stimulation in the noble crayfish

Transcription factors (TFs) have an important role in the regulation of the gene expression network. The role of TFs in the immune response of freshwater crayfish is poorly understood, but leveraging the regulatory mechanisms of immune response could augment the resistance against the invasive oomycete pathogen, Aphanomyces astaci. Previous studies indicated that the TFs CCAAT/enhancer-binding protein (C/EBP) and putative Krüppel homolog-1 protein (Kr-h1) might play a role in immune and stress response of the noble crayfish (Astacus astacus). Here, we aimed to further characterise these two gene products to gain a better understanding of their evolutionary origin, domain organisation and expression patterns across different crayfish tissues. Furthermore, we conducted an immune stimulation experiment to observe the potential changes in the gene expression of C/EBP and Kr-h1 under immune challenge in different crayfish tissues. Our results showed that both C/EBP and Kr-h1 are closely related to other C/EBPs and Kr-h1s in Malacostraca. Gene expression analysis revealed that both TFs are present in all analysed tissues, with higher expression of C/EBP in the gills and Kr-h1 in the abdominal muscle. Immune stimulation with laminarin (mimicking β-1-3-glucan in the oomycete cell wall) showed an activation of the crayfish immune system, with an overall increase in the total haemocyte count (THC) compared to untreated control and crayfish buffered saline (CBS) treatment. On the gene expression level, an up-regulation of the C/EBP gene was detected in the laminarin treated group in hepatopancreas and heart, while no changes were observed for the Kr-h1 gene. Our results indicate an early change in C/EBP expression in multiple tissues during immune stimulation and suggest its involvement in the immune response of the noble crayfish.

The Crayfish Plague Pathogen Aphanomyces astaci in Ireland

Crayfish plague is a devastating disease of European freshwater crayfish and is caused by the oomycete Aphanomyces astaci (Ap. astaci), believed to have been introduced to Europe around 1860. All European species of freshwater crayfish are susceptible to the disease, including the white-clawed crayfish Austropotamobius pallipes. Ap. astaci is primarily spread by North American crayfish species and can also disperse rapidly through contaminated wet gear moved between water bodies. This spread, coupled with competition from non-indigenous crayfish, has drastically reduced and fragmented native crayfish populations across Europe. Remarkably, the island of Ireland remained free from the crayfish plague pathogen for over 100 years, providing a refuge for A. pallipes. However, this changed in 1987 when a mass mortality event was linked to the pathogen, marking its introduction to the region. Fortunately, crayfish plague was not detected again in Ireland until 2015 when a molecular analysis linked a mass mortality event in the Erne catchment to Ap. astaci. Since then, the pathogen has appeared across the island. Between 2015 and 2023, Ap. astaci was detected in 18 water catchments, revealing multiple genotypes. Intriguingly, the pathogen in Ireland is present without its natural host species. The uneven distribution of various genetic lineages strongly suggests the human-mediated transport of zoospores via contaminated water equipment as a primary cause of spread. This review details the timeline of these events, Ap. astaci's introduction into Ireland, and its rapid spread. As well, this review references the genotypes that have been determined, and discusses the issue of non-indigenous crayfish species in Ireland and management efforts.

Body size mediates latitudinal population differences in the response to chytrid fungus infection in two amphibians

Factors behind intraspecific variation in sensitivity to pathogens remain poorly understood. We investigated how geographical origin in two North European amphibians affects tolerance to infection by the chytrid fungus Batrachochytrium dendrobatidis (Bd), a generalist pathogen which has caused amphibian population declines worldwide. We exposed newly metamorphosed individuals of moor frog Rana arvalis and common toad Bufo bufo from two latitudinal regions to two different BdGPL strains. We measured survival and growth as infections may cause sub-lethal effects in fitness components even in the absence of mortality. Infection loads were higher in B. bufo than in R. arvalis, and smaller individuals had generally higher infection loads. B. bufo had high mortality in response to Bd infection, whereas there was little mortality in R. arvalis. Bd-mediated mortality was size-dependent and high-latitude individuals were smaller leading to high mortality in the northern B. bufo. Bd exposure led to sub-lethal effects in terms of reduced growth suggesting that individuals surviving the infection may have reduced fitness mediated by smaller body size. In both host species, the Swedish Bd strain caused stronger sublethal effects than the British strain. We suggest that high-latitude populations can be more vulnerable to chytrids than those from lower latitudes and discuss the possible mechanisms how body size and host geographical origin contribute to the present results.

