Evaluation of Short-Term Mussel Test for Estimating Toxicity.

Effect concentrations of ammonia, nickel, sodium chloride, and potassium chloride from short-term 7-day tests were compared to those from standard chronic 28-day toxicity tests with juvenile mussels (fatmucket, Lampsilis siliquoidea) to evaluate the sensitivities of the 7-day tests. The effect concentrations for nickel (59 µg Ni/L), chloride (316-519 mg Cl/L, a range from multiple tests), and potassium (15 mg K/L) obtained from the 7-day tests were within a range of effect concentrations for each corresponding chemical in the 28-day tests (41-91 µg Ni/L, 251->676 mg Cl/L, 15-23 mg K/L), whereas the 7-day ammonia effect concentration (0.40 mg/L total ammonia nitrogen; TAN) was up to 3.3-fold greater than the 28-day effect concentrations (0.12-0.36 mg TAN/L) but with overlapped 95% confidence limits. These results indicate that the 7-day tests produced similar estimates compared to the 28-day tests. Further studies are needed to evaluate the 7-day test sensitivity using additional chemicals with different modes of toxic action. Environ Toxicol Chem 2024;43:2020-2025. Published 2024. This article is a U.S. Government work and is in the public domain in the USA.

Molecular phylogenetic position and the life cycle of Phyllodistomum cyprini Feng et Wang, 1995 in Japan, with a note of a larval Phyllodistomum sp. from unionid mussels in North America.

A gorgoderid species, Phyllodistomum cyprini Feng et Wang, 1995 obtained from ureters of common carp, Cyprinus carpio in Japan is described both morphologically and molecularly. Its larval stage is a macrocercous cercaria found in a unionid mussel, Nodularia nipponensis first described by Urabe et al. (2015). A molecular phylogenetic study revealed that P. cyprini is not closely related to rhopalocercous cercariae from unionid mussels both in Europe and North America. This result indicates that there are several distinct clades in Phyllodistomum species that use unionid mussels as a first intermediate host, and suggests that the cercarial morphology may be a more accurate indicator of the phylogeny of Phyllodistomum than molluscan host identity.

Targeted metabolomics characterizes metabolite occurrence and variability in stable freshwater mussel populations.

Freshwater mussels (order Unionida) play a key role in freshwater systems as ecosystem engineers and indicators of aquatic ecosystem health. The fauna is globally imperilled due to a diversity of suspected factors; however, causes for many population declines and mortality events remain unconfirmed due partly to limited health assessment tools. Mussel-monitoring activities often rely on population-level measurements, such as abundance and age structure, which reflect delayed responses to environmental conditions. Measures of organismal health would enable preemptive detection of declining condition before population-level effects manifest. Metabolomic analysis can identify shifts in biochemical pathways in response to stressors and changing environmental conditions; however, interpretation of the results requires information on inherent variability of metabolite concentrations in mussel populations. We targeted metabolites in the haemolymph of two common mussels, Lampsilis cardium and Lampsilis siliquoidea, from three Indiana streams (USA) using ultra-high-performance liquid chromatography combined with quadrupole time-of-flight mass spectroscopy. The influence of species, stream and sex on metabolite variability was examined with distance-based redundancy analysis. Metabolite variability was most influenced by species, followed by site and sex. Inter- and intraspecies metabolite variability among sexes was less distinct than differences among locations. We further categorized metabolites by occurrence and variability in mussel populations. Metabolites with high occurrence (Categories 1 and 2) included those indicative of energy status (catabolism versus anabolism; arginine, proline, carnitine, nicotinic acid, pantothenic acid), oxidative stress (proline, glutamine, glutamate) and protein metabolism (thymidine, cytidine, inosine). Metabolites with lower occurrence (Category 3) are constituents of assorted metabolic pathways and can be important biomarkers with additional temporal sampling to characterize their variability. These data provide a reference for future temporal (before/after) monitoring and for studies of stressor-metabolite linkages in freshwater mussels.

Freshwater mussel conservation: A global horizon scan of emerging threats and opportunities.

