North American Freshwater Research Articles

Description and potential sources of a shell deformity in North American freshwater mussels (Unionoida)

AbstractObjectiveFreshwater mussels of the order Unionoida are among the most imperiled taxa in North America, and many species are undergoing enigmatic decline without fully understood causation. Disease pathology and parasitology have been identified as areas with significant knowledge gaps in relation to these declines. We investigated a shell deformity of unknown cause that is widespread in northeastern North America by adding to the clinical description from a mussel assemblage in Massachusetts with a deformity prevalence exceeding 50%. We build upon previous qualitative descriptions of this deformity with investigations of shell morphology and mussel age.MethodsWe conducted a qualitative survey of the mussel community to evaluate the prevalence of deformity. Mussels were classified as deformed based on the presence of a distinct truncation of the posterior margin of the shell. For the eastern elliptio Elliptio complanata, we evaluated the shell height, shell length, and height : length ratio of animals classified as deformed versus normal and we conducted a comparison to a reference population. We also incorporated shell thin sectioning and aging to qualitatively describe the deformity in cross section and to compare age distributions between deformed and normal eastern elliptio.ResultWe observed the presence of this deformity in four species, including the eastern elliptio, eastern lampmussel Lampsilis radiata, eastern pearlshell Margaritifera margaritifera, and creeper Strophitus undulatus. In cross section, the deformity appeared to be caused by repeated disturbance in growth in the posterior portion of the shell. Deformed eastern elliptio had markedly shorter shells for a given shell height when compared to normal and reference mussels, and they tended to be older at shorter shell lengths than normal mussels from the same site.ConclusionThe cause of the shell deformity in the United States remains unknown, although it appears similar in description to the deformity caused by a commensal midge, Xenochironomus canterburyensis, which infects a distantly related freshwater mussel in New Zealand. We highlight potential causes and the need for further investigation.

Legacy copper/nickel mine tailings potentially harbor novel iron/sulfur cycling microorganisms within highly variable communities.

The oxidation of sulfide-bearing mine tailings catalyzed by acidophilic iron and sulfur-oxidizing bacteria releases toxic metals and other contaminants into soil and groundwater as acid mine drainage. Understanding the environmental variables that control the community structure and metabolic activity of microbes indigenous to tailings (especially the abiotic stressors of low pH and high dissolved metal content) is crucial to developing sustainable bioremediation strategies. We determined the microbial community composition along two continuous vertical gradients of Cu/Ni mine tailings at each of two tailings impoundments near Sudbury, Ontario. 16S rRNA amplicon data showed high variability in community diversity and composition between locations, as well as at different depths within each location. A temporal comparison for one tailings location showed low fluctuation in microbial communities across 2 years. Differences in community composition correlated most strongly with pore-water pH, Eh, alkalinity, salinity, and the concentration of several dissolved metals (including iron, but not copper or nickel). The relative abundances of individual genera differed in their degrees of correlation with geochemical factors. Several abundant lineages present at these locations have not previously been associated with mine tailings environments, including novel species predicted to be involved in iron and sulfur cycling.IMPORTANCEMine tailings represent a significant threat to North American freshwater, with legacy tailings areas generating acid mine drainage (AMD) that contaminates rivers, lakes, and aquifers. Microbial activity accelerates AMD formation through oxidative metabolic processes but may also ameliorate acidic tailings by promoting secondary mineral precipitation and immobilizing dissolved metals. Tailings exhibit high geochemical variation within and between mine sites and may harbor many novel extremophiles adapted to high concentrations of toxic metals. Characterizing the unique microbiomes associated with tailing environments is key to identifying consortia that may be used as the foundation for innovative mine-waste bioremediation strategies. We provide an in-depth analysis of microbial diversity at four copper/nickel mine tailings impoundments, describe how communities (and individual lineages) differ based on geochemical gradients, predict organisms involved in AMD transformations, and identify taxonomically novel groups present that have not previously been observed in mine tailings.

Simultaneous species detection and discovery with environmental DNA metabarcoding: A freshwater mollusk case study.