Protocols for studying the crayfish plague pathogen, Aphanomyces astaci, and its host-pathogen interactions

The crayfish plague caused by the pathogen Aphanomyces astaci has decimated the European and Asian populations of freshwater crayfish and represents an important threat to the other highly susceptible crayfish species in the world, such as the Australian, Madagascar, and South American species. The development and application of molecular methods addressed to the identification of A. astaci has increased exponentially during the last decades in contrast to a slow trend of the pathogen biology and host interaction. There is still a need for a better comprehension of the A. astaci-crayfish interactions, specifically the resistance and tolerance immune mechanism. These types of studies required a robust basic knowledge on the developmental biology of the pathogen in order to reproduce life stages and to perform infection experiments. A great piece of work in this area was carried out during the 1960 s to 80 s in University of Uppsala. Thus, the purpose of this work was to update previous protocols as well as to generate new guidelines to reproduce key developmental biology stages of A. astaci, to eventually identify crayfish populations with higher resistance and tolerance to this pathogen. This work also refers to other methodologies and guidelines for the diagnosis of crayfish plague, the pathogen isolation, and the in vitro production of zoospores.

Austropotamobius pallipes can be infected by two haplotypes of Aphanomyces astaci: A key example from an outbreak at an ex-situ conservation facility

The crayfish plague, caused by the pathogen Aphanomyces astaci, is a pandemic disease endemic to North America that has been devastating susceptible crayfish populations in Europe since the 19th century. In Spain, this disease has decimated populations of the native crayfish species Austropotamobius pallipes due to introductions of North American crayfish, which act as vectors of the pathogen. To combat against these losses, several regional governments have established ex-situ breeding programs to restock wild populations of the species. In this study, we report on an outbreak of A. astaci that occurred in one of the most important A. pallipes aquaculture centers in Spain. Using a variety of detection methods, we analyzed affected crayfish and environmental samples from the facilities over a period of six months and determined that the outbreak was caused by two haplotypes of A. astaci, d1 and d2, which are both associated with the North American crayfish species Procambarus clarkii. To our knowledge, this is the first report of a two-haplotype coinfection of A. astaci outside the native range of this pathogen.

On the conservation of white-clawed crayfish in the Iberian Peninsula: Unraveling its genetic diversity and structure, and origin

European crayfish species are a clear example of the drastic decline that freshwater species are experiencing. In particular, the native species of the Iberian Peninsula, the white clawed-crayfish (WCC) Austropotamobius pallipes, is listed as "endangered" by the IUCN and included in Annex II of the EU Habitat Directive and requires especially attention. Currently, implemented conservation management strategies require a better understanding of the genetic diversity and phylogeographic patterns, as well as of its evolutionary history. For this purpose, we have generated the largest datasets of two informative ribosomal mitochondrial DNA regions, i.e., cytochrome oxidase subunit I and 16S, from selected populations of the WCC covering its geographical distribution. These datasets allowed us to analyze in detail the (i) genetic diversity and structure of WCC populations, and (ii) divergence times for Iberian populations by testing three evolutionary scenarios with different mtDNA substitution rates (low, intermediate, and high rates). The results indicate high levels of haplotype diversity and a complex geographical structure for WCC in the Iberian Peninsula. The diversity found includes new unique haplotypes from the Iberian Peninsula and reveals that most of the WCC genetic variability is concentrated in the northern and central-eastern regions. Despite the fact that molecular dating analyses provided divergence times that were not statistically supported, the proposed scenarios were congruent with previous studies, which related the origin of these populations with paleogeographic events during the Pleistocene, which suggests an Iberian origin for these WCC. All results generated in this study, indicate that the alternative hypothesis of an introduced origin of the Iberian WCC is highly improbable. The result of this study, therefore, has allowed us to better understand of the genetic diversity, structure patterns, and evolutionary history of the WCC in the Iberian Peninsula, which is crucial for the management and conservation needs of this endangered species.

