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Abstract The gut microbiome is influenced by host species and the environment, but how the environment influences the microbiome of animals introduced into a new ecosystem has rarely been investigated. Freshwater mussels are aquatic fauna, with some threatened or endangered species propagated in hatcheries and introduced into natural systems as part of conservation efforts. The effects of the environment on the freshwater mussel gut microbiome were assessed for two hatchery-propagated species (Lampsilis ovata, Lampsilis ornata) introduced into rivers within their natural range. Mussels were placed in rivers for 8 weeks, after which one subset was collected, another subset remained in that river, and a third subset was reciprocally transplanted to another river in the same river basin for a further 8 weeks. Gut microbiome composition and diversity were characterized for all mussels. After the initial 8 weeks, mussels showed increased gut bacterial species richness and distinct community composition compared to hatchery mussels, but gut microbiome diversity then decreased for mussels that remained in the same river for all 16 weeks. The gut bacterial community of mussels transplanted between rivers shifted to resemble that of mussels placed initially into the recipient river and that remained there for the whole study. All mussels showed high proportions of Firmicutes in their gut microbiome after 8 weeks, suggesting an essential role of this phylum in the gut of Lampsilis species. These findings show that the mussel gut microbiome shifts in response to new environments and provide insights into conservation strategies that involve species reintroductions. 

Abstract We present a newly developed design for a self‐contained benthic chamber for conducting in situ ecosystem experiments in streams, with a focus on biogeochemical processes such as ecosystem metabolism and nutrient cycling. Our design expands upon smaller, portable chamber designs and is meant to answer questions at larger scales. These new chambers allow for a high level of experimental control in the field and can be used to generate spatially explicit data regarding ecosystem processes and to test mechanistic hypotheses. They are built to be deployed within the stream over periods of weeks to months and to withstand natural hydraulic forces of the benthic zone. First, we describe the materials and steps that are needed to construct these chambers in detail. Then, we report the methods and results of a multi‐part, diagnostic field study meant to demonstrate the performance and utility of the design. We quantified solute dynamics using a conservative tracer injection, then we estimated ecosystem metabolism across the study site and performed nutrient additions. We detected asymptotic declines in tracer concentrations, calculated nutrient removal rates, and mapped hotspots of ecosystem metabolism. Flow velocity and water depth imposed limitations, but with appropriate methodological forethought these limitations can be minimized. The capacity of our design to accommodate complex, three‐dimensional habitats and macrofauna, along with the capability to generate spatially explicit data, are the main advances we present. These advances provide a novel method whereby motivated users can connect mechanistic hypothesis testing with natural ecological processes through ecosystem‐level field experiments. 

ABSTRACT Niche partitioning promotes species coexistence. Yet, it remains unclear how phylogeny and morphology influence the trophic niches of closely related aquatic species with shared feeding modes. Freshwater mussels (Family: Unionidae) are a group of filter‐feeding bivalves that are ideal for investigating mechanisms of niche partitioning. Particle size selection and patterns of ingestion are controlled by gill latero‐frontal cirri density (CD) and the number of cilia per cirrus (CC). We investigated trophic assimilation and niche area using stable isotope signatures (𝛿¹³C and 𝛿¹⁵N) and gill morphology with scanning‐electron microscopy for a diverse mussel assemblage from the Sipsey River, Alabama, USA. We predicted that (1) trophic niches and gill morphology would differ within and among species across sites; (2) co‐occurring species would partition food resources; (3) greater phylogenetic distances among species would result in increased trophic dissimilarity; (4) more CC and higher CD would result in a narrower trophic niche area, or more constrained range of food items assimilated. We found that (1) species identity and site influenced gill morphology and stable isotope signatures but that the trophic niche area of a species was only affected by species identity; (2) the average proportion of niche area overlap between co‐occurring species was low across sites (0.04 to 0.18); (3) trophic dissimilarity among species increased with phylogenetic distance; (4) CD but not the number of CC negatively related to trophic niche area. Our results indicate that gill morphology and evolutionary history are likely key factors governing the trophic niches of mussels. In addition, intraspecific variation in gill morphology across sites may either reflect a phenotypic response to differences in local resource availability or suggest that other mechanisms shape particle selection. Examining the interplay among the trophic niche, phylogeny, and morphology among functionally similar species further informs our understanding of the mechanisms facilitating their coexistence. 

ABSTRACT Freshwater mussels (Bivalvia: Unionida) are among the most imperilled freshwater taxa. Yet, there is a lack of basic life history information for mussels, including data on their growth and longevity. These data help inform conservation efforts, as they can indicate whether species or populations may be vulnerable to decline and inform which species may be best adapted to certain habitats. We aimed to quantify growth and longevity in five mussel species from four river systems in the southeastern United States and test whether growth was related to stream flow. We also interpreted our findings in the context of life history theory.To model mussel growth and longevity, we cut radial thick sections from the shells of mussels and used high‐resolution photography to image the shells. We identified annual growth rings (annuli) and used von Bertalanffy growth models to estimate growth rate (K) and maximum age (A_max) across 13 mussel populations. We then used biochronological methods to remove age‐related variation in annual growth in each shell. We tested whether annual growth was correlated with stream flow using discharge‐based statistics.We found substantial variation inKandA_maxamong species and among populations of the same species.Kwas negatively related toA_max. We did not find consistent correlations between annual growth and stream flow.Our estimates ofKandA_maxalign with previous studies on closely related species and populations. They also match the eco‐evolutionary prediction that growth rate and longevity are negatively related. Life history theory predicts that short‐lived species with higher growth rates should be better adapted to environments with cyclical disturbance regimes, whereas longer‐lived species with low growth rates should be better adapted to stable environments. The lack of correlation between annual growth and stream flow suggests that mussel growth may be limited by other factors in our study system.While some species seem to have relatively narrow ranges for growth and longevity, other species show wide variation among populations. This highlights the need for species‐ and population‐specific conservation efforts. Fundamental life history information can be integrated with other species traits to predict how freshwater taxa may respond to ecological threats. 