Environmental DNA (eDNA) sampling is a powerful tool for rapidly characterizing biodiversity patterns for specious, cryptic taxa with incomplete taxonomies. One such group that are also of high conservation concern are North American freshwater gastropods. In particular, springsnails of the genus Pyrgulopsis (Family: Hydrobiidae) are prevalent throughout the western United States where >140 species have been described. Many of the described species are narrow endemics known from a single spring or locality, and it is believed that there are likely many additional species which have yet to be described. The distribution of these species across the landscape is of interest because habitat loss and degradation, climate change, groundwater mining, and pollution have resulted in springsnail imperilment rates as high as 92%. Determining distributions with conventional sampling methods is limited by the fact that these snails are often <5 mm in length with few distinguishing morphological characters, making them both difficult to detect and to identify. We developed an eDNA metabarcoding protocol that is both inexpensive and capable of rapid, accurate detection of all known Pyrgulopsis species. When compared with conventional collection techniques, our pipeline consistently resulted in detection at sites previously known to contain Pyrgulopsis springsnails and at a cost per site that is likely to be substantially less than the conventional sampling and individual barcoding that has been done historically. Additionally, because our method uses eDNA extracted from filtered water, it is non-destructive and suitable for the detection of endangered species where "no take" restrictions may be in effect. This effort represents both a tool which is immediately applicable to taxa of high conservation concern across western North America and a case study in the broader application of eDNA sampling for landscape assessments of cryptic taxa of conservation concern.

Genetic structure of the Silver Chub indicates distinctiveness of Lake Erie population

AbstractObjectiveSilver Chub Macrhybopsis storeriana is a small riverine minnow endemic to North American fresh waters. Its range extends from the southern USA to southcentral Canada; the latter includes a rare lacustrine population in Lake Erie. Anthropogenic activities pose an immediate threat to several Silver Chub populations, currently categorized from special concern to threatened at the state level in the USA and federally and provincially not‐at‐risk to endangered in Canada. Several studies have examined the anthropogenic causes for the decline of Silver Chub populations, but conservation efforts have been hindered by the lack of knowledge of the population genetics of this species.MethodsHere, we provide an assessment of the genetic diversity of Silver Chub populations across the USA and Canada using a fast‐evolving mitochondrial gene, with particular focus on the Lake Erie population.ResultWe found the Lake Erie population to be divergent from all other populations, with nearly all the haplotypes sampled there being private.ConclusionOur study provides genetic evidence that the Silver Chub population in Lake Erie could be considered a separate conservation unit.

Coevolution With Host Fishes Shapes Parasitic Life Histories in a Group of Freshwater Mussels (Unionidae: Quadrulini).

Ecological interactions among species often lead to parasitic lineages coevolving with host resources, which is often suggested as the primary driver of parasite diversification. Freshwater mussels are bivalves that possess a parasitic life cycle requiring larval encystment on freshwater vertebrates to complete metamorphosis. The North American freshwater mussel tribe Quadrulini has a suite of life history adaptations including highly specialized patterns of host use, infection strategies, and variable larval morphologies. However, the evolution of life histories has yet to be explored using phylogenetic comparative methods. In this study, we use a holistic approach incorporating biogeographical, ecological, molecular, and morphological datasets to reconstruct the evolution of Quadrulini. Comparative phylogenetic analyses suggested the diversification of Quadrulini has been driven, at least in part, by codiversification with their primary host fishes in Ictaluridae. Major diversification events in both ictalurids and quadrulines were estimated to have occurred in the Mississippi River basin throughout the Miocene. Life history characteristics associated with parasitism were supported to have coevolved with host repertories, supporting the hypothesis that ecological interactions with host fishes have shaped the evolution of highly specialized traits in this group. Our findings demonstrate the importance of ecological interactions with host resources in shaping the evolutionary history of freshwater mussels.

The history of ice-sheet retreat on North America during Termination 5: Implications for the origin of the sea-level highstand during interglacial stage 11

