Invasive Crayfish Species Research Articles

Temporal and spatial patterns in the shifting of otter diet to invasive prey after river damming

AbstractRiver damming promotes profound changes in aquatic biodiversity, including the facilitation of biological invasions, with subsequent impacts on native communities. The Eurasian otter (Lutra lutra) is a generalist aquatic predator that often incorporates new prey types when they become available. We studied the short‐ and long‐term effects of river damming on the trophic ecology of the otter across a riverine landscape gradient, from the flooded area to upstream tributaries, focusing on the largest artificial lake of Western Europe. Through the analysis of otter feces (spraints), we describe otter diet composition in 14 sites at different distances from the impounded area in three periods: (i) the pre‐filling phase (i.e., lotic conditions); (ii) the immediate filling phase; and (iii) 16 years after the dam closure. The diet of the otter drastically shifted in the short term, from being dominated by native cyprinids to being mainly composed of invasive species of fish and crayfish. In the long term, the number of invasive species consumed by the otter continued to increase while native fish completely disappeared as prey near the reservoir. Presumably, the construction of the dam generated clear spatial gradients of abundances of native and invasive fish species. These patterns arose in the short term but became accentuated in the long term and were mirrored in otter diet composition. Our results demonstrate the potential of otter diet analyses as a surrogate for the impacts of human alterations on aquatic ecosystems, including those of river damming and the associated facilitation of biological invasions.

Effect of dietary supplementation with fatty acids on growth, survival, and fatty acid patterns in Procambarus clarkii and Procambarus virginalis: the first comparison of two invasive crayfish species

The red swamp crayfish, Procambarus clarkii, and the marbled crayfish, Procambarus virginalis, are invasive species expanding their ranges on a global scale. Their spread is favored by the fact that these crayfish are also aquaculture species of great economic importance (P. clarkii) or promising prospects in this regard (P. virginalis). This study represents the first attempt to assess the impact of diet supplementation with fatty acids on growth, survival, and fatty acid content in these crayfish species, as well as the first record of fatty acid patterns in the obligate parthenogenetic marbled crayfish. Our results showed that supplementation with saturated palmitic acid increased P. clarkii survival, which has not previously been reported for crustaceans. This positive effect on the part of palmitic acid supplementation was not observed for P. virginalis individuals. Marbled crayfish had significantly higher survival rates than red swamp crayfish regardless of the diet used. The growth of both species was not affected by fatty acid supplementation. Our study indicated that young marbled crayfish had a poorer fatty acid profile than juvenile conspecifics and red swamp crayfish. Fatty acid supplementation and type of diet significantly affected the fatty acid composition in the crayfish tissues. There was an increase in unsaturated α-linolenic acid for P. clarkii and palmitic acid for P. virginalis. Palmitic acid quantitatively dominated in both species regardless of the experimental treatment, with the exception of juvenile P. virginalis, which was fed an oligochaete diet rich in EPA.

Integrating population genetics and species distribution modelling to guide conservation of the noble crayfish, Astacus astacus, in Croatia

The noble crayfish, Astacus astacus, is an indigenous European freshwater species. Its populations show significant declines caused by anthropogenic pressure on its habitats, climate change and the spread of invasive species. Diminishing populations’ trends and loss of genetic diversity highlight the need for effective conservation that will ensure their long-term survival. We combined population genetics and species distribution modelling (SDM) to reveal the impact of climate change and invasive species on the noble crayfish, and to guide future conservation programs of current populations. Our study showed that Croatian populations of A. astacus harbour an important part of species genetic diversity and represent significant genetic reservoir at the European level. The SDM results predicted substantial reductions of suitable habitats for A. astacus by the 2070; only 13% of its current potential distribution is projected to remain stable under pessimistic Representative Concentration Pathway (RCP 8.5) emission scenario. Moreover, most of the populations with high genetic diversity are located in the areas predicted to become unsuitable, and consequently have a high probability of being lost in the future. Further, SDM results also indicated considerable decrease of future habitat suitability for invasive crayfish species in Croatia, suggesting that climate change poses a major threat to already endangered A. astacus. The obtained results help in the identification of populations and areas with the highest conservation value which should be given the highest priority for protection. In order to preserve present diversity in areas that are predicted as suitable, we propose assisted migration and repopulation approaches, for enhancing populations’ size and saving maximum genetic variability. The result of our research emphasizes once again the benefits of multidisciplinary approach in the modern biodiversity conservation.

