Pacifastacus Leniusculus Research Articles

Getting rid of rain and stars: mitigating inhibition effects in ddPCR assays, the case of the invasive crayfish Pacifastacus leniusculus in the streams of Luxembourg

ddPCR is getting more and more popular in the field of eDNA-based aquatic monitoring. Even if emulsion PCR used in ddPCR confers a partial resistance to inhibition due to the high number of reactions for the same sample (between 10K and 20K), it is not impervious to it. Inhibition impacts the fluorescence amplitude of positive droplets, affecting both their dispersion and their position relatively to the negative droplets cloud. This fluctuation could jeopardize the use of a shared threshold among several samples and thus the objective assignation of the positive droplets. This is even more critical for low concentration samples such as eDNA samples: the positive droplets are scarce and it is thus crucial to objectively discriminate if they can be counted as positive by establishing an appropriate threshold. Another issue is the artifactual generation of high fluorescence droplets that could be counted as positive with a single threshold solution. Here we propose a double threshold method to take both high fluorescence droplets and PCR inhibition impact into account allowing for an objective sorting of the positive and negative droplets in ddPCR assays.

Modulation of endogenous antioxidants by zinc and copper in signal crayfish (Pacifastacus leniusculus)

Metal pollution is a long-standing concern and bioindicators are commonly used in ecotoxicological studies to monitor impacted wildlife populations for evidence of sublethal effects. Significant variation in the response of common biomarkers to metals is reported across taxa, thus necessitating careful characterization in model organisms. In this study, we describe the regulation of glutathione S-transferase (GST), glutathione (GSH), and metallothionein (MT) by zinc chloride (0.6, 0.9, 1.2, 2.4, 4.8, 9.6 μg g−1) and copper chloride (0.6, 0.9, 1.2 μg g−1) in signal crayfish (Pacifastacus leniusculus). Zinc chloride did not alter GST activity relative to controls in the hepatopancreas. Crayfish exposed to copper chloride exhibited decreased GST activity at the lowest dose tested (0.6 μg g−1) with no change observed at the higher doses. GSH did not change in response to either metal when sexes were grouped together. MT concentrations increased in response to zinc (2.4, 4.6, and 9.6 μg g−1 doses) and copper (0.6, 0.9, and 1.2 μg g−1 doses) in gill tissue. In tail tissue, MT increased at the 2.4 and 4.8 μg g−1 zinc chloride doses and all the concentrations of copper tested. Sex-specific differences in endogenous antioxidant expression were also analyzed with no clear patterns emerging. We concluded that these endpoints are sensitive to zinc and copper in signal crayfish, although careful interpretation is needed when applying them in field studies given the variation in responses, non-monotonic dose responses, and differences in biotic and abiotic factors that inevitably exist in different aquatic ecosystems.

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.

Signal crayfish burrowing, bank retreat and sediment supply to rivers: A biophysical sediment budget

AbstractBurrowing into riverbanks by animals transfers sediment directly into river channels and has been hypothesised to accelerate bank erosion and promote mass failure. A field monitoring study on two UK rivers invaded by signal crayfish (Pacifastacus leniusculus) assessed the impact of burrowing on bank erosion processes. Erosion pins were installed in 17 riverbanks across a gradient of crayfish burrow densities and monitored for 22 months. Bank retreat increased significantly with crayfish burrow density. At the bank scale (<6 m river length), high crayfish burrow densities were associated with accelerated bank retreat of up to 253% and more than a doubling of the area of bank collapse compared with banks without burrows. Direct sediment supply by burrowing activity contributed 0.2% and 0.6% of total sediment at the reach (1.1 km) and local bank (<6 m) scales. However, accelerated bank retreat caused by burrows contributed 12.2% and 29.8% of the total sediment supply at the reach and bank scales. Together, burrowing and the associated acceleration of retreat and collapse supplied an additional 25.4 t km−1 a−1 of floodplain sediments at one site, demonstrating the substantial impact that signal crayfish can have on fine sediment supply. For the first time, an empirical relation linking animal burrow characteristics to riverbank retreat is presented. The study adds to a small number of sediment budget studies that compare sediment fluxes driven by biotic and abiotic energy but is unique in isolating and measuring the substantial interactive effect of the acceleration of abiotic bank erosion facilitated by biotic activity. Biotic energy expended through burrowing represents an energy surcharge to the river system that can augment sediment erosion by geophysical mechanisms.