Abstract Increases in species richness with habitat area (species–area relationship, or SAR) and increases in ecosystem function with species richness (biodiversity–ecosystem functioning, or BEF) are widely studied ecological patterns. Incorporating functional trait analysis into assemblage datasets may help clarify interpretations of SAR and BEF relationships in natural ecological systems. For example, life history theory can be used to make predictions about what species are most important in generating ecosystem function given a certain set of environmental conditions. We used quantitative assemblage data for freshwater mussels at nine sites in western Alabama, USA, to test for SAR and BEF relationships. At each site, we calculated species richness, mussel assemblage density, and two fundamental metrics of ecosystem function: biomass and secondary production. We also tested whether the proportional biomass and production contributions from species belonging to each of three life history strategies—opportunistic strategistsadapted to unstable or frequently disturbed habitats,periodic strategistsadapted to habitats subject to predictable large‐scale disturbances, andequilibrium strategistsadapted to stable habitats—varied longitudinally with stream drainage area, a proxy for habitat area. Species richness increased with stream size (SAR), and both biomass and production increased with species richness (BEF) and mussel density. There were few longitudinal changes in the proportional contributions of the different life history strategy classifications that we used, but the invasive clamCorbicula flumineacontributed proportionally more biomass and production at sites that had smaller drainage areas. This study provides further evidence for a clear longitudinal SAR in stream‐dwelling taxa. It also suggests BEF relationships for biomass and secondary production in natural assemblages but underscores the importance of assemblage density in BEF studies that use observational field data. Variation in proportional biomass and production contributions by different life history strategies was likely limited by the size of the stream size gradient in our study, as contributions were uniformly high for species with life history traits better adapted to stable and productive habitats such as mid‐sized rivers with low or predictable hydrologic disturbance frequencies. This highlights the need to understand how organisms' functional traits govern their relationships to the environment at different scales. 

Abstract Accelerating the design and implementation of environmental flows (e-flows) is essential to curb the rapid, ongoing loss of freshwater biodiversity and the benefits it provides to people. However, the effectiveness of e-flow programs may be limited by a singular focus on ensuring adequate flow conditions at local sites, which overlooks the role of other ecological processes. Recent advances in metasystem ecology have shown that biodiversity patterns and ecosystem functions across river networks result from the interplay of local (environmental filtering and biotic interactions) and regional (dispersal) ecological processes. No guidelines currently exist to account for these processes in designing e-flows. We address this gap by providing a step-by-step operational framework that outlines how e-flows can be designed to conserve or restore metasystem dynamics. Our recommendations are relevant to diverse regulatory contexts and can improve e-flow outcomes even in basins with limited in situ data. 

Abstract Evolutionary and ecological hypotheses of the freshwater mussel subfamily Ambleminae are intensely geographically biased—a consequence of the complete exclusion of Mesoamerican taxa in phylogenetic reconstructions of the clade. We set out to integrate a portion of the Mesoamerican freshwater mussel assemblage into existing hypotheses of amblemine classification and evolution by generating a molecular phylogeny that includes four previously unsampled Mesoamerican genera and nine species endemic to that region. Given the traditionally hypothesized affinity to Nearctic mussels and the understanding that classification should reflect common ancestry, we predicted that (a) Mesoamerican genera would be recovered as members of the recognized tribes of the Ambleminae, and (b) genera would be supported as monophyletic. The mutilocus phylogeny (COI + 28S + 16S) reported herein does not fully support either of those hypotheses. NeitherCyrtonaiasnorPsorulawere supported as monophyletic and we predict several other Mesoamerica genera are also non‐monophyletic. The reconstructed phylogeny recovered four independent lineages of Mesoamerican freshwater mussels and these clades are distributed across the phylogeny of the Ambleminae, including the tribe Quadrulini (Megalonaias), Lampsilini (two lineages:Cyrtonaias explicata/Sphenonaias microdon, andPachynaias), and a previously unrecognized, exclusively Mesoamerican and Rio Grande clade consisting of the generaPsoronaias,PsorulaandPopenaias. The latter clade possesses several morphological characteristics that distinguish it from its sister taxon, tribe Lampsilini, and we recognize this newly identified Mesoamerican clade as a fifth tribe of the Ambleminae attributable to the Popenaiadini Heard & Guckert, 1970. This revised classification more completely recognizes the suprageneric diversity of the Ambleminae.