Host-pathogen coevolution drives innate immune response to Aphanomyces astaci infection in freshwater crayfish: transcriptomic evidence

BACKGROUND

For over a century, scientists have studied host-pathogen interactions between the crayfish plague disease agent Aphanomyces astaci and freshwater crayfish. It has been hypothesised that North American crayfish hosts are disease-resistant due to the long-lasting coevolution with the pathogen. Similarly, the increasing number of latent infections reported in the historically sensitive European crayfish hosts seems to indicate that similar coevolutionary processes are occurring between European crayfish and A. astaci. Our current understanding of these host-pathogen interactions is largely focused on the innate immunity processes in the crayfish haemolymph and cuticle, but the molecular basis of the observed disease-resistance and susceptibility remain unclear. To understand how coevolution is shaping the host's molecular response to the pathogen, susceptible native European noble crayfish and invasive disease-resistant marbled crayfish were challenged with two A. astaci strains of different origin: a haplogroup A strain (introduced to Europe at least 50 years ago, low virulence) and a haplogroup B strain (signal crayfish in lake Tahoe, USA, high virulence). Here, we compare the gene expression profiles of the hepatopancreas, an integrated organ of crayfish immunity and metabolism.

RESULTS

We characterised several novel innate immune-related gene groups in both crayfish species. Across all challenge groups, we detected 412 differentially expressed genes (DEGs) in the noble crayfish, and 257 DEGs in the marbled crayfish. In the noble crayfish, a clear immune response was detected to the haplogroup B strain, but not to the haplogroup A strain. In contrast, in the marbled crayfish we detected an immune response to the haplogroup A strain, but not to the haplogroup B strain.

CONCLUSIONS

We highlight the hepatopancreas as an important hub for the synthesis of immune molecules in the response to A. astaci. A clear distinction between the innate immune response in the marbled crayfish and the noble crayfish is the capability of the marbled crayfish to mobilise a higher variety of innate immune response effectors. With this study we outline that the type and strength of the host immune response to the pathogen is strongly influenced by the coevolutionary history of the crayfish with specific A. astaci strains.

Shrimp bacterial and parasitic disease listed in the OIE: A review

Shrimp aquaculture industry has steadily increased with demand and development of aquaculture technology. In recent years, frequent diseases have become a major risk factor for shrimp aquaculture, such as a drastically reduced the production of shrimp and causing national economic loss. Among them, shrimp bacterial diseases such as hepatopancreatic necrosis disease (AHPND) and necrotizing hepatopancreatitis (NHP-B) and parasitic disease such as Aphanomyces astaci (crayfish plague) are emerging and evolving into new types. OIE (World Organization for Animal Health) regularly updates information on diseases in the Aquatic Code and Aquatic Manual, but in-depth information on the shrimp diseases are lacking. Therefore, the purpose of this review is to provide information necessary for the response and prevention of shrimp diseases by understanding the characteristics and diagnosis of shrimp diseases designated by OIE.