Human health risk from consumption of aquatic species in arsenic-contaminated shallow urban lakes

Arsenic (As) causes cancer and non-cancer health effects in humans. Previous research revealed As concentrations over 200 μg g−1 in lake sediments in the south-central Puget Sound region affected by the former ASARCO copper smelter in Ruston, WA, and significant bioaccumulation of As in plankton in shallow lakes. Enhanced uptake occurs during summertime stratification and near-bottom anoxia when As is mobilized from sediments. Periodic mixing events in shallow lakes allow dissolved As to mix into oxygenated waters and littoral zones where biota reside. We quantify As concentrations and associated health risks in human-consumed tissues of sunfish [pumpkinseed (Lepomis gibbosus) and bluegill (Lepomis macrochirus)], crayfish [signal (Pacifastacus leniusculus) and red swamp (Procambarus clarkii)], and snails [Chinese mystery (Bellamya chinensis)] from lakes representing a gradient of As contamination and differing mixing regimes. In three shallow lakes with a range of arsenic in profundal sediments (20 to 206 μg As g−1), mean arsenic concentrations ranged from 2.9 to 46.4 μg g−1 in snails, 2.6 to 13.9 μg g−1 in crayfish, and 0.07 to 0.61 μg g−1 in sunfish. Comparatively, organisms in the deep, contaminated lake (208 μg g−1 in profundal sediments) averaged 11.8 μg g−1 in snails and 0.06 μg g−1 in sunfish. Using inorganic As concentrations, we calculated that consuming aquatic species from the most As-contaminated shallow lake resulted in 4–10 times greater health risks compared to the deep lake with the same arsenic concentrations in profundal sediments. We show that dynamics in shallow, polymictic lakes can result in greater As bioavailability compared to deeper, seasonally stratified lakes. Arsenic in oxygenated waters and littoral sediments was more indicative of exposure to aquatic species than profundal sediments, and therefore we recommend that sampling methods focus on these shallow zones to better indicate the potential for uptake into organisms and human health risk.

Three crayfish species of different origin in a medium-sized river system: a new state of affairs

In view of contemporary changes in aquatic environments, determining the distribution of both native and emerging invasive crayfish species is increasingly important. In central Europe, the three invasive crayfish species of the signal crayfish Pacifastacus leniusculus (Dana, 1852), the spiny-cheek crayfish Faxonius limosus (Rafinesque, 1817) and the red swamp crayfish Procambarus clarki (Girard, 1852) are of North American origin. The spiny-cheek crayfish was first brought to the southern Baltic basin at the end of nineteenth century, and its expansion ensued rapidly. At the same time, the indigenous species of the noble crayfish Astacus astacus (Linnaeus, 1758) began to disappear. The spread of the signal crayfish started in the second half of twentieth century; however, it has progressed strongly in recent years. Latest studies of fish fauna in the Drwęca River system, a tributary of the lower Vistula River, have simultaneously revealed new information on the occurrence of crayfish. The most widespread was spiny-cheek crayfish found at ten sites throughout the river basin. The second alien species, the signal crayfish, was noted in four locations in the upper part of the river system, but no mixed populations were noted. A particularly valuable result of the study was the discovery of an unknown site of noble crayfish in a small stream.