Termination (T) 5, ∼424 ka, involved the biggest deglaciation of land-ice mass during the Quaternary. Warming and ice-sheet retreat during T5 led to an exceptionally long period of interglacial warmth known as Marine Isotope Stage (MIS) 11, ∼424–395 ka. A detailed understanding of the history of continental ice-sheet decay during T5 is required to disentangle regional contributions of ice-sheet retreat to sea-level rise (that range between ∼1 and 13 m above present day) and to correct it for glacio-isostatic adjustments (GIA). Yet little is known about the timing and magnitude of retreat during this time of the volumetrically most important continental ice sheet in the Northern Hemisphere, the Laurentide Ice Sheet (LIS). Here we present new authigenic Fe-Mn oxyhydroxide-derived high-resolution records of Pb isotope data and associated rare earth element profiles for samples spanning T5 from Labrador Sea IODP Site U1302/3. These records feature astronomically-paced radiogenic Pb isotope excursions that track increases in chemical weathering of North American bedrock and freshwater routing to the Labrador Sea via Hudson Straits associated with LIS retreat. Our records show that LIS retreat during T5 began 429. 2 ± 7.9 ka (2σ) and likely occurred over a longer timescale (by ∼10 to 5 kyr) than that observed for T2 and T1. They also show that Hudson Bay Ice Saddle collapse (and therefore LIS break-up) occurred ∼419 ± 4.7 ka (2σ), around the same time as best estimates of southern Greenland deglaciation, but ∼12 kyr before LIS deglaciation and the sea-level high-stand associated with the latter half of MIS 11 likely occurred. Our findings therefore highlight that ice-mass loss on North America likely played an important role in the seemingly protracted nature of T5 sea-level rise. A comparison of the deglaciation histories of the LIS and the southern Greenland Ice Sheet during T5, T2 and T1 also demonstrates that the well-constrained history of regional ice-sheet retreat during T1 is not always applicable as a template for older late Pleistocene terminations in GIA modelling.

A cylindrospermopsin-producing cyanobacterium isolated from a microbial mat in the Baltic Sea

Toxic benthic mats of cyanobacteria are associated with water quality problems and animal poisonings around the world. A strain of the filamentous cyanobacterial genus Kamptonema was isolated from a water bloom in the Baltic Sea four decades ago and later shown to produce cylindrospermopsins. However, the exact habitat of this strain remains unclear and cylindrospermopsins have not yet been reported from water blooms in the Baltic Sea. Here, we report the isolation of Kamptonema sp. UHCC 0994 from a benthic microbial mat collected in shallow water on the coast of Helsinki. We obtained draft genome sequences for the Kamptonema spp. PCC 7926 and UHCC 0994 strains that were isolated from the Baltic Sea. These genomes were 90–96% similar to previously studied Kamptonema sp. PCC 6506 and Kamptonema formosum PCC 6407, which were isolated from benthic and North American freshwater environments, respectively. The genomes of all four Kamptonema strains encode complete cylindrospermopsin biosynthetic gene clusters. We detected the production of cylindrospermopsin and 7-epi-cylindrospermopsin in the four Kamptonema strains using high-resolution liquid chromatography mass spectrometry. The four strains encode genes for producing gas vesicles distributed in two to three different regions of their genomes. Kamptonema spp. UHCC 0994 and PCC 7926 have both retained the ability to regulate their buoyancy when grown in liquid culture. Together this suggests that these toxic cyanobacteria may exhibit a tychoplanktic lifestyle in the Baltic Sea. This study suggests that microbial mats containing cyanobacteria could be a source of environmental toxins in the Baltic Sea.

Multi-locus phylogeny of the catfish genus Ictalurus Rafinesque, 1820 (Actinopterygii, Siluriformes) and its systematic and evolutionary implications

BackgroundIctalurus is one of the most representative groups of North American freshwater fishes. Although this group has a well-studied fossil record and has been the subject of several morphological and molecular phylogenetic studies, incomplete taxonomic sampling and insufficient taxonomic studies have produced a rather complex classification, along with intricate patterns of evolutionary history in the genus that are considered unresolved and remain under debate.ResultsBased on four loci and the most comprehensive taxonomic sampling analyzed to date, including currently recognized species, previously synonymized species, undescribed taxa, and poorly studied populations, this study produced a resolved phylogenetic framework that provided plausible species delimitation and an evolutionary time framework for the genus Ictalurus.ConclusionsOur phylogenetic hypothesis revealed that Ictalurus comprises at least 13 evolutionary units, partially corroborating the current classification and identifying populations that emerge as putative undescribed taxa. The divergence times of the species indicate that the diversification of Ictalurus dates to the early Oligocene, confirming its status as one of the oldest genera within the family Ictaluridae.