We identified 14 emerging and poorly understood threats and opportunities for addressing the global conservation of freshwater mussels over the next decade. A panel of 17 researchers and stakeholders from six continents submitted a total of 56 topics that were ranked and prioritized using a consensus-building Delphi technique. Our 14 priority topics fell into five broad themes (autecology, population dynamics, global stressors, global diversity, and ecosystem services) and included understanding diets throughout mussel life history; identifying the drivers of population declines; defining metrics for quantifying mussel health; assessing the role of predators, parasites, and disease; informed guidance on the risks and opportunities for captive breeding and translocations; the loss of mussel-fish co-evolutionary relationships; assessing the effects of increasing surface water changes; understanding the effects of sand and aggregate mining; understanding the effects of drug pollution and other emerging contaminants such as nanomaterials; appreciating the threats and opportunities arising from river restoration; conserving understudied hotspots by building local capacity through the principles of decolonization; identifying appropriate taxonomic units for conservation; improved quantification of the ecosystem services provided by mussels; and understanding how many mussels are enough to provide these services. Solutions for addressing the topics ranged from ecological studies to technological advances and socio-political engagement. Prioritization of our topics can help to drive a proactive approach to the conservation of this declining group which provides a multitude of important ecosystem services.

Freshwater Mussels Show Elevated Viral Richness and Intensity during a Mortality Event.

Freshwater mussels (Unionida) are among the world's most imperiled taxa, but the relationship between freshwater mussel mortality events and infectious disease is largely unstudied. We surveyed viromes of a widespread and abundant species (mucket, Actinonaias ligamentina; syn: Ortmanniana ligamentina) experiencing a mortality event of unknown etiology in the Huron River, Michigan, in 2019-2020 and compared them to viromes from mucket in a healthy population in the St. Croix River, Wisconsin and a population from the Clinch River, Virginia and Tennessee, where a mortality event was affecting the congeneric pheasantshell (Actinonaias pectorosa; syn: Ortmanniana pectorosa) population. We identified 38 viruses, most of which were associated with mussels collected during the Huron River mortality event. Viral richness and cumulative viral read depths were significantly higher in moribund mussels from the Huron River than in healthy controls from each of the three populations. Our results demonstrate significant increases in the number and intensity of viral infections for freshwater mussels experiencing mortality events, whereas individuals from healthy populations have a substantially reduced virome comprising a limited number of species at low viral read depths.

Biofouling of a unionid mussel by dreissenid mussels in nearshore zones of the Great Lakes.

In North America, native unionid mussels are imperiled due to factors such as habitat degradation, pollution, and invasive species. One of the most substantial threats is that posed by dreissenid mussels, which are invasive mussels that attach to hard substrates including unionid shells and can restrict movement and feeding of unionids. This dreissenid mussel biofouling of unionids varies spatially in large ecosystems, such as the Great Lakes, with some areas having low enough biofouling to form effective refugia where unionid mussels might persist. Here, we measured biofouling on mussels suspended in cages over the growing season (generally first week in June to last week of August) over 3 years in nearshore areas in Lake Erie (2014-2016), Lake Michigan (Grand Traverse Bay, 2015 and Green Bay, 2016), and Lake Huron (2015). Biofouling varied substantially by years within Lake Erie, with increasingly higher biofouling rates each year. Although dreissenid mussels are present throughout these lakes, we observed very low biofouling in Grand Traverse Bay (Lake Michigan) and Saginaw Bay (Lake Huron), with no dreissenid mussels in 8 of 9 sites across these two bays. Sampling in the rivermouth of the Fox River (Wisconsin) and the Maumee River (Ohio) both showed very high biofouling in areas adjacent to the outlet of these tributaries into Green Bay and Maumee Bay (Lake Erie), respectively. These watersheds are dominated by agriculture, and we would expect high growth of primary producers (i.e., mussel food) and primary consumers (unionids and zebra mussels) in these areas compared to the other sampled bays or the open waters of the Great Lakes.

Mussel parasite richness and risk of extinction.