Risk of invasiveness of non-native aquatic species in the eastern Mediterranean region under current and projected climate conditions

Human-induced biological introductions pose a major threat to global biodiversity, and this is especially frequent in the eastern Mediterranean region, which is a globally important biodiversity hotspot area of high conservation value. To predict at which level introduced species in this region might become invasive under current and projected climate conditions, 232 non-native aquatic organisms were screened using the Aquatic Species Invasiveness Screening Kit. Based on receiver operative characteristic curve analysis, thresholds were identified to distinguish between low, medium and high risk species. The “top invasive” (very high risk) species identified were: brown bullhead Ameiurus nebulosus, blue crab Callinectes sapidus, gibel carp Carassius gibelio, Philippine catfish Clarias batrachus, Chinese mitten crab Eriocheir sinensis, bluespotted cornetfish Fistularia commersonii, silver carp Hypophthalmichthys molitrix, silver-cheeked toadfish Lagocephalus sceleratus, half-smooth golden pufferfish Lagocephalus spadiceus, Suez pufferfish Lagocephalus suezensis, signal crayfish Pacifastacus leniusculus, fathead minnow Pimephales promelas, channeled applesnail Pomacea canaliculata, red swamp crayfish Procambarus clarkii, devil firefish Pterois miles and European catfish Silurus glanis. The risk of being invasive of more than half of the screened species increased after taking global warming predictions into account, and several species considered to be globally invasive (cf. high risk) were classified as posing only a medium risk for the eastern Mediterranean region. Region-specific risk screenings, as implemented in this study, are therefore essential for setting priorities in preventative management for the conservation of key biodiversity hotspots and the optimal allocation of resources in view of full risk assessment for the species identified as (very) high risk.

The effect of LED lights on trap catches in signal crayfish fisheries

The effect of LED lights on trap catches in signal crayfish fisheries

Vibrio areninigrae as a pathogenic bacterium in a crustacean

The occurrence of infectious diseases poses a significant threat to the aquaculture industry worldwide. Therefore, characterization of potentially harmful pathogens is one of the most important strategies to control disease outbreaks. In the present study, we investigated for the first time the pathogenicity of two Vibrio species, Vibrio metschnikovii, a foodborne pathogen that causes fatalities in humans, and Vibrio areninigrae, a bacteria isolated from black sand in Korea, using a crustacean model, the signal crayfish Pacifastacus leniusculus. Mortality challenges indicated that injection of V. metschnikovii (108 CFU/crayfish) has a mortality percentage of 22% in crayfish. In contrast, injection of P. leniusculus with 108 or 107 CFU of V. areninigrae resulted in 100% mortality within one and two days post-injection, respectively. V. areninigrae was successfully re-isolated from hepatopancreas of infected crayfish and caused 100% mortality when reinjected into new healthy crayfish. As a consequence of this infection, histopathological analysis revealed nodule formation in crayfish hepatopancreas, heart, and gills, as well as sloughed cells inside hepatopancreatic tubules and atrophy. Moreover, extracellular crude products (ECP’s) were obtained from V. areninigrae in order to investigate putative virulence factors. In vivo challenges with ECP’s caused >90% mortalities within the first 24 h. In vitro challenges with ECP’s of hemocytes induced cytotoxicity of hemocytes within the first hour of exposure. These findings represent the first report that V. areninigrae is a highly pathogenic bacterium that can cause disease in crustaceans. On the contrary, V. metschnikovii could not represent a threat for freshwater crayfish.

Prolonged phenanthrene exposure reduces cardiac function but fails to mount a significant oxidative stress response in the signal crayfish (Pacifastacus leniusculus)

Crustaceans are important ecosystem bio-indicators but their response to pollutants such as polyaromatic hydrocarbons (PAHs) remains understudied, particularly in freshwater habitats. Here we investigated the effect of phenanthrene (at 0.5, 1.0 and 1.5 mg L−1), a 3-ringed PAH associated with petroleum-based aquatic pollution on survival, in vivo and in situ cardiac performance, the oxidative stress response and the tissue burden in the signal crayfish (Pacifastacus leniusculus). Non-invasive sensors were used to monitor heart rate during exposure. Phenanthrene reduced maximum attainable heart rate in the latter half (days 8–15) of the exposure period but had no impact on routine heart rate. At the end of the 15-day exposure period, the electrical activity of the semi-isolated in situ crayfish heart was assessed and significant prolongation of the QT interval of the electrocardiogram was observed. Enzyme pathways associated with oxidative stress (superoxide dismutase and total oxyradical scavenging capacity) were also assessed after 15 days of phenanthrene exposure in gill, hepatopancreas and skeletal muscle; the results suggest limited induction of protective antioxidant pathways. Lastly, we report that 15 days exposure caused a dose-dependent increase in phenanthrene in hepatopancreas and heart tissues which was associated with reduced survivability. To our knowledge, this study is the first to provide such a thorough understanding of the impact of phenanthrene on a crustacean.