Hybridization between two introduced, invasive crayfish species in the upper Juniata River system, Pennsylvania, USA

Abstract Hybridization of non-native crayfishes with native crayfishes can facilitate invasion success of the non-native. We report novel genetic evidence of hybridization between rusty crayfish, Faxonius rusticus (Girard, 1852), and Allegheny crayfish, F. obscurus (Hagen, 1870), two non-native species in the upper Juniata River basin of central Pennsylvania. We used double-digest restriction-associated DNA sequencing, which yielded 28,031 single nucleotide polymorphisms, to characterize the extent of hybridization. Two of eight individuals with mixed character traits were identified as the progeny of hybrids backcrossed into rusty crayfish. One individual, presumed to be a rusty crayfish in the field, was genetically identified as a hybrid backcross to rusty crayfish. The three crayfish taxa (rusty, Allegheny, and intermediates) are virtually indistinguishable based on typical morphological characters. Key character traits that distinguish rusty and Allegheny crayfishes can be mixed atypically in certain individuals (i.e., intermediates). Our results indicate that introgressive hybridization has occurred between rusty and Allegheny crayfishes, and that at least some of the character-trait intermediate individuals have hybrid ancestry. Hybridization may be an additional means by which invasive rusty crayfish increase their fitness and dominance at the expense of existing species and their habitat.

Physiological performance of native and invasive crayfish species in a changing environment: insights from Dynamic Energy Budget models.

Crayfish are keystone species important for maintaining healthy freshwater ecosystems. Crayfish species native to Europe, such as Astacus astacus and Austropotamobius torrentium, are facing decline and are increasingly endangered by changing climate and invasions of non-native crayfish, such as Pacifastacus leniusculus and Procambarus virginalis. The success of these invasions largely depends on differences in ontogeny between the native species and the invaders and how changes in the environment will affect the ontogeny. Dynamic Energy Budget (DEB) models can be used to investigate such differences because the models capture dependence of metabolism, and therefore ontogeny, on environmental conditions. We develop DEB models for all four species and investigate key elements of ontogeny and metabolism affecting interspecific competition. We then use the DEB models to predict individual growth and reproduction in current and new conditions that are expected to arise from climate change. Although observations suggest that P. leniusculus poses the major threat to native species, our analysis identifies P. virginalis, in spite of its smaller size, as the superior competitor by a large margin—at least when considering metabolism and ontogeny. Our simulations show that climate change is set to increase the competitive edge of P. virginalis even further. Given the prospects of P. virginalis dominance, especially when considering that it is able to withstand and spread at least some crayfish plague strains that severely affect native species, additional research into P. virginalis is necessary.

Insight into the noble crayfish morphological diversity: a geometric morphometric approach

The noble crayfish (Astacus astacus), a keystone species of high ecological, economic, and cultural importance in Europe, is threatened due to a long-term population decline caused by anthropogenic pressure on its habitats, the presence of non-indigenous invasive crayfish species and climate change. Since the effective protection of the remaining populations requires conservation measures based on the comprehensive knowledge of the species, including good understanding of its genetic and morphological variability, our aim was to study morphological features of the noble crayfish in Croatia using geometric morphometrics for the first time. We applied two-dimensional geometric morphometrics to find morphological differences among 15 populations of the noble crayfish from Croatian freshwater habitats, grouped according to previously established (a) mitochondrial (genetic) lineages, (b) genetic clusters inferred from nuclear microsatellites, as well as (c) river basins and (d) habitat types (lotic, lentic). Overall, the results indicated the existence of morphological diversity among the studied populations of the noble crayfish in Croatia. Shape analysis showed differences in cephalon based on crayfish affiliation to different genetic lineages, genetic clusters, river basins and habitat types. Our study provided novel insights into morphological diversity of the endangered noble crayfish in the area of its high genetic diversity.