Parasite conservation is important for the maintenance of ecosystem diversity and function. Conserving parasites relies first on understanding parasite biodiversity and second on estimating the extinction risk to that biodiversity. Although steps have been taken independently in both these areas, previous studies have overwhelmingly focused on helminths in vertebrate hosts over broad scales, providing low resolution and excluding a large proportion of possible host and parasite diversity. We estimated both total obligate parasite richness and parasite extinction risk in freshwater mussels (Unionidae and Margaritiferidae) from Europe and the United States to provide a case study for considering parasite conservation in a severely understudied system. We used currently reported host-parasite relationships to extrapolate parasite diversity to all possible mussel hosts and then used the threat levels of those hosts to estimate the extinction risk for both described and undescribed parasites. An estimated 67% of parasite richness in freshwater mussels is undescribed and over 80% of the most host-specific groups (digenean trematodes and ciliates) are undescribed. We estimated that 21% of this total parasite fauna is at immediate risk of extinction, corresponding to 60 unique species, many of which will likely go extinct before being described. Given the important roles parasites play in community structure and function and the strong ecosystem engineering capacities of freshwater mussels, such extinctions are likely to severely affect freshwater ecosystems. Our detailed study of mussel parasites provides compelling evidence for the hidden conservation threat to parasites through extinction cascades and shows parasites are deserving of immediate attention.

La conservación de parásitos es importante para el mantenimiento de la diversidad y funcionamiento de los ecosistemas. La conservación de parásitos depende en primera instancia del entendimiento de la biodiversidad de parásitos y, en segunda, de la estimación del riesgo de extinción de esa biodiversidad. Mientras que se han tomado medidas en ambas áreas, estudios previos se han enfocado abrumadoramente en helmintos de hospederos vertebrados, proporcionando baja resolución y excluyendo a una amplia proporción de una probable diversidad de hospederos y parásitos. Estimamos tanto la riqueza de parásitos obligados como el riesgo de extinción de mejillones de agua dulce (Unionidae y Margaritiferidae) de Europa y Estados Unidos para proporcionar un caso de estudio para considerar la conservación de parásitos en un sistema severamente poco estudiado. Utilizamos las relaciones hospedero-parásito registradas hasta la fecha para extrapolar la diversidad de parásitos a todas las especies posibles de mejillones hospederos y luego utilizamos los niveles de amenaza de aquellos hospederos para estimar el riesgo de extinción tanto para parásitos descritos y no descritos. Alrededor de 67% de la riqueza de parásitos de mejillones de agua dulce no esta descrito, así como mas de 80% de los grupos específicos de hospederos (trematodos digeneos y ciliados) tampoco están descritos. Estimamos que 21% del total de esta fauna de parásitos está en riesgo inminente de extinción, correspondiendo a 60 especies únicas, muchas de las cuales probablemente se extinguirán antes de ser descritas. Dado la importancia del papel que desempeñan los parásitos en la estructura y función de la comunidad y las notables capacidades de los mejillones de agua dulce para la ingeniería del ecosistema, es muy probable que tales extinciones afecten severamente a los ecosistemas dulceacuícolas. Nuestro estudio detallado de los parásitos de mejillones proporciona evidencia convincente de la amenaza oculta para los parásitos mediante cascadas de extinción y muestra que los parásitos son merecedores de atención inmediata.

Threats to freshwater mussels: The interactions of water temperature, velocity and total suspended solids on ecophysiology and growth.

Freshwater unionid mussels are ecosystem engineers that are highly endangered in part because of land-use changes that have altered their habitat and negatively impacted their ecophysiology. The environmental factors that affect mussels do not act alone and may be better understood using a multiple-stressor approach. We examine how changes in water temperature, turbidity (total suspended solids; TSS) and water velocity affected the clearance rates (CR), oxygen consumption rates (OC), and resultant scope for growth (SFG) of Lampsilis siliquoidea in laboratory flow chamber experiments. The CR, OC and SFG of L. siliquoidea increased with acclimation temperature and velocity, and decreased with TSS concentration and acute temperature exposure, although these responses were more complicated when factors were combined. The primary factor affecting CR and OC varied with acclimation temperature, with warmer temperature and high TSS leading to strong declines in clearance rates. A worst-case scenario would involve a summer season where temperatures and TSS loads are above-average, and water velocities are either below- or above- average, which are likely under increased frequencies of storm, flood, or drought events due to climate change. Conservation measures should focus on protecting aquatic systems during these times and also use a multistressor approach to determine how environmental factors interact in efforts to protect and recover freshwater mussel populations.