The role of abiotic and biotic factors in interspecific competition of Polish crayfish – comprehensive literature review

Abstract Invasive species are those that have been transferred by humans out of their natural range. Native crayfish species in Polish waters include: Astacus astacus and Pontastacus leptodactylus, whereas invasive species are: Pacifastacus leniusculus, Faxonius limosus, Procambarus clarkii and Procambarus virginalis. The objective of this study was to determine how abiotic and biotic environmental factors contribute to interspecific competition of Polish crayfish based on the available literature. Abiotic factors affecting the interspecific competition include tolerance to extreme pH values, calcium ion content, temperature, oxygenation, water salinity, preferred substrate and the type of water bodies. Biotic factors are, inter alia, pathogens, food base, plant cover and interactions in the prey–predator system, as well as interactions between crayfish species. The most important abiotic factors are water temperature and oxygenation, while the most important biotic factor is the crayfish plague – a deadly disease for native species. Each invasive species has a different set of traits and adaptations that enable a successful invasion. However, a successful invasion of a given species is not determined by one, but many adaptations that coexist.

The effect of chronic exposure to chloridazon and its degradation product chloridazon-desphenyl on signal crayfish Pacifastacus leniusculus

The effects of chloridazon (Ch) and its metabolite chloridazon-desphenyl (Ch-D) at the environmentally relevant concentrations of 0.45 µg/L and 2.7 µg/L on signal crayfish Pacifastacus leniusculus were assessed in a 30-day exposure followed by a 15-day depuration period. Locomotion, biochemical haemolymph profile, oxidative and antioxidant parameters, and histopathology were evaluated. Crayfish exposed to Ch at 0.45 µg/L and 2.7 µg/L showed significantly (p < 0.01) higher CAT activity and GSH level in hepatopancreas and gill compared to controls. The concentration of Ch at 2.7 µg/L was associated with significantly (p < 0.01) higher levels of GLU, LACT, ALT, AST in haemolymph compared to controls. Chloridazon-desphenyl exposure at both tested concentrations caused significantly higher (p < 0.01) GLU, LACT, ALT, AST, NH3, and Ca in haemolymph; lipid peroxidation (TBARS) levels in hepatopancreas; and CAT activity and GSH level in hepatopancreas and gill. Alterations of structure including focal dilatation of tubules, increased number of fibrillar cells, and haemocyte infiltration in the interstitium were observed with 2.7 µg/L Ch and with both Ch-D exposures. Locomotion patterns did not vary significantly among groups. A 15-day recovery period was insufficient to restore normal physiological parameters in exposed groups. Chloridazon and its metabolite Ch-D exerts harmful effects on crayfish.

Selective fish passage: Restoring habitat connectivity without facilitating the spread of a non-native species