Parasite DNA detection in water samples enhances crayfish plague monitoring in asymptomatic invasive populations

Invasive species can facilitate the spread of pathogens by first providing asymptomatic host reservoirs, and then driving disease outbreaks in native populations through pathogen spillover. An example of this are invasive crayfish species in Europe (Faxonius limosus, Pacifastacus leniusculus, Procambarus clarkii), which carry the deadly plague agent (Aphanomyces astaci). Effective disease management requires comprehensive monitoring, however, pathogen detection in carrier populations with low pathogen prevalence and intensities is challenging. We simultaneously collected and analysed crayfish tissue samples of invasive crayfish populations and water samples to compare A. astaci detection in different sample types using quantitative PCR. Combined, the two sampling methods revealed A. astaci presence with DNA concentrations above limit of detection (LOD; the lowest concentration which can be detected with reasonable certainty) in 13 of 23 invasive crayfish populations. In four additional sites, A. astaci DNA concentrations below LOD were found in water. In four populations only were A. astaci concentrations above LOD detected in both sample types and in three populations in concentrations above LOD in tissue but below LOD in water. The likely reason for these discrepancies is the low A. astaci prevalence and concentration in resistant invasive crayfish, which limit detection reliability. Consistency may be improved by timing surveys with seasonal periods of high A. astaci abundance and by increasing water sampling effort. Considering the ease of collecting eDNA samples, compared to crayfish tissue sampling, eDNA methods would facilitate frequent and comprehensive surveys. However, remaining uncertainties in eDNA-based detection reveal the relevance of combining monitoring tools to improve detection of invasive pathogens and their management.

Genetic diversity of redclaw crayfish Cherax quadricarinatus von Martens 1868 using 16S mitochondrial DNA marker

There have been reported around 11 aquatic invasive alien species (IAS) found in Malaysian waters. Cherax quadricarinatus, or commonly known as Australian redclaw crayfish, is one of the invasive species. C. quadricarinatus has the potential of causing negative impacts ecologically and economically in the local environment as it has reported in several countries if they get established and not monitored properly. Habitat alteration, native species depletion and spreading of diseases are among the reported negative impacts of C. quadricarinatus. This study was conducted with the aim to assess the genetic diversity of C. quadricarinatus from 4 different populations. Uncovering the diversity and population structure of the redclaw crayfish will help in enhancing the understanding of adaptation and survival of C. quadricarinatus. Thus, the information can be used in monitoring and management of this invasive crayfish in future. DNA of C. quadricarinatus was successfully extracted from its tissue and amplified via polymerase chain reaction (PCR) using mitochondrial DNA (mtDNA) 16S gene then proceeded for sequencing and analysed using several genetic analysis software to understand the diversity, phylogeny and population structure of this invasive crayfish species. A total of 493 bp fragments of 32 samples from four sampling sites were obtained. Four haplotypes were observed which Hap-1 was the most common haplotype. The highest genetic variation is Selangor (Pi = 0.00248, Hd = 0.694). However, low levels of both haplotype and nucleotide diversity indicates the loss of genetic diversity. Analysis of molecular variance (AMOVA) results revealed that the percentage of genetic variation within the population was 69.58% while among populations was 30.42%, indicating significant genetic differentiation among population (P < 0.05). The maximum likelihood tree showed that all haplotypes were clustered and grouped together with United States, Czech Republic, China and Australia.

Using eDNA to simultaneously detect the distribution of native and invasive crayfish within an entire country

The introduction of invasive crayfish has led to a decline of many European native species of crayfish across their range. In this study, novel duplex assays for all crayfish occurring in Switzerland were developed. We aimed to identify the distribution of the seven species using a traditional trap surveillance method as well by collecting water samples to detect eDNA by species-specific quantitative real-time PCR. We reveal our overall experience in finding optimal field and laboratory techniques to discover the distribution and abundance of native and invasive species in order to enhance knowledge of early invasive species invasion and highlight important pockets of populations where native species remain, for implementation of conservation strategies. Using eDNA, important populations of native noble and white-clawed crayfish were revealed in multiple waters across various cantons. The successful identification of native and invasive crayfish species in Switzerland using eDNA can be applied to future nationwide projects. This method which has the ability to detect all species simultaneously across an entire country, will allow an improvement in freshwater crayfish conservation management.