Mussel Mass Mortality and the Microbiome: Evidence for Shifts in the Bacterial Microbiome of a Declining Freshwater Bivalve.

Freshwater mussels (Unionida) are suffering mass mortality events worldwide, but the causes remain enigmatic. Here, we describe an analysis of bacterial loads, community structure, and inferred metabolic pathways in the hemolymph of pheasantshells (Actinonaias pectorosa) from the Clinch River, USA, during a multi-year mass mortality event. Bacterial loads were approximately 2 logs higher in moribund mussels (cases) than in apparently healthy mussels (controls). Bacterial communities also differed between cases and controls, with fewer sequence variants (SVs) and higher relative abundances of the proteobacteria Yokenella regensburgei and Aeromonas salmonicida in cases than in controls. Inferred bacterial metabolic pathways demonstrated a predominance of degradation, utilization, and assimilation pathways in cases and a predominance of biosynthesis pathways in controls. Only two SVs correlated with Clinch densovirus 1, a virus previously shown to be strongly associated with mortality in this system: Deinococcota and Actinobacteriota, which were associated with densovirus-positive and densovirus-negative mussels, respectively. Overall, our results suggest that bacterial invasion and shifts in the bacterial microbiome during unionid mass mortality events may result from primary insults such as viral infection or environmental stressors. If so, bacterial communities in mussel hemolymph may be sensitive, if generalized, indicators of declining mussel health.

Reproductive aspects of freshwater unionid mussel sperm: Seasonal dynamics, male-to male variability, and cell quantification.

Freshwater unionid mussel diversity is decreasing because of species extirpation or extinction. While little can be done to recover lost species, there is an opportunity to develop techniques to save other species. This can be facilitated through gene banking and assisted reproduction. Unfortunately, limited information is available on mussel reproduction, especially relating to sperm quality. Objectives, therefore, were to quantify seasonal changes in sperm concentration and morphology for two unionid mussels, Ligumia subrostrata and Lampsilisstraminea, measure intraspecific heterogeneity for sperm morphometry, and develop an efficient method to quantify sperm concentration using a microspectrophotometer. There were no differences in sperm concentration when cells were extracted from the center or at a half centimeter on either side of the visceral mass, during the spawning season. There was a seasonal change in sperm concentration, such that concentration for L. subrostrata ranged from 1.1 × 109 to 19.60 × 109 cells/mL with there being the largest counts between 26 September to 7 November. L. straminea sperm concentration was greatest (20.0 × 109 cells/mL) on 13 September and subsequently decreased. Sperm were uniflagellated and SEM results for L. subrostrata and L. straminea showed mean head length and width (mid-spawning) were 3.38 ± 0.04 µm and 1.61 ± 0.01 µm and 3.37 ± 0.04 µm and 1.61 ± 0.01 µm, respectively. There were close (R2 ≥ 0.85) quadratic associations between hemocytometer counts and absorbance (300, 600, 700 nm). These results provide baseline information to further investigate sperm quality, fertilizing capacity, and cryopreservation for freshwater mussels.

Filter-feeders have differential bottom-up impacts on green and brown food webs.