River managers are challenged to address two key threats to freshwater biodiversity. The first is the effects of habitat fragmentation by instream structures, such as dams and weirs, that disrupt migrations and impact species distributions. The second is the impact of non-native species on native species and ecological processes. However, mitigating anthropogenic habitat fragmentation through the installation of passage facilities can facilitate the invasion and spread of non-native species. This study compared the potential of two existing low-cost fish passage technologies designed for sloping weirs, a cylindrical bristle cluster (CBC) array and horizontally oriented studded tiles, to facilitate upstream movement of native European fish while preventing dispersal of non-native American signal crayfish (Pacifastacus leniusculus); thus providing a selective fish passage solution. Crayfish movement and passage was experimentally quantified at a Crump weir installed in a recirculating flume under two velocity regimes (low and high), without (control) and with the addition of either a CBC array or studded tiles. Results were compared to passage efficacy (PE) data for native fish species for both technologies (existing data). Most (84.4%) crayfish were active during the trials, exhibiting frequent up and downstream movements below the weir. During control conditions under the high velocity regime, high velocities (ca. 2.39 m s−1) prevented crayfish reaching the foot of the weir (PE: 0%). Under the low velocity regime, relatively low velocities (ca. 0.74 m s−1) at the weir crest prevented most crayfish from passing (PE: 10–16%). Crayfish movement speed and total distance moved were lower under the high than the low velocity regime. Neither fish pass technology improved crayfish maximum distance of ascent on the downstream weir face or PE under either velocity regime. Under comparable conditions to the high velocity regime tested here, previous studies have shown both technologies improve PE for native fish. Hence, both CBC arrays and studded tiles would likely function as suitable selective fish passes where the conservation objective is not to aid the spread of non-native crayfish. Additional passage inhibiting technologies will be required at sites where complete blockage of crayfish movement is required.

Cuticle-associated bacteria can inhibit crayfish pathogen Aphanomyces astaci: Opening the perspective of biocontrol in astaciculture

Pathogenic oomycete Aphanomyces astaci, the causative agent of crayfish plague, is listed among the world's 100 worst invasive alien species due to its fast spread and detrimental impact on native European crayfish populations, as well as losses in crayfish aquaculture. During the infection process, A. astaci mycelium invades the host through its cuticle where it interacts with the resident epibiotic bacteria. Since host microbial communities may have an important role in the defense against pathogens, in this study we aimed to i) identify the epibiotic bacterial isolates from the cuticle of two commercially relevant crayfish – the signal crayfish Pacifastacus leniusculus and the narrow-clawed crayfish Pontastacus leptodactylus, and ii) distinguish those that can inhibit the growth of A. astaci mycelium. Results of 16S rRNA gene sequencing and MALDI–TOF mass spectrometry revealed that the majority of collected isolates belonged to the phylum Proteobacteria (69%), followed by Bacteroidetes (17%) and Firmicutes (8%). Using the in vitro plate assays, we tested the potential of the isolates to inhibit A. astaci mycelial growth and classified them as inhibitors (33%) or non-inhibitors (67%). More than half of the identified inhibitors (56%) belonged to the genus Pseudomonas. Our results represent the first step in the understanding of interactions between A. astaci and epibiotic bacteria on the crayfish cuticle and open the perspective for testing in vivo the application of selected bacterial inoculums for crayfish plague prevention and/or treatment. The development of such A. astaci biocontrol measures is urgently needed in astaciculture.

Effective investments due to seasonal morphological changes? Possible reasons and consequences of allometric growth and reproduction in adult signal crayfish (Pacifastacus leniusculus)

Signal crayfish (Pacifastacus leniusculus (Dana, 1852)) are among the most invasive macroinvertebrates in Europe, representing economic value as well as serious threats to ecosystems. Despite decades of research, uncertainties with respect to its biology remain. The present study describes the observations of adult signal crayfish during a single growing season, with a sex ratio of 1:1, reared under natural ambient conditions, in which moulting events, growth increments, morphological changes, mortality, and reproductive output were monitored. Two moults were observed in the majority of both sexes, with significant differences in moult growth increments of once- and twice-moulted animals. These led to modifications in morphology during the growing season. Females showed isometric growth at the first moult, while, at the second moult, they strengthened the functional state of structures linked to reproduction. In males, the effect was similar, but to a lesser extent, with extensive chelae development at the second moult. There were no differences in relative fecundity of once- and twice-moulted females. However, signal crayfish have different needs during the growing and reproductive seasons. The presented results indicate the ability to use resources according to current needs (development of strategical body parts) via their seasonal morphological plasticity as seen in representatives of the family Cambaridae.