Resistance to Crayfish Plague: Assessing the Response of Native Iberian Populations of the White-Clawed Freshwater Crayfish

Crayfish plague, caused by the oomycete pathogen Aphanomyces astaci, is one of the most devastating of the emerging infectious diseases. This disease is responsible for the decline of native European and Asian freshwater crayfish populations. Over the last few decades, some European crayfish populations were reported to display partial to total resistance to the disease. The immune response in these cases was similar to that exhibited by the natural carriers of the pathogen, North American freshwater crayfish, e.g., weak-to-strong melanization of colonizing hyphae. We tested the degree of resistance displayed by 29 native Iberian populations of Austropotamobius pallipes that were challenged by zoospores of the pathogen. We measured the following parameters: (i) mean survival time, (ii) cumulative mortality, and (iii) immune response, and found that the total cumulative mortality of all the challenged populations was 100%. The integration of the results from these parameters did not allow us to find differences in resistance towards A. astaci among the northern and central populations of the Iberian Peninsula. However, in the southern populations, we could identify four distinct population responses based on an evaluation of a GLM analysis. In the first case, the similar response could be explained by the effect of a pathogen strain with a lower-than-expected virulence, and/or an actual increase in resistance. In the Southern populations, these differences appear to be the consequence of either whole population or individual resistance. Individuals that survived for a longer period than the others showed a stronger immune response, i.e., presence of partially or fully melanized hyphae, which is similar to that of North American crayfish species. This might be the consequence of different mechanisms of resistance or/and tolerance towards A. astaci.

Native European crayfish Astacus astacus competitive in staged confrontation with the invasive crayfish Faxonius limosus and Procambarus acutus

The European native, noble crayfish (Astacus astacus) has suffered from a serious and long term population decline due to habitat destruction, water pollution and the impact of the invasive North American crayfish that are carriers of the crayfish plague (Aphanomyces astaci). The latter being the major factor currently confining noble crayfish to uninvaded (parts of) waterbodies. However, recently wild populations of apparently healthy noble crayfish carrying the crayfish plague have been found. As crayfish are known for their inter- and intraspecific agonistic behaviour which may be key for their competitive success, this raised the interesting question what would happen if the crayfish plague would not be a dominant factor anymore in the interaction between native and invasive species. Since the outcome of those encounters is still unclear, this study explores whether the noble crayfish can stand its ground towards invasive species in such agonistic interactions. Furthermore, the ability of the noble crayfish and invasive crayfish to acquire shelter through agonistic interaction is also assessed. Through pairwise staged interactions, agonistic behaviour and shelter competition between the native A. astacus and the invasive Faxonius limosus and Procambarus acutus were examined. The results showed that A. astacus triumphs over F. limosus and P. acutus in agonistic encounters and in competition for shelter. In turn, P. acutus dominates F. limosus in staged encounters and shelter. In possible future situations were crayfish plague does no longer eradicate noble crayfish populations, our results show that the native noble crayfish might still have a promising future when confronted with invasive species.

High-Throughput Sequencing of Environmental DNA as a Tool for Monitoring Eukaryotic Communities and Potential Pathogens in a Coastal Upwelling Ecosystem

The marine environment includes diverse microeukaryotic organisms that play important functional roles in the ecosystem. With molecular approaches, eukaryotic taxonomy has been improved, complementing classical analysis. In this study, DNA metabarcoding was performed to describe putative pathogenic eukaryotic microorganisms in sediment and marine water fractions collected in Galicia (NW Spain) from 2016 to 2018. The composition of eukaryotic communities was distinct between sediment and water fractions. Protists were the most diverse group, with the clade TSAR (Stramenopiles, Alveolata, Rhizaria, and Telonemida) as the primary representative organisms in the environment. Harmful algae and invasive species were frequently detected. Potential pathogens, invasive pathogenic organisms as well as the causative agents of harmful phytoplanktonic blooms were identified in this marine ecosystem. Most of the identified pathogens have a crucial impact on the aquacultural sector or affect to relevant species in the marine ecosystem, such as diatoms. Moreover, pathogens with medical and veterinary importance worldwide were also found, as well as pathogens that affect diatoms. The evaluation of the health of a marine ecosystem that directly affects the aquacultural sector with a zoonotic concern was performed with the metabarcoding assay.