Acoustic Variation in Ictalurid Catfishes

More than 35,000 ray-finned fish (Actinopterygii) species are potentially using acoustic communication. However, of the approximately 1200 known soniferous fish species, few include North American freshwater fish. To help fill this knowledge gap in fish acoustic communication, which holds great promise for conservation monitoring, I document acoustic measurements (duration 90%, bandwidth 90%, number of pulses, center frequency, and peak time) across 4 species (Ameiurus nebulosus, Ameiurus natalis, Noturus flavus, Ictalurus punctatus) from 3 genera of the North American catfish family, Ictaluridae. This was done by recording 10 trials of disturbance calls from 28 individuals and analyzing 1294 sounds using Raven Pro 1.6 software. I hypothesized that: 1) more phylogenetically/morphologically related species would have more similar acoustic features, 2) acoustic features would correlate with one another, and 3) acoustic features would correlate with standard length (cm). For hypothesis 1, I instead found that Ameriurus nebulosus was the most acoustically dissimilar, despite having the highest level of phylogenetic/morphological similarity with Ameirus natalis. However, only Ameriurus nebulosus' number of pulses were significantly different from other species. For hypothesis 2, it was found that many acoustic measurements were correlated with one another as predicted. For hypothesis 3, only the number of pulses was found to be significantly correlated with standard length, but minimally so. These findings further support that pulsation measurements may contain a high level of phylogenetic signal, given that it is the most crucial characteristic to differentiate species.

Species invasiveness and community invasibility of North American freshwater fish fauna revealed via trait-based analysis

While biological invasions are recognized as a major threat to global biodiversity, determining non-native species’ abilities to establish in new areas (species invasiveness) and the vulnerability of those areas to invasions (community invasibility) is challenging. Here, we use trait-based analysis to profile invasive species and quantify the community invasibility for >1,800 North American freshwater fish communities. We show that, in addition to effects attributed to propagule pressure caused by human intervention, species with higher fecundity, longer lifespan and larger size tend to be more invasive. Community invasibility peaks when the functional distance among native species was high, leaving unoccupied functional space for the establishment of potential invaders. Our findings illustrate how the functional traits of non-native species determining their invasiveness, and the functional characteristics of the invaded community determining its invasibility, may be identified. Considering those two determinants together will enable better predictions of invasions.

Assessing climate change impacts on North American freshwater habitat of wild Atlantic salmon - urgent needs for collaborative research

Climate change and human activities have dramatically affected all ecosystems inhabited by Atlantic salmon, causing drastic population declines. Change in river temperature dynamics (e.g. daily variability, frequency, and duration of summer maximum, warmer thermal regimes) is of special concern as it impacts growth rates, reproductive success, prey abundance and phenology, timing of migration, and ultimately survival. The Atlantic Salmon Research Joint Venture held a workshop to address the effects of climate change on freshwater habitats of Atlantic salmon and identify research gaps and priorities. Here we summarize the state of the science for three key themes identified by workshop participants: (1) Effects of climate change on in-river habitat conditions, (2) Physiological and behavioral responses of salmon to temperature, and (3) Population-level responses of salmon to climate change. The group highlighted the crucial importance of understanding and monitoring the links between river temperature dynamics and physiological requirements of Atlantic salmon across different life stages and habitat conditions, with a focus on freshwater life stages. Climate change will undoubtedly continue to affect instream habitats across all seasons and render challenging conditions for all freshwater Atlantic salmon life stages. Hence, we call for urgent interdisciplinary collaborations and partnerships among scientists and managers to address the pressing research gaps that require large-scale data integration across life cycle stages and ecosystems. More collaboration between scientists, managers, and interest groups is needed to ensure that fundamental science directly addresses the knowledge-action gap to enhance evidence-based decision-making and conservation.

The first record of the North American freshwater limpet Ferrissia californica (Mollusca, Gastropoda) in Morocco

Invasive species are a major threat to global biodiversity. One of the most effective invaders among freshwater snails is the North American ancyline gastropod Ferrissia californica (= fragilis). A new population of this species has been found in Lake Zerrouka (Northern Morocco), a Protected Area, which is also a site of ecological and biological interest (known as SIBE). Thus, this record expands the known distribution range of this invasive species in the Mediterranean area. Despite the fact it is located in only one out of more than 150 localities sampled by us, ongoing investigations could soon reveal more finds of the species in Morocco. Lake Zerrouka is known for recently described endemic snail species, i.e. Ifrania zerroukansis and Gyraulus marocana. Therefore, the monitoring of the presence and expansion of F. californica within invaded areas, as well as studies improving the knowledge on its biology and ecology is an urgent need.