Thermal sensitivity of feeding and burrowing activity of an invasive crayfish in UK waters

AbstractClimate change and invasive species are among the biggest threats to global biodiversity and ecosystem function. Although the individual impacts of climate change and invasive species are commonly assessed, we know far less about how a changing climate may impact invading species. Increases in water temperature due to climate change are likely to alter the thermal regime of UK rivers, and this in turn may influence the performance of invasive species such as signal crayfish (Pacifastacus leniusculus), which are known to have deleterious impacts on native ecosystems. We evaluate the relationship between water temperature and two key performance traits in signal crayfish—feeding and burrowing rate—using thermal experiments on wild‐caught individuals in a laboratory environment. Although water temperature was found to have no significant influence on burrowing rate, it did have a strong effect on feeding rate. Using the thermal performance curve for feeding rate, we evaluate how the thermal suitability of three UK rivers for signal crayfish may change as a result of future warming. We find that warming rivers may increase the amount of time that signal crayfish can achieve high feeding rate levels. These results suggest that elevated river water temperatures as a result of climate change may promote higher signal crayfish performance in the future, further exacerbating the ecological impact of this invasive species.

Strong impacts of signal crayfish invasion on upland stream fish and invertebrate communities

Abstract Impacts of invasive signal crayfish Pacifastacus leniusculus on native species and ecosystems are widely recognised, but mostly through small‐scale studies and laboratory experiments that may not always reflect impacts in nature. Recorded effects of signal crayfish on fish populations are equivocal. In this study, using the before–after/control–impact and control–impact approaches, the effects of signal crayfish invasion on native fishes, particularly benthic fishes and young‐of‐year (YoY) salmonids, and macroinvertebrate communities, were determined on several spatial and temporal scales through three correlated study elements (S1–S3), in upland streams of the River Tees, England. In S1, we sampled fish and benthic macroinvertebrates of 18 streams identically in 2011 and 2018. These streams were categorised into two groups: (1) uninvaded (without signal crayfish in both sampling years; n = 7); and (2) invaded (with signal crayfish) streams, comprising pre‐invaded (invaded before 2011; n = 8) and newly invaded (invaded between 2011 and 2018, n = 3). Despite similar habitat conditions in both years (all variables p &gt; 0.05) fish and macroinvertebrate communities changed over time in pre‐invaded streams and by comparison to uninvaded streams. A decline in the abundance of benthic fish and YoY salmonids was observed in pre‐invaded and newly invaded streams. Complete disappearance of bullhead Cottus perifretum following signal crayfish invasion was recorded in two pre‐invaded streams. In the second study, S2, we assessed within‐stream differences in fishes and macroinvertebrates in two Tees streams by comparing sections with (invaded) and without (uninvaded) signal crayfish. Compared to uninvaded sections, taxonomic richness and abundance of fish and macroinvertebrates were significantly lower in invaded sections, and the overall communities also differed significantly. In S3, long‐term data series (since 1990) of water quality and macroinvertebrates of six Tees streams comprising those invaded by signal crayfish (n = 3) and uninvaded (n = 3) were analysed. Water quality showed little change, or an improvement, over time but significant changes in the macroinvertebrate taxonomic richness and community structure occurred following signal crayfish invasion. Long‐term changes in macroinvertebrate communities in invaded streams tended to be due to declines in more sedentary taxa such as molluscs and cased trichopterans. Widespread and long‐term ecological disruption is occurring because of signal crayfish invasion in upland streams of the Tees catchment that may lead to a complete disappearance of some benthic fish species, as well as reduced densities of YoY salmonids and a shift towards less diverse macroinvertebrate communities, dominated by more mobile, crayfish‐resistant taxa.