Dispersion of adeleid oocysts by vertebrates in Gran Canaria, Spain: report and literature review

Within the family Adeleidae, Adelina spp. belong to a group of arthropod pathogens. These parasites have been reported to have a wide geographic distribution, however, there are no reports of these protists in the Canary Islands, Spain. One of the peculiarities of the life cycle of Adelina spp. is the participation of a predator, because fecundation and sporulation occur inside the body cavity, and so necessitate destruction of the definitive host. The involvement therefore of a ‘dispersion host’, which eats the definitive host and spreads the oocysts through its faeces, is critical for the maintenance of certain Adelina spp. On the island of Gran Canaria, adeleid oocysts have been found in stool samples from four animals, three California kingsnakes (Lampropeltis californiae), and one feral cat. These animals were part of a larger coprological study of vertebrate parasites (117 snakes, 298 cats), where pseudoparasitic elements were also recorded. L. californiae and feral cats are invasive species which are widespread across the island and this novel finding of Adelina spp. oocysts in their faeces suggests that they could also serve as potential sentinel species for arthropod parasites.

Controlled Infection Experiment With Aphanomyces astaci Provides Additional Evidence for Latent Infections and Resistance in Freshwater Crayfish

For 150 years the crayfish plague disease agent Aphanomyces astaci has been the cause of mass mortalities among native European crayfish populations. However, recently several studies have highlighted the great variability of A. astaci virulence and crayfish resistance toward the disease. The main aim of this study was to compare the response of two crayfish species, the European native noble crayfish (Astacus astacus) and the invasive alien marbled crayfish (Procambarus virginalis), to an A. astaci challenge with a highly virulent strain from haplogroup B and a lowly virulent strain from haplogroup A. In a controlled infection experiment we showed a high resistance of marbled crayfish against an A. astaci infection, with zoospores from the highly virulent haplogroup B strain being able to infect the crayfish, but unable to cause signs of disease. Furthermore, we demonstrated a reduced virulence in the A. astaci strain belonging to haplogroup A, as shown by the light symptoms and the lack of mortality in the generally susceptible noble crayfish. Interestingly, in both marbled crayfish and noble crayfish challenged with this strain, we observed a significant decrease of the detected amount of pathogen’s DNA during the experiment, suggesting that this A. astaci haplogroup A strain has a decreased ability of penetrating into the cuticle of the crayfish. Our results provide additional evidence of how drastically strains belonging to A. astaci haplogroup B and haplogroup A differ in their virulence. This study confirmed the adaptation of one specific A. astaci haplogroup A strain to their novel European hosts, supposedly due to reduced virulence. This feature might be the consequence of A. astaci’s reduced ability to penetrate into the crayfish. Finally, we experimentally showed that marbled crayfish are remarkably resistant against the crayfish plague disease and could potentially be latently infected, acting as carriers of highly virulent A. astaci strains.

Money Kills Native Ecosystems: European Crayfish as an Example

Native European crayfish conservation was triggered by invasion of crayfish plague disease agent, Aphanomyces astaci, starting 1860s in Northern Italy. Resulting crayfish plague epidemics quickly spread over Continental Europe, then to Finland, Sweden and finally, after running amok around Europe, A. astaci was discovered also in Iberian Peninsula, Norway, Ireland, and United Kingdom in 1970s and 1980s. By that time significant proportion of native crayfish stocks had been lost, and while crayfish plague epidemics were still recorded, also industrialization and waterways construction were causing damage to remaining native crayfish stocks. While alien crayfish introductions, at least Faxonius limosus, already gave rise to first wave of crayfish plague epidemics in late 19th century, later in 1960s it was decided that introductions of alien Pacifastacus leniusculus should be initiated to replace native European crayfish populations. Decisions were based on presumed advantages for fishery, suitable habitat requirements and supposed immunity against A. astaci. Furthermore, conservation of native European crayfish species was sidelined and focus shifted toward alien crayfish stocking routine and consumption. Alien crayfish species introductions resulted in repeated waves of crayfish plague epidemics among remaining native crayfish stocks. It was soon discovered that alien crayfish of North American origin were, as suspected, permanent reservoirs for A. astaci, that some of those alien species were losing their resistance against selected strains of A. astaci and struggled in European aquatic ecosystems. In this article, we introduce numerous motives behind grand mistake of introducing alien crayfish species to Europe and then promoting their stocks instead of focusing on conservation of native crayfish species. We outline how false economical, biological and ecologic assumptions were used to justify a hasty introduction of alien crayfish, which has further devastated native crayfish and also permanently changed European aquatic ecosystems, both with disastrous consequences. Lesson to be learnt is that science-based warnings about alien species damage to native ecosystems and native crayfish must be taken with utmost caution. Protection of native European crayfish should be core issue, not commercial activities. Finally, we summarize main threats and actions needed to protect remaining native freshwater crayfish fauna in Europe.