Short-term paleogeographic reorganizations and climate events shaped diversification of North American freshwater gastropods over deep time

What controls species diversity and diversification is one of the major questions in evolutionary biology and paleontology. Previous studies have addressed this issue based on various plant and animal groups, geographic regions, and time intervals. However, as most previous research focused on terrestrial or marine ecosystems, our understanding of the controls on diversification of biota (and particularly invertebrates) in freshwater environments in deep time is still limited. Here, we infer diversification rates of North American freshwater gastropods from the Late Triassic to the Pleistocene and explore potential links between shifts in speciation and extinction and major changes in paleogeography, climate, and biotic interactions. We found that variation in the speciation rate is best explained by changes in continental fragmentation, with rate shifts coinciding with major paleogeographic reorganizations in the Mesozoic, in particular the retreat of the Sundance Sea and subsequent development of the Bighorn wetland and the advance of the Western Interior Seaway. Climatic events in the Cenozoic (Middle Eocene Climate Optimum, Miocene Climate Optimum) variably coincide with shifts in speciation and extinction as well, but no significant long-term association could be detected. Similarly, no influence of diversity dependence was found across the entire time frame of ~ 214 Myr. Our results indicate that short-term climatic events and paleogeographic changes are relevant to the diversification of continental freshwater biota, while long-term trends have limited effect.

Giant gar from directly above the Cretaceous-Palaeogene boundary suggests healthy freshwater ecosystems existed within thousands of years of the asteroid impact.

The Cretaceous-Palaeogene (K-Pg) mass extinction was responsible for the destruction of global ecosystems and loss of approximately three-quarters of species diversity 66 million years ago. Large-bodied land vertebrates suffered high extinction rates, whereas small-bodied vertebrates living in freshwater ecosystems were buffered from the worst effects. Here, we report a new species of large-bodied (1.4-1.5 m) gar based on a complete skeleton from the Williston Basin of North America. The new species was recovered 18 cm above the K-Pg boundary, making it one of the oldest articulated vertebrate fossils from the Cenozoic. The presence of this freshwater macropredator approximately 1.5-2.5 thousand years after the asteroid impact suggests the rapid recovery and reassembly of North American freshwater food webs and ecosystems after the mass extinction.

Body size is not correlated with the evolution of male coloration in darters (Percidae: Etheostomatinae)

Abstract Coloration and body size are among the many morphological traits that vary among fish lineages. Elaborate coloration and body size covary in other animal groups, but relationships between these two morphological characteristics have not been rigorously examined in fishes. We formally test for correlations between coloration and body size in darters (Percidae: Etheostomatinae), a group of North American freshwater fishes that vary in the presence of male coloration and maximum body size. Although uncorrected analyses indicate a significant correlation between colour traits and body size in darters, phylogenetically corrected logistic regression models and ANOVAs revealed no significant correlations, suggesting body size does not act as a constraint on elaborate coloration or vice versa. These results are discussed in an ecological and behavioural context.

Compensating for Small Body Size: The Reproductive Ecology of Southern Spotted Turtle (Clemmys guttata) Populations

Gradients in environmental conditions across a species' geographic distribution can drive variability in a variety of life history traits. In North American freshwater turtles, both body and clutch size have commonly been shown to vary latitudinally, and these two traits are often directly related, with larger individuals producing larger clutches. We studied the reproductive ecology in two Georgia populations of Spotted Turtles (Clemmys guttata) from 2016–2020 by attaching radio transmitters to female turtles during the breeding season. We x-rayed turtles to determine clutch sizes and used thread bobbins to locate nesting locations, allowing us to determine nest fates. Across all Spotted Turtle clutches (n = 41), mean clutch size was 2.1 (range: 1–4) eggs per clutch. Approximately 92% of individuals that we monitored produced at least one clutch during the breeding season, and we identified 16 instances of individuals producing more than one clutch in a single year, including six turtles triple clutching during 2018. We located 24 Spotted Turtle nests during the study, nine (37.5%) of which either hatched or partially hatched. The other nests were either depredated (41.7%), did not hatch due to infertility or environmental reasons (8.3%), or had an undetermined fate (12.5%). Our results indicate that annual reproductive output in southern Spotted Turtle populations can exceed that of northern populations where individuals produce a single larger clutch per reproductive season. Finally, opportunistic observations in Florida from 2014–2021 indicated that the reproductive season can begin over a month earlier than in southern Georgia, highlighting the variability in reproductive ecology even across a relatively short latitudinal distance.