Nutrient recycling by consumers can strongly impact nutrient availability for autotrophic and heterotrophic microbes, thus impacting functions such as primary production and decomposition. Filter-feeding freshwater mussels form dense, multispecies assemblages in aquatic ecosystems and have been shown to play a critical role in nutrient cycling. Mussel excretion can enhance benthic primary production and influence algal species composition. However, the role of mussels in brown or detritus-based food webs and species-specific differences has received considerably less attention. Here, using mesocosm experiments, we assessed how three species of freshwater mussels that occupy three different phylogenetic tribes influenced benthic algal accrual, ecosystem metabolism, cotton strip decomposition, leaf litter (Acer saccharum) decomposition, and litter-associated fungal biomass measured as ergosterol. Additionally, we measured mussel excretion and biodeposition rates and assessed the stoichiometry (C:N, C:P, and N:P) of the benthic algae, cotton strips, and leaf litter. In comparison to controls without mussels, generally, mussel treatments had higher benthic algal biomass composed of more diatoms, higher gross primary productivity and net ecosystem production rates, and higher cotton strip tensile strength loss, but there was not a difference in ecosystem respiration rates, leaf litter decomposition rates, or fungal biomass. Benthic algae had lower C:N and higher N:P in mussel treatment tanks and cotton strip C:N was lower in mesocosms with mussels. Our results suggest that nutrient regeneration by mussels most strongly regulates green food webs, with some impacts to brown food webs, suggesting that consumers have interactive effects on microbial functioning in freshwaters.

Bioaccumulation of sediment-associated dinonylnaphthalene sulfonates in the freshwater mussel Lampsilis siliquoidea and oligochaete Tubifex tubifex.

Naphthalene sulfonic acids (NSAs) are used primarily as additives in a wide range of industrial products (e.g., rubber materials, coatings, sealants, fuels, paints). Based on modeled physicochemical properties, NSAs would likely partition into sediments or the tissues of biota in an aquatic system. This study examined the potential for three NSAs, dinonylnaphthalene disulfonic acid (DNDS), barium dinonylnaphthalene sulfonate (BaDNS), and calcium dinonylnaphthalene sulfonate (CaDNS), to accumulate in the tissue of a freshwater mussel (Lampsilis siliquoidea) and oligochaete worm (Tubifex tubifex). The ability of L. siliquoidea to depurate accumulated chemical was also assessed. Mussels were exposed via sand spiked with CaDNS for 25 d, and then transferred to clean water where their ability to depurate the chemical over an additional 28 d was monitored. Worms were exposed to each of the three NSAs via spiked sediment for 28 d. NSA concentrations were measured separately in gill, foot, and remaining soft tissues (viscera) for mussels and in whole body tissue samples of worms. For L. siliquoidea, the largest concentration of CaDNS was measured in the gill tissue; once removed from CaDNS exposure, mussels were able to depurate up to 87% of the CaDNS from their tissues in 28 days. The biota-sediment accumulation factors (28-d BSAFs) for T. tubifex were 2.8-5.2, 0.53-0.76, and 0.83-1.11 for DNDS, BaDNS, and CaDNS, respectively. For mussel gill and viscera, BCFK values were 14.07 and 16.39, respectively. When BAFKs were calculated using the concentration of CaDNS in sand, they were 1.11 and 1.29 for mussel gill and viscera, respectively. These values are much lower than what would be necessary to classify this chemical as bioaccumulative; however, the BSAFs for DNDS in T. tubifex indicated a potential biomagnification concern if this compound were to occur in the aquatic environment.

Evaluation of Juvenile Freshwater Mussel Sensitivity to Multiple Forms of Florpyrauxifen-Benzyl.

ProcellaCOR® (active ingredient [ai], florpyrauxifen-benzyl) is an aquatic herbicide registered for use in 2018 for managing invasive and nuisance macrophyte species. Registration studies evaluating its acute toxicity revealed a favorable environmental profile; however, prior to this study, no information existed on the toxicity of florpyrauxifen-benzyl to native freshwater mussels (Family Unionidae), one of the most sensitive and imperiled faunal groups globally. We followed standard acute (96 h) toxicity test guidelines and exposed juvenile Fatmucket (Lampsilis siliquoidea) and Eastern Lampmussel (Lampsilis radiata) to the following formulations or compounds: ProcellaCOR SC and EC formulations, technical grade active ingredient (TGAI, florpyrauxifen-benzyl), and an analytical-grade sample of the weaker florpyrauxifen acid (FA). In all tests, the estimated median lethal concentrations to produce 50% mortality (LC50) were greater than the highest concentration tested of each formulation or compound. The no observable adverse effect concentrations (NOAEC, based on analytical recoveries measured at the highest concentration tested where no toxicity was observed) were TGAI = 26 µg/L, FA = 100,000 µg/L, ProcellaCOR® SC = 193 µg ai/L ProcellaCOR® EC = 585 µg ai/L and the NOAEC values for the registered commercial formulation products (ProcellaCOR® SC and ProcellaCOR® EC) were orders of magnitude greater (3.9× and 11.7×, respectively) than the maximum application rate (50 µg/L). Our results show that the herbicide formulations and compounds tested were not acutely toxic to juveniles of these two species of freshwater mussels, indicating minimal risk of short-term exposure from florpyrauxifen-benzyl applications in the environment for aquatic weed control. However, potential chronic or sublethal effects remain uncharacterized and warrant additional investigation.