Inference of local invasion pathways in two invasive crayfish species displaying contrasting genetic patterns

Abstract The efficient management of invasive alien species (IAS) requires the identification of their introduction pathways. Genetic assessments have proven useful to inform invasion pathways at large (national to worldwide) scales, but studies at local scales are still rare, despite their importance for guiding management. In this study, genetic analyses were used to identify local invasion pathways of two invasive crayfish species (the spiny‐cheek crayfish Faxonius limosus and the red swamp crayfish Procambarus clarkii) in a dense network of artificial lakes. We first characterized the spatial patterns of genetic variability, effective population sizes (Ne) and among‐lakes recent migration events for each species using neutral microsatellite markers. We then identified the environmental factors affecting genetic variability and inferred the potential local invasion pathways. Results revealed different patterns of genetic variability between the two species: F. limosus displayed very low levels of genetic diversity, Ne and spatial structuring compared to P. clarkii, which displayed high genetic diversity, Ne and spatial genetic structuring. We also demonstrated context‐dependent effects of different environmental factors (fishery management, spatial distribution and lake size) on genetic variability indices. We did not identify local invasion pathways for F. limosus due to limited genetic variability, likely caused by a strong founder effect and potential parthenogenetic reproduction. Contrastingly, multiple invasion pathways (release, contaminant, unaided/corridor spread and stowaway) were identified for P. clarkii. Synthesis and applications. Although limited in some particular cases (e.g. for species having experienced strong shaping events and/or displaying asexual reproductive modes), neutral genetic variation assessments can provide important insights for inferring local invasion pathways in complex landscapes for invasive alien species displaying short generation times and complex invasion histories.

Can fish lifts aid upstream dispersal of the invasive red swamp crayfish (Procambarus clarkii) past high‐head hydropower plants?

AbstractMany types of fish passage facilities have been built worldwide to mitigate river fragmentation by instream barriers and partially restore upstream movements of fish. Among these, fish lifts are a design that enables fish to pass through high‐head obstacles (&gt;15 m height) and can be considered the most cost‐effective fishway. However, fishways may also be used by non‐native invasive species to disperse and impact new areas and organisms upstream. As far as the authors are aware, this short communication is the first record of the red swamp crayfish (Procambarus clarkii) passing through a fish lift. The lift, built at the Touvedo hydropower plant on the Lima River (northwestern Portugal), was mainly designed for the upstream passage of diadromous fish. Fish‐lift use and upstream passage of two crayfish occurred over 3 years (November 2017–October 2020) during periods of turbine shutdown and high water temperatures (18°C). In the present case, the crayfish passage should not pose a serious threat due to the limiting characteristics of the upstream lotic environment. Nevertheless, despite the small sample size, this study shows the importance of considering the passage of invasive crayfish species while planning future fishways and the need to continuously monitor such facilities to detect the passage of this and other non‐native invasive species.

Differences in the functional responses of four invasive and one native crayfish species suggest invader‐specific ecological impacts

AbstractInvasive non‐native species represent a leading threat to global freshwater biodiversity and non‐native crayfish species frequently cause extensive ecological damage. However, the extent to which their impact: (1) depends on invader identity and (2) differs from the natural state with native crayfish remains unclear. Comparison of the functional responses of invasive and native species represents a promising approach in this regard.Here, we explored whether four invasive crayfish species (calico crayfishFaxonius immunis, spiny‐cheek crayfishFaxonius limosus, signal crayfishPacifastacus leniusculus, and marbled crayfishProcambarus virginalis) in European freshwaters and the most widespread native species (noble crayfishAstacus astacus) overlap in function in their potential effects on key resources of benthic food webs. First, the impact on gammarids and zebra mussels was assessed by means of comparative functional response analysis using the functional response ratio as impact metric; second, the consumption of macrophytes (Charasp.) and detritus (leaf litter) was quantified and compared using feeding experiments.Both invader‐ and resource‐specific effects were observed. Invasive calico crayfish and signal crayfish exhibited the strongest per capita effects on gammarids and zebra mussels, respectively, with functional response ratios being 2‐fold higher than those of native noble crayfish. Marbled crayfish showed an intermediate effect on both prey species, whereas spiny‐cheek crayfish had lower impacts than noble crayfish. In the feeding experiment, calico crayfish consumed the highest amount of detritus, while the consumption of macrophytes did not differ among the five crayfish species.Our work demonstrates as‐yet unrecognised differences in functional responses among the four North American crayfish invaders and the European noble crayfish. The lack of congruence across the observed impacts suggests a mostly species‐specific pattern and stresses the importance of species and resource identity when considering the ecological impact of crayfish. An initial assessment of invader‐specific potential impacts positions calico crayfish and signal crayfish among the most impactful invaders.