A Comprehensive Review on Crustaceans' Immune System With a Focus on Freshwater Crayfish in Relation to Crayfish Plague Disease

Freshwater crayfish immunity has received great attention due to the need for urgent conservation. This concern has increased the understanding of the cellular and humoral defense systems, although the regulatory mechanisms involved in these processes need updating. There are, however, aspects of the immune response that require clarification and integration. The particular issues addressed in this review include an overall description of the oomycete Aphanomyces astaci, the causative agent of the pandemic plague disease, which affects freshwater crayfish, and an overview of crustaceans' immunity with a focus on freshwater crayfish. It includes a classification system of hemocyte sub-types, the molecular factors involved in hematopoiesis and the differential role of the hemocyte subpopulations in cell-mediated responses, including hemocyte infiltration, inflammation, encapsulation and the link with the extracellular trap cell death pathway (ETosis). In addition, other topics discussed include the identity and functions of hyaline cells, the generation of neoplasia, and the emerging topic of the role of sessile hemocytes in peripheral immunity. Finally, attention is paid to the molecular execution of the immune response, from recognition by the pattern recognition receptors (PRRs), the role of the signaling network in propagating and maintaining the immune signals, to the effector elements such as the putative function of the Down syndrome adhesion molecules (Dscam) in innate immune memory.

Value-conflicts in the conservation of a native species: a case study based on the endangered white-clawed crayfish in Europe

The future of the native European crayfish Austropotamobius pallipes depends on accurate conservation management. The goal of this paper is to attempt an investigation of the major ethical conflicts that can emerge in the conservation of this endangered crayfish threatened by invasive competitors, introduced diseases, and landscape alteration. To assess this issue, we will employ the Ethical Matrix, in a version explicitly tailored for its use in conservation. The filled Ethical Matrix will highlight several potential conflicts between values such as environmental protection, social and economic interests, animal welfare, cultural and aesthetic value, etc. We will discuss these conflicts, alongside some potential mitigating strategies present in the literature. We will stress in particular the need to take into account the ethical principle of fairness when assessing the economic and recreational value of invasive species, especially concerning the unfair distribution of costs. Moreover, we will assert the importance of conservation of A. pallipes both for its existence value and for its role as an umbrella and keystone species. Beyond its focus on A. pallipes, the Ethical Matrix here discussed might also provide insights on the value conflicts relative to analogous in situ conservation efforts involving a native species threatened by invasive alien competitors.

Graphic abstract

Microbial pathogens of freshwater crayfish: A critical review and systematization of the existing data with directions for future research

Despite important ecological role and growing commercial value of freshwater crayfish, their diseases are underresearched and many studies examining potential crayfish pathogens do not thoroughly address their epizootiology, pathology or biology. This study reviews over 100 publications on potentially pathogenic viruses, bacteria, fungi and fungal-like microorganisms reported in crayfish and systematizes them based on whether pathogenicity has been observed in an analysed species. Conclusions on pathogenicity were based on successful execution of infectivity trials. For 40.6% of examined studies, microbes were successfully systematized, while for more than a half (59.4%) no conclusion on pathogenicity could be made. Fungi and fungal-like microorganisms were the most studied group of microbes with the highest number of analysed hosts, followed by bacteria and viruses. Our analysis demonstrated the need for: (a) inclusion of higher number of potential host species in the case of viruses, (b) research of bacterial effects in tissues other than haemolymph, and (c) more research into potential fungal and fungal-like pathogens other than Aphanomyces astaci. We highlight the encountered methodological challenges and biases and call for a broad but standardized framework for execution of infectivity trials that would enable systematic data acquisition on interactions between microbes and the host.