Pilferer, murderer of innocents or prey? The potential impact of killer shrimp (Dikerogammarus villosus) on crayfish

Freshwater ecosystems worldwide are facing the establishment of non-native species, which, in certain cases, exhibit invasive characteristics. The impacts of invaders on native communities are often detrimental, yet, the number and spread of non-native invasive species is increasing. This is resulting in novel and often unexpected combinations of non-native and native species in natural communities. While the impact of invaders on native species is increasingly well-documented, the interactions of non-native invaders with other non-native invaders are less studied. We assessed the potential of an invasive amphipod, the killer shrimp Dikerogammarus villosus (Sowinsky, 1894), to cope with other established invaders in European waters: North American crayfish of the Astacidae family—represented by signal crayfish Pacifastacus leniusculus (Dana, 1852), and the Cambaridae family—represented by marbled crayfish Procambarus virginalis Lyko, 2017. The main goal of this study was to investigate if killer shrimp, besides their role as prey of crayfish, can significantly influence their stocks by predating upon their eggs, hatchlings and free-moving early juveniles. Our results confirmed that killer shrimp can predate on crayfish eggs and hatchlings even directly from females abdomens where they are incubated and protected. As marbled crayfish have smaller and thinner egg shells as well as smaller juveniles than signal crayfish, they were more predated upon by killer shrimp than were signal crayfish. These results confirmed that the invasive killer shrimp can feed on different developmental stages of larger freshwater crustaceans and possibly other aquatic organisms.

A novel ‘triple drawdown’ method highlights deficiencies in invasive alien crayfish survey and control techniques

Abstract Freshwater crayfish can be successful invaders that threaten native biota and aquatic ecosystems in numerous countries worldwide. Nonetheless, the inability of conventional crayfish survey techniques like trapping and handsearching to yield quantitative population data has limited the understanding of crayfish invasion biology and associated ecological impacts. Here, we employed a novel ‘triple drawdown’ (TDD) method to sample invasive populations of signal crayfish Pacifastacus leniusculus in a headwater stream in Northern England. The method was compared with conventional techniques of trapping and handsearching. The TDD method proved to be an effective technique with high capture efficiency, reporting signal crayfish densities from 20.5 to 110.4 animals/m2 at our study sites. These numbers exceed any previous estimates for similar streams. The TDD showed the vast majority of individuals across all sites were juvenile or sub‐adult (&lt;26 mm CL), with only 2.3% of the population large enough (≥35 mm CL) to be caught in standard traps. Synthesis and applications. The triple drawdown (TDD) method demonstrates strong inefficiencies and biases in conventional crayfish survey and management techniques. Trapping is not recommended for representative sampling or control of juvenile dominated populations. TDDs, which can be adapted and modified to operate in multiple habitat types and freshwater systems, generate robust quantitative data on invasive crayfish population demographics in situ. This can advance our understanding of the biology of an important invader of freshwater systems around the world. Obtaining this data prior and post‐intervention is fundamental to evaluate invasive crayfish management, and we recommend the TDD method to assess the effectiveness of future control measures.

Modelling the geographical distributions of one native and two introduced species of crayfish in the French Alps

The French Alps is an important area for several species of crayfish; some species are native in this region and several were introduced by humans. The present study aimed at modelling the distributions of these two categories of species and find the significant environmental variables to explain these distributions. Field sampling was performed in this region in 2014 resulting in enough data for modelling the distribution of three species, one native (the white-clawed crayfish, Austropotamobius pallipes) and two introduced from North America (the spiny-cheek crayfish, Faxonius limosus, and the signal crayfish, Pacifastacus leniusculus). Environmental data were collected from different sources and included altitude, slope, land cover, and several water quality measures such as nitrate and ammonium concentrations. The two introduced species showed different influential variables compared to the white-clawed crayfish. Currently, it is unlikely that the two introduced species affect negatively the native white-clawed crayfish. The most important threat for this species seems to be the increase of temperature caused by global warming. If the introduced species continue to spread, they may provide a synergy with the increase of temperature and threaten the native populations. It appears crucial to continue the monitoring of these species in this area. Future investigations should be carried out to understand the relationships between present crayfish spatial distribution and pathogens carried by alien species as these might be critical factors mediating the interactions among these species.