Patterns and determinants of phenotypic variability within two invasive crayfish species

Abstract Knowledge on the ecological determinants and evolutionary processes shaping intraspecific variability in the wild remains scarce. It is particularly needed in the context of biological invasions to fully understand the consequences of invasive species on the functioning of recipient ecosystems. Using geometric morphometrics, stable isotopes, and elemental composition analyses, we quantified phenotypic variability (morphological, trophic, and stoichiometric traits) within and among invasive populations of two crayfish species with distinct invasion histories over 23 gravel pit lakes in southwest France. We sampled 12 populations of the red swamp crayfish (Procambarus clarkii Girard, 1852) and 11 populations of the spiny‐cheek crayfish (Faxonius limosus Rafinesque, 1817). We aimed at unravelling the ecological determinants and the mechanisms (neutral or adaptive) underlying the phenotypic variability among invasive populations. We demonstrate that, for each group of traits, P. clarkii and F. limosus display contrasting patterns of variance distribution across three ecological scales (population, sex, individual). Then, we demonstrate that P. clarkii trait variation in body morphology and stoichiometry is associated with both ecological and historical determinants (i.e. predation pressure, intraspecific invasion, and invasion age), and morphological traits in F. limosus vary with ecological factors only (i.e. predation pressure and abundance of P. clarkii). Finally, we highlight that different combinations of neutral and adaptive processes shaped the phenotypic variability in the two species, with a higher contribution of adaptive processes in F. limosus. Overall, these results indicate that F. limosus has already gone through local adaptation in the meta‐population while this has not yet occurred for P. clarkii, which was introduced later. This highlights that these two invasive species might have contrasting effects across ecological scales. Our study emphasises that studying invasive species can provide great knowledge on intraspecific variability and its ecological determinants and evolutionary processes in the wild. Our results also stress the need to focus on intraspecific variability in the context of biological invasions as it can be substantial across wide geographic areas.

Genetic Haplotype Mapping of the Allegheny Crayfish (Faxonius obscurus)

Non-indigenous species are a significant threat to freshwater biodiversity. In the Potomac Basin there are several known invasive crayfish species including F. virilis (virile crayfish) and F.rusticus (rusty crayfish). It is imperative to discover whether F. obscurus is in fact an invasive crayfish that needs a management plan or simply another native species that needs protection. F. obscurus is a wide-ranging crayfish species that is native to the northeastern United States. Populations found in the Atlantic Slope Drainage basins are considered invasive. Using CO1 and 16S primers, we will be creating a range-wide haplotype map to investigate these invasions.

EDNA Detection of Native and Invasive Crayfish Species Allows for Year-Round Monitoring and Large-Scale Screening of Lotic Systems

Effective management of both endangered native and invasive alien crayfishes requires knowledge about distribution, monitoring of existing and early detection of newly established populations. Complementary to traditional survey methods, eDNA sampling has recently emerged as a highly sensitive non-invasive detection method to monitor crayfish populations. To advance the use of eDNA as detection tool for crayfish we used a twofold approach: 1) we designed a novel set of specific eDNA-assays for all native (Austropotamobius torrentium, Austropotamobius pallipes, Astacus astacus) and the most relevant invasive crayfish species (Pacifastacus leniusculus, Faxonius limosus, Faxonius immunis) in Central Europe. To ensure specificity each primer pair was tested in silico, in vitro, and in situ; 2) we assessed the influence of spatio-temporal variables (distance to upstream population, season, stream size) on eDNA detection in seven streams using two different detection methods (qualitative endpoint PCR and quantitative droplet digital PCR, ddPCR). The newly developed eDNA assays successfully detected all crayfish species across different lotic and lentic habitats. eDNA detection rate (endpoint PCR) and eDNA-concentration (ddPCR) were significantly influenced by distance and season. eDNA detection was successful up to 7 km downstream of the source population and across all seasons, although detectability was lowest in winter. eDNA detection rate further decreased with increasing stream size. Finally, eDNA-concentration correlated positively with estimated upstream population size. Overall, we provide near operational eDNA assays for six crayfish species, enabling year-round detection, which represents a clear benefit over conventional methods. Due to its high sensitivity, eDNA detection is also suitable for the targeted search of as-yet unrecorded or newly emerging populations. Using quantitative ddPCR might further allow for a rough estimation of population size, provided that the identified spatio-temporal factors are accounted for. We therefore recommend implementing eDNA-detection as a complementary survey tool, particularly for a large-scale screening of data-deficient catchments or a year-round monitoring.