Development and application of eDNA-based tools for the conservation of white-clawed crayfish

eDNA-based methods represent non-invasive and cost-effective approaches for species monitoring and their application as a conservation tool has rapidly increased within the last decade. Currently, they are primarily used to determine the presence/absence of invasive, endangered or commercially important species, but they also hold potential to contribute to an improved understanding of the ecological interactions that drive species distributions. However, this next step of eDNA-based applications requires a thorough method development. We developed an eDNA assay for the white-clawed crayfish (Austropotamobius pallipes), a flagship species of conservation in the UK and Western Europe. Multiple subsequent in-situ and ex-situ validation tests aimed at improving method performance allowed us to apply eDNA-based surveys to evaluate interactions between white-clawed crayfish, crayfish plague and invasive signal crayfish. The assay performed well in terms of specificity (no detection of non-target DNA) and sensitivity, which was higher compared to traditional methods (in this case torching). The eDNA-based quantification of species biomass was, however, less reliable. Comparison of eDNA sampling methods (precipitation vs. various filtration approaches) revealed that optimal sampling method differed across environments and might depend on inhibitor concentrations. Finally, we applied our methodology together with established assays for crayfish plague and the invasive signal crayfish, demonstrating their significant interactions in a UK river system. Our analysis highlights the importance of thorough methodological development of eDNA-based assays. Only a critical evaluation of methodological strengths and weaknesses will allow us to capitalise on the full potential of eDNA-based methods and use them as decision support tools in environmental monitoring and conservation practice.

Crayfish plague in Czechia: Outbreaks from novel sources and testing for chronic infections

The crayfish plague pathogen Aphanomyces astaci, which is among the most studied pathogens of aquatic invertebrates, co-evolved with North American crayfish species but threatens crayfish on other continents. The pathogen causes mass mortalities, particularly in Europe. In this study we document 12 crayfish plague outbreaks that occurred from 2014 to 2019 in Czechia and, by using available molecular techniques (microsatellite and mtDNA markers), we reveal the A. astaci genotypes involved. Our results provide the first evidence of strains from genotype group D, originally associated with the host Procambarus clarkii, causing Astacus astacus and Austropotamobius torrentium mass mortalities in Czechia. Moreover, mtDNA sequencing confirmed two distinct haplotypes of the D haplogroup, indicating two independent sources of infection, presumably originating from ornamental crayfish in the pet trade or spreading from crayfish established in neighbouring countries. Genotype group A was recorded in two As. astacus mortalities, and genotype group E, associated with Faxonius limosus, in two Au. torrentium and three As. astacus mortalities. Microsatellite genotyping also reidentified the unusual genotype SSR-Up in two As. astacus outbreaks, ten years after its first documented occurrence. In addition, we tested healthy-appearing indigenous crayfish from 25 localities for potential chronic infections. No traces of A. astaci DNA were detected; chronic infections in European crayfish species thus do not seem a pervasive phenomenon in Czechia. However, their role as A. astaci latent reservoirs, especially in Pontastacus leptodactylus populations introduced to the country since the late 19th century, cannot be excluded.

Ecological plasticity and commercial impact of invasive marbled crayfish populations in Madagascar

Background

The marbled crayfish (Procambarus virginalis) is a monoclonal, parthenogenetically reproducing freshwater crayfish species that has formed multiple stable populations worldwide. Madagascar hosts a particularly large and rapidly expanding colony of marbled crayfish in a unique environment characterized by a very high degree of ecological diversity.