Invasive zebra mussel (Dreissena polymorpha) threatens an exceptionally large population of the depressed river mussel (Pseudanodonta complanata) in a postglacial lake.

Freshwater mussels are in decline worldwide, with the depressed river mussel Pseudanodonta complanata being one of the rarest and most endangered species in Europe. Invasive mussels are suspected to be an important factor of decline, but there is little information on their interaction with native species.This study analyzed densities, depth distribution, and individual sizes and weights in one of the largest known populations of P. complanata in Europe in relation to the co-occurring invasive zebra mussel Dreissena polymorpha and other mussel species, using a systematic transect analysis. Pseudanodonta complanata was the dominant unionid species in Lake Siecino reaching densities of up to 26 ind/m2, with half of the specimens found at a water depth of 2.0-4.0 m. Densities were highest on sandy substrates in areas of underwater currents. In contrast, 67% of native Unio tumidus were found at depths < 1 m, indicating different habitat preference.In the study area, 91% of P. complanata, 92% of U. tumidus, and all Anodonta individuals were fouled by D. polymorpha. The dreissenid:unionid mass ratio (mean ± SD; maximum) was 0.43 ± 0.56; 4.22 and 0.86 ± 1.87; 8.76 in P. complanata and U. tumidus, respectively. Pseudanodonta complanata fouled with D. polymorpha were impaired in their anchoring capability and had shell deformations potentially affecting shell closing and filtration activity. Fouling intensity was negatively correlated with unionid density, potentially leading to accelerated population declines.The observed adverse effects of invasive zebra mussels on the depressed river mussel and the difficulties in eradicating established populations of invasive mussels suggest that D. polymorpha should be considered a serious threat to P. complanata. Therefore, the further spread of zebra mussels into habitats with native unionids needs to be avoided by all means.

Temperature-Related Responses of an Invasive Mussel and 2 Unionid Mussels to Elevated Carbon Dioxide.

Zebra mussels (Dreissena polymorpha) have exacerbated the decline of native freshwater mussels (order Unionida) in North America since their arrival in the 1980s. Options for controlling invasive mussels, particularly in unionid mussel habitats, are limited. Previously, carbon dioxide (CO2 ) showed selective toxicity for zebra mussels, relative to unionids, when applied in cool water (12 °C). We first determined 96-h lethal concentrations of CO2 at 5 and 20 °C to zebra mussels and responses of juvenile plain pocketbook (Lampsilis cardium). Next, we compared the time to lethality for zebra mussels at 5, 12, and 20 °C during exposure to partial pressure of CO2 (PCO2 ) values of 110 to 120 atm (1 atm = 101.325 kPa) and responses of juvenile plain pocketbook and fragile papershell (Leptodea fragilis). We found efficacious CO2 treatment regimens at each temperature that were minimally lethal to unionids. At 5 °C, plain pocketbook survived 96-h exposure to the highest PCO2 treatment (139 atm). At 20 °C, the 96-h lethal concentration to 10% of animals (LC10) for plain pocketbook (173 atm PCO2 , 95% CI 147-198 atm) was higher than the LC99 for zebra mussels (118 atm PCO2 , 95% CI 109-127 atm). Lethal time to 99% mortality (LT99) of zebra mussels in 110 to 120 atm PCO2 ranged from 100 h at 20 °C to 300 h at 5 °C. Mean survival of both plain pocketbook and fragile papershell juveniles exceeded 85% in LT99 CO2 treatments at all temperatures. Short-term infusion of 100 to 200 atm PCO2 at a range of water temperatures could reduce biofouling by zebra mussels with limited adverse effects on unionid mussels. Environ Toxicol Chem 2020;39:1546-1557. Published 2020. This article is a U.S. Government work and is in the public domain in the USA. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Salinity tolerance of a rare and endangered unionid mussel, Popenaias popeii (Texas Hornshell) and its implications for conservation and water management.