Invasive crayfish outperform Potamonautid crabs at higher temperatures

Abstract Data on ecological impacts of freshwater crayfish invasions in Africa are scarce but invasion history suggests the likelihood of negative implications for biodiversity. To evaluate the potential for ecological impacts we describe the consumer‐resource dynamics of two established crayfish species (Cherax quadricarinatus and Procambarus clarkii) in comparison with the native trophic analogue, the freshwater crab Potamonautes perlatus preying upon Clarias gariepinus fry using comparative functional responses (FRs) and FR ratio (FRR). Experiments were conducted under dark and light conditions as well as low (19°C) and high (28°C) temperature treatments. All three species exhibited potentially population destabilising Type II (hyperbolic) FRs towards prey, which was significantly higher in the dark than in the light. At low temperatures, P. perlatus exhibited the highest maximum feeding estimate although the FR curve was not significantly different from C. quadricarinatus at the highest prey densities. Both crayfish species had higher attack rates at both temperatures and consumed significantly more prey at the high temperature than P. perlatus. The FRR of both crayfish species at the high temperature treatment was higher than that of P. perlatus due to high attack rate and low handling parameters. At 19°C, P. perlatus had a higher or negligible relative FR magnitude compared to P. clarkii and C. quadricarinatus respectively, which suggests some degree of biotic resistance at this temperature; however, this resistance is decreased at 28°C. Cherax quadricarinatus consistently had disruptive FR parameters across both temperatures. Findings from this study represent an important step towards understanding the impacts of crayfish in Africa for public authorities and environmental managers. The two invasive crayfish species have the potential to exert greater per capita impacts on benthic prey communities in invaded systems as shown by their high FRs and FRRs. Furthermore, C. quadricarinatus is an emerging invader globally and our results provide evidence of potential for negative ecological impact regardless of thermal conditions. African countries and their respective environmental managers should therefore maximise efforts to prevent the introduction and spread of these invaders to conserve native biodiversity.

Year-round monitoring and large-scale screening of native and invasive crayfishes in lotic systems

Freshwater crayfish are among the most threatened animal taxa in Central Europe. Effective conservation measures for endangered native and management of invasive alien crayfishes requires knowledge about distribution, monitoring of existing and early detection of newly established populations. eDNA has recently emerged as a promsing, highly sensitive, and non-invasive detection tool in this regard. To evaluate eDNA as detection tool for freshwater crayfish, we developed a novel set of specific eDNA-assays for all native (Austropotamobius torrentium, Austropotamobius pallipes, Astacus astacus) and the most relevant invasive crayfish species (Pacifastacus leniusculus, Faxonius limosus, Faxonius immunis) in Central Europe. To ensure specificity each primer pair was tested in silico, in vitro, and in situ, including a total of 13 lotic and lentic waterbodies (Fig. 1). Moreover, we assessed the influence of spatio-temporal variables (distance to upstream population, season, and stream size) on eDNA detection in seven streams using two different detection methods (qualitative endpoint PCR and quantitative droplet digital PCR, ddPCR). The newly developed eDNA assays successfully detected all crayfish species across different lotic and lentic habitats. Multiple linear mixed-effects analysis revealed a significant effect of distance and season on eDNA detection rate (endpoint PCR) and eDNA-concentration (ddPCR). Year-round detection was successful up to 7 km downstream of the source population, although detectability was lowest in winter. eDNA detection rate further decreased with increasing stream size. Finally, eDNA-concentration correlated positively with estimated upstream population size. Overall, our study provides easily applicable eDNA assays for six crayfish species, enabling year-round detection, which represents a clear benefit over conventional methods. Due to its high sensitivity, eDNA detection is also suitable for the targeted search of as-yet unrecorded or newly emerging populations. Using quantitative ddPCR might further allow for a rough estimation of population size, provided that the identified spatio-temporal factors are accounted for. We therefore recommend implementing eDNA-detection as a complementary survey tool, particularly for a large-scale screening of data-deficient catchments or a year-round monitoring (Chucholl et al. 2021).