Results

Here we provide a detailed characterization of five marbled crayfish populations in Madagascar and their habitats. Our data show that the animals can tolerate a wide range of ecological parameters, consistent with their invasive potential. While we detected marbled crayfish in sympatry with endemic crayfish species, we found no evidence for the transmission of the crayfish plague pathogen, a potentially devastating oomycete. Furthermore, our results also suggest that marbled crayfish are active predators of the freshwater snails that function as intermediate hosts for human schistosomiasis. Finally, we document fishing, farming and market sales of marbled crayfish in Madagascar.

Conclusions

Our results provide a paradigm for the complex network of factors that promotes the invasive spread of marbled crayfish. The commercial value of the animals is likely to result in further anthropogenic distribution.

Electronic supplementary material

The online version of this article (10.1186/s12898-019-0224-1) contains supplementary material, which is available to authorized users.

Italian acacia honey exhibits lytic effects against the crayfish pathogens Aphanomyces astaci and Fusarium avenaceum

This study purpose was to evaluate the in vitro inhibitory properties of Italian acacia honey extracts against pathogenic aquatic oomycete/fungal isolates that cause different diseases in crayfish, resulting in an elevated mortality rate. The antimycotic activity of acacia honey aqueous extracts was evaluated against the strain UEF88662 of Aphanomyces astaci (oomycete) and the strain SMM2 of Fusarium avenaceum (fungus). The extracts preparation was carried out with water by a cheap, not complex and organic solvent-free procedure, with low environmental impact and the higher possibility of large-scale reproducibility. The anti-oomycete and antifungal activities were quantitatively evaluated by growth, survival and sporulation microbiological assays. The extracts displayed a dose-dependent inhibitory efficacy on oomycete and fungal growth and survival, as well as on the production of oomycete and fungal spores. Supported by future in vivo studies, our results encourage the use of natural extracts like honey as innovative tools to counteract mycotic infections. SIGNIFICANCE AND IMPACT OF THE STUDY: The continuous spread of aquatic fungal disease as the 'crayfish plague' and the 'burn spot disease' has severe ecological and commercial repercussions. Critical factor to prevent further spread is the availability of effective antifungals possibility derived from local natural resources to use in innovative strategies of control and eradication of these diseases. This study provides relevant information about the in vitro anti-oomycete and antifungal activity of Italian acacia honey aqueous extracts against two highly infectious and dangerous pathogenic species, Aphanomyces astaci and Fusarium avenaceum, that are responsible for important crayfish diseases.

Hidden sites in the distribution of the crayfish plague pathogen Aphanomyces astaci in Eastern Europe: Relicts of genetic groups from older outbreaks?

The crayfish plague agent Aphanomyces astaci is one of the world's most threatening invasive species. Originally from North America, the pathogen is being imported alongside American crayfish species, which are used for various purposes. In this study, we investigated the marginal, currently known distribution area of the pathogen in Eastern Europe by sampling narrow-clawed crayfish (Astacus leptodactylus) and spiny-cheek crayfish (Orconectes limosus) populations. In addition, using specific real-time PCR, we tested several marine decapod species, which also occur in brackish waters of the Danube at the West coast of the Black Sea and the Dniester River basin. By sequencing the nuclear chitinase gene, mitochondrial rnnS/rnnL DNA and by genotyping using microsatellite markers, we identified the A. astaci haplogroups of highly infected specimens. The A. astaci DNA was detected in 9% of the investigated A. leptodactylus samples, both in invaded and non-invaded sectors, and in 8% of the studied O. limosus samples. None of the marine decapods tested positive for A. astaci. The results revealed that narrow-clawed crayfish from the Dniester River carried the A. astaci B-haplogroup, while A. astaci from the Danube Delta belonged to the A- and B-haplogroups. In the invaded sector of the Danube, we also identified the A-haplogroup. Microsatellite analysis revealed a genotype identical to the genotype Up. It might be that some of the detected A. astaci haplogroups are relics from older outbreaks in the late 19th century, which may have persisted as a chronic infection for several decades in crayfish populations.