Unionid mussels are considered sensitive to salinity and there is growing concern in arid and semi-arid regions that declining flows coupled with anthropogenic impacts are amplifying natural salinity levels. In this study, we tested the effects of varying salinity concentrations (3.0, 4.0, 5.0, 6.0, 7.0 and 10.0 ppt NaCl) on survival of adult Popenaias popeii, (Texas Hornshell). This species occurs in the Rio Grande basin of Texas and northern Mexico, an arid to semi-arid stream plagued by salinization, and was recently listed as Endangered under the U.S. Endangered Species Act. We performed 2, 4, and 10-day toxicity tests on individuals from two disjunct populations: Laredo, TX, and the Lower Canyons of the Rio Grande near Big Bend National Park. We found no significant differences in LC50 estimates between populations at 96-hrs or 10-days but significant differences in TUD50s at 5 ppt between populations, which indicates that tolerance does not vary but sensitivity may between these populations. Overlaying LC50 estimates at 10-days for both populations on plots of salinity (ppt) measured over time, we show parts of the Rio Grande periodically approach or exceed 4.0 ppt, indicating these reaches are becoming unsuitable for P. popeii and populations within them at risk.

Modelling the invasion history of Sinanodonta woodiana in Europe: Tracking the routes of a sedentary aquatic invader with mobile parasitic larvae.

Understanding the invasive potential of species outside their native range is one of the most pressing questions in applied evolutionary and ecological research. Admixture of genotypes of invasive species from multiple sources has been implicated in successful invasions, by generating novel genetic combinations that facilitate rapid adaptation to new environments. Alternatively, adaptive evolution on standing genetic variation, exposed by phenotypic plasticity and selected by genetic accommodation, can facilitate invasion success. We investigated the population genetic structure of an Asian freshwater mussel with a parasitic dispersal stage, Sinanodonta woodiana, which has been present in Europe since 1979 but which has expanded rapidly in the last decade. Data from a mitochondrial marker and nuclear microsatellites have suggested that all European populations of S. woodiana originate from the River Yangtze basin in China. Only a single haplotype was detected in Europe, in contrast to substantial mitochondrial diversity in native Asian populations. Analysis of microsatellite markers indicated intensive gene flow and confirmed a lower genetic diversity of European populations compared to those from the Yangtze basin, though that difference was not large. Using an Approximate Bayesian Modelling approach, we identified two areas as the probable source of the spread of S. woodiana in Europe, which matched historical records for its establishment. Their populations originated from a single colonization event. Our data do not support alternative explanations for the rapid recent spread of S. woodiana; recent arrival of a novel (cold-tolerant) genotype or continuous propagule pressure. Instead, in situ adaptation, facilitated by repeated admixture, appears to drive the ongoing expansion of S. woodiana. We discuss management consequences of our results.

Biomass distribution of fishes and mussels mediates spatial and temporal heterogeneity in nutrient cycling in streams.