Beaver: The North American freshwater climate action plan

AbstractRivers and streams, when fully connected to their floodplains, are naturally resilient systems that are increasingly part of the conversation on nature‐based climate solutions. Reconnecting waterways to their floodplains improves water quality and quantity, supports biodiversity and sensitive species conservation, increases flood, drought and fire resiliency, and bolsters carbon sequestration. But, while the importance of river restoration is clear, beaver‐based restoration—for example, strategic coexistence, relocation, and mimicry—remains an underutilized strategy despite ample data demonstrating its efficacy. Climate‐driven disturbances are actively pushing streams into increasingly degraded states, and the window of opportunity for restoration will not stay open forever. Therefore, now is the perfect time to apply the science of beaver‐based low‐tech process‐based stream restoration to support building climate resilience across the landscape. Not every stream will be a good candidate for beaver‐based restoration, but we have the tools to know which ones are. Let us use them.This article is categorized under: Science of Water &gt; Hydrological Processes Water and Life &gt; Nature of Freshwater Ecosystems Water and Life &gt; Conservation, Management, and Awareness

The North American Freshwater Migratory Fish Database (NAFMFD): Characterizing the migratory life histories of freshwater fishes of Canada, the United States and Mexico

AbstractAimMigratory freshwater fishes are those that must access discrete habitats to complete their life cycles. Freshwater fish migrations occur around the world and provide numerous ecosystem services for humans and natural systems; however, many migratory species are in decline globally. A limiting factor to successfully conserve freshwater migratory fishes is that the migratory life histories of many species are unknown or only partially described. To provide researchers with critical and comprehensive information to conserve migratory fishes, we developed the North American Freshwater Migratory Fish Database (NAFMFD).LocationCanada, Mexico and the United States.TaxonFreshwater fish.MethodsTo develop this database, we assigned migratory status, pattern and behaviour to a comprehensive list of freshwater fish species found throughout North America. We assembled the database which included assignments (i.e. migratory status, pattern and behaviour) as well as the sources used to make the assignments. Researchers and managers from across North America reviewed the database for completeness and accuracy on the migratory life histories of fishes.ResultsThe database synthesizes current knowledge of migratory status, pattern and behaviour of native and non‐native freshwater fishes throughout North America, including 1250 species representing 79 families and 325 genera. Results showcase the diversity of migratory life histories of freshwater fishes on the continent, including that at least 25% of North American freshwater fishes are migratory, 23% are non‐migratory and 44% have undetermined migratory status.Main conclusionsNAFMFD improves the quality of migratory data accessible to researchers, which supports a more holistic understanding of the threats encountered by migratory fishes, including habitat fragmentation. The approach we used in developing NAFMFD can provide guidance for developing similar databases in other regions. Collectively, our work offers new insights into the range of freshwater fish migratory life histories, stimulating a need to better understand this diversity globally.

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.

Population structure and gene flow in the Sheepnose mussel (Plethobasus cyphyus) and their implications for conservation.

North American freshwater mussel species have experienced substantial range fragmentation and population reductions. These impacts have the potential to reduce genetic connectivity among populations and increase the risk of losing genetic diversity. Thirteen microsatellite loci and an 883 bp fragment of the mitochondrial ND1 gene were used to assess genetic diversity, population structure, contemporary migration rates, and population size changes across the range of the Sheepnose mussel (Plethobasus cyphyus). Population structure analyses reveal five populations, three in the Upper Mississippi River Basin and two in the Ohio River Basin. Sampling locations exhibit a high degree of genetic diversity and contemporary migration estimates indicate that migration within river basins is occurring, although at low rates, but there is no migration is occurring between the Ohio and Mississippi river basins. No evidence of bottlenecks was detected, and almost all locations exhibited the signature of population expansion. Our results indicate that although anthropogenic activity has altered the landscape across the range of the Sheepnose, these activities have yet to be reflected in losses of genetic diversity. Efforts to conserve Sheepnose populations should focus on maintaining existing habitats and fostering genetic connectivity between extant demes to conserve remaining genetic diversity for future viable populations.