Force and Boldness: Cumulative Assets of a Successful Crayfish Invader

Multiple causes can determine the disturbance of natural equilibrium in a population of a species, with a common one being the presence of invasive competitors. Invasives can drive native species to the resettlement of the trophic position, changing reproduction strategies or even daily normal behaviours. Here, we investigated the hypothesis that more effective anatomical features of an intruder (Faxonius limosus) come with increased boldness behaviour, contributing to their invasion success in competition against the native species (Pontastacus leptodactylus). We tested the boldness of specimens representing the two species by video-based assessment of crayfish individuals’ attempts to leave their settlement microenvironment. The experiment was followed by a series of measurements concerning chelae biometry, force and muscle energetics. The native species was less expressive in terms of boldness even if it had larger chelae and better muscular tissue performance. In contrast, because of better biomechanical construction of the chelae, the invasive species was capable of twice superior force achievements, which expectedly explained its bolder behaviour. These findings suggest that, in interspecific agonistic interactions, the behaviour strategy of the invasive crayfish species is based on sheer physical superiority, whereas the native crayfish relies on intimidation display.

Environmental nitrate impacts foraging and agonistic behaviours of invasive non-native crayfish (Pacifastacus leniusculus and Faxonius virilis)

Invasive species often co-occur in nature and positive or negative interactions among them can affect community structure and ecosystem functioning. Outcomes depend on the intrinsic traits of the species involved and on how species’ performance is affected by the environmental conditions of the recipient ecosystem. Here, we investigated the impact of nitrate, a nutrient common in urban and agricultural sewage discharges, on two invasive crayfish species (Pacifastacus leniusculus and Faxonius virilis) co-occurring in a Nitrate Vulnerable Zone in the UK. Crayfish were exposed to two ecologically relevant nitrate concentrations (≤ 10 and 50 mg NO3−/l) and seven traits relating to foraging success and interspecific interactions were assessed. During monospecific experimental trials, elevated nitrate had a similar impact on the foraging efficacy of P. leniusculus and F. virilis, reducing their ability to sense and find food. However, during dyadic interspecific competition experiments, higher nitrate caused a greater reduction in the number of aggressive behaviours of F. virilis towards P. leniusculus. As such, changes in environmental nitrate regime may affect interactions between these invasive species in regions of sympatry with consequences for their respective invasion dynamics and the wider ecosystem.

Review of Dam Effects on Native and Invasive Crayfishes Illustrates Complex Choices for Conservation Planning

Dams are among the most prevalent and extreme alterations humans have perpetrated on fluvial systems. The dramatic physical and biological changes caused by dams have been synthesized for many aquatic faunal groups, but not for crayfishes. In addition, invasive crayfish species are an increasing threat to global biodiversity, and dams have both costs and benefits with respect to crayfish invasions. North American crayfishes have imperiled native crayfishes in Europe, largely by hosting and spreading the crayfish plague pathogen Aphanomyces astaci that is lethal to European crayfishes. The differential effects of A. astaci on North American vs. European crayfishes contribute to differences between the continents in the costs and benefits of dams. We reviewed literature on both the detrimental and beneficial effects of dams on crayfishes, with emphasis on conservation of European crayfishes. We also suggested additional potential dam effects that warrant investigation. Our review illustrates the challenges and opportunities dams create for crayfish conservation. Dams create detrimental effects to native crayfishes, including reducing suitable habitats necessary for native habitat-specialist species and creating habitats suitable for non-native habitat-generalist species; fragmenting crayfish populations; and reducing species' ability to recolonize upstream habitats. Conversely, dams can have beneficial effects by creating barriers that slow or halt upstream invasions by non-native crayfishes and spread of the crayfish plague. The complexity of the issues and the limited ecological information available highlights the need for future studies on the effects of dams on crayfishes. Crayfishes are one of the most imperiled groups of aquatic fauna globally; therefore, understanding the beneficial and detrimental effects of dams is essential for effective conservation of many crayfish species.