Animals can play important roles in cycling nutrients [hereafter consumer-driven nutrient dynamics (CND)], but researchers typically simplify animal communities inhabiting dynamic environments into single groups that are tested under relatively static conditions. We propose a conceptual framework and present empirical evidence for CND that considers the potential effects of spatially overlapping animal groups within dynamic ecosystems. Because streams can maintain high biomass of mussels and fish, we were able to evaluate this framework by testing if biogeochemical hotspots generated by stable aggregations of mussels attract fishes. We predicted that spatial overlap between these groups may increase the flux of mineralized nutrients. We quantified how different fish assemblage biomass was between mussel bed reaches and reaches without mussels. We compared fish and mussel biomass at mussel beds to test whether differences in animal biomass mediate their contributions to nutrient cycling through nitrogen and phosphorous excretion. We estimated areal excretion rates for each group by combining biomass estimates with measured excretion rates. Fish biomass was homogeneously distributed, except following a period of low flow when fish were more concentrated at mussel beds. Mussel biomass was consistently an order of magnitude greater than fish biomass and mussel areal excretion rates exceeded fish excretion rates. However, the magnitude of those differences varied spatially and temporally. Mussel excretion stoichiometry varied with changes in assemblage composition, while fish excretion stoichiometry varied little. Biogeochemical hotspots associated with mussels did not generally overlap with fish aggregations, thus, under these conditions, animal processes appear to exert additive ecosystem effects.

A Comparison of the chemical sensitivities between in vitro and in vivo propagated juvenile freshwater mussels: Implications for standard toxicity testing.

Unionid mussels are ecologically important and are globally imperiled. Toxicants contribute to mussel declines, and toxicity tests using juvenile mussels-a sensitive life stage-are valuable in determining thresholds used to set water quality criteria. In vitro culture methods provide an efficient way to propagate juveniles for toxicity testing, but their relative chemical sensitivity compared with in vivo propagated juveniles is unknown. Current testing guidelines caution against using in vitro cultured juveniles until this sensitivity is described. Our objective was to evaluate the relative sensitivity of juvenile mussels produced from both in vitro and in vivo propagation methods to selected chemicals. We conducted 96-h acute toxicity tests according to ASTM International guidelines with 3 mussel species and 6 toxicants: chloride, nickel, ammonia, and 3 copper-based compounds. Statistically significant differences between in vitro and in vivo juvenile 96-h median effect concentrations were observed in 8 of 17 tests, and in vitro juveniles were more sensitive in 6 of the 8 significant differences. At 96 h, 4 of the 8 statistically different tests for a given chemical were within a factor of 2, which is the intralaboratory variation demonstrated in a recent evaluation of mussel toxicity tests. We found that although differences in chemical sensitivity exist between in vitro and in vivo propagated juvenile mussels, they are within normal toxicity test variation. Therefore, in vitro propagated juvenile mussels may be appropriate for use in ASTM International-based toxicity testing. Environ Toxicol Chem 2018;37:3077-3085. © 2018 SETAC.

Effects of nitrate on freshwater mussel glochidia attachment and metamorphosis success to the juvenile stage.

Water quality and contaminants have been frequently identified as critical stressors for freshwater mussels, many species of which are highly imperiled throughout North America and the world. Nutrient pollution, specifically nitrate, has become one of the most prevalent causes of water quality degradation globally, with increasing anthropogenic input from suburban and agricultural runoff, municipal wastewater, and industrial waste. Nitrate acute toxicity is generally low for aquatic species, but the potential effects of nitrate exposure are largely unknown for freshwater mussels, particularly during the parasitic stage of their complex lifecycle. Therefore, this study was designed to determine the effects of short-term nitrate exposure at environmentally relevant concentrations on juvenile production in two freshwater mussel species. Lampsilis siliquoidea and L. fasciola glochidia were exposed to nitrate (0, 11, or 56 mg NO3-N/L) for 24 h before inoculation on a primary host, Largemouth Bass (Micropterus salmoides). Glochidia attachment, metamorphosis success, and total number of juveniles produced were monitored on individual fish. Exposure of L. siliquoidea glochidia to 56 mg NO3-N/L nitrate resulted in a significant (p = 0.02) 35% reduction of total juveniles produced, a combined result of moderate decreases in both glochidia attachment and metamorphosis success. A similar trend (28% reduction; p = 0.06) was evident with 11 mg NO3-N/L. No effects were apparent for L. fasciola, suggesting species-specific differences in responses even among closely related species. These results are the first to suggest that glochidia exposure to nitrate may adversely affect juvenile recruitment in some species. Findings from these studies are important for improving characterization of the hazards of nitrate pollution to aquatic life and this work will help better define the role of water quality in assessing habitat suitability for mussel conservation efforts.