Alan D. Christian

Population genetics of the freshwater mussel, Amblema plicata (Say 1817) (Bivalvia: Unionidae): Evidence of high dispersal and post-glacial colonization

Over 70% of North American freshwater mussel species (families Unionidae and Margaritiferidae) are listed as threatened or endangered. Knowledge of the genetic structure of target species is essential for the development of effective conservation plans. Because Ambelma plicata is a common species, its population genetic structure is likely to be relatively intact, making it a logical model species for investigations of freshwater mussel population genetics. Using mtDNA and allozymes, we determined the genotypes of 170+ individuals in each of three distinct drainages: Lake Erie, Ohio River, and the Lower Mississippi River. Overall, within-population variation increased significantly from north to south, with unique haplotypes and allele frequencies in the Kiamichi River (Lower Mississippi River drainage). Genetic diversity was relatively low in the Strawberry River (Lower Mississippi River drainage), and in the Lake Erie drainage. We calculated significant among-population structure using both molecular markers (A.p. Φstxa0=xa00.15, θstxa0=xa00.12). Using a hierarchical approach, we found low genetic structure among rivers and drainages separated by large geographic distances, indicating high effective population size and/or highly vagile fish hosts for this species. Genetic structure in the Lake Erie drainage was similar to that in the Ohio River, and indicates that northern populations were founded from at least two glacial refugia following the Pleistocene. Conservation of genetic diversity in Amblema plicata and other mussel species with similar genetic structure should focus on protection of a number of individual populations, especially those in southern rivers.

Trophic position and potential food sources of 2 species of unionid bivalves (Mollusca:Unionidae) in 2 small Ohio streams

Abstract We determined the trophic positions of 2 species of freshwater mussels, Elliptio dilatata and Ptychobranchus fasciolaris, from 2 small streams in central Ohio by measuring stable C and N isotope ratios and digestive fluid enzyme activities. We also examined stable C and N isotopes, microbial biomass, microbial community structure, nutrient (i.e., C, N, and P) concentrations, and contribution of microbial C to total fine particulate organic C (FPOC). We hypothesized that 1) allochthonous inputs compose most of FPOC, 2) mussels use fine particulate organic material (FPOM) as a food source, and 3) mussels respond to the low-protein content of FPOM by showing high protease activity. Microbial C composed 35 to 86% of total FPOC during the autumn sampling period. FPOM stable isotope values varied seasonally, whereas δ13C and δ15N content in mussel tissue was spatially (i.e., among sites) and temporally similar. Mussels were 2 to 4‰ more depleted in δ13C than seasonal FPOM. Digestive fluid enzymes were spatially and temporally stable across species, with activity of esterase > protease > lipase > glucosidase. Lipase:protease of digestive fluids from mussels were <1, indicating a low-protein diet. Our results suggest that microbial biomass in FPOM constitutes a large portion of mussel diet and that mussels assimilate significant amounts of C from this source.

Population genetics and phylogeography of freshwater mussels in North America, Elliptio dilatata and Actinonaias ligamentina (Bivalvia: Unionidae).

Extrinsic and intrinsic forces combined shape the population structure of every species differently. Freshwater mussels are obligate parasites to a host fish during a juvenile stage (glochidia). Elliptio dilatata (ED) and Actinonaias ligamentina (AL) are co‐occurring freshwater mussel taxa with similar North American distribution and share some potential host fish. Using mitochondrial DNA, we determined the genotypes of 190 + individuals from collection sites in at least two tributaries in the Lake Erie and Ohio River watersheds, along with the Ouachita and Strawberry rivers in the southeast. Both species had followed a stepping‐stone model of dispersal, with greater pairwise genetic structure among collection sites of ED. Also, phylogeographical analysis for ED found significant geographical structuring of haplotype diversity. Overall, within‐population variation increased significantly from north to south, with low genetic diversity in the Strawberry River. We calculated significant among‐population structure for both species (ED: ΦST = 0.62, P < 0.001; AL: ΦST = 0.16, P < 0.001). Genetic analysis identified the Ouachita River as an area of significant reproductive isolation for both species. Results for AL indicated dispersal into northern areas from two genetically distinct glacial refugia, where results for ED indicated dispersal followed by low gene flow in northern areas. The conservation strategies for mussels that co‐occur in the same ‘bed’ could be species specific. Species such as ED have management units on the population scale, where AL has a more homogeneous genetic structure across its range.

Nutrient release and ecological stoichiometry of freshwater mussels (Mollusca:Unionidae) in 2 small, regionally distinct streams

Abstract Ecological stoichiometry is the study of the balance of multiple elements in ecological interactions and processes. We investigated the ecological stoichiometry of freshwater mussels across 3 seasons at 2 sites each in an agricultural watershed (Little Darby Creek [LD], Ohio) and a forested watershed (Upper Ouachita River [OR], Arkansas) (2 species/stream). We used nutrient-release experiments to determine C, N, and P content and elemental ratios in seston and in consumer-driven nutrient recycling (CNR) components (mussel soft tissues, shells, biodeposited material, and excreted nutrients). We focused on seasonal patterns of: 1) biodeposition and excretion rates; 2) seston and CNR component % C, % N, % P, C:N, C:P, and N:P; and 3) degree of homeostasis in mussels. Differences in mass-specific biodeposition and excretion rates were driven largely by seasonal factors. Percent P, C:N, C:P, and N:P of seston were seasonally variable in LD, and % C, % N, % P, C:N, C:P, and N:P of seston were seasonally variable in OR. Mussel tissues and biodeposited materials were variable for 3 to 5 of the 6 nutrient metrics in both LD and OR. Mussel shell and soft-tissue nutrient stoichiometry were relatively homeostatic and fell within stoichiometric ranges of other macroinvertebrates, except that mussel tissue had higher % P and lower C:P and N:P than are usually observed in other macroinvertebrates. Mussels can repackage nutrients in stream seston and fine benthic organic matter in the form of biodeposited material and excreted nutrients, thereby providing a significant source of C, N, and P for other benthic organisms.

Development and Assessment of a Sampling Design for Mussel Assemblages in Large Streams

Abstract Freshwater mussel beds of the lower 68 km of the Cache River, Arkansas, were delineated, sampled using dive techniques and a stratified random sampling methodology and analyzed for density and species richness. A total of 38 mussel beds were delineated, 14 major beds (Mbeds) and 24 minor beds (mbeds), and defined by areal extent and mussel density. Analysis of our sampling precision indicated 80% or better confidence levels for a majority of our sites and suggested that a sample size of 15 1-m2 quadrats is sufficient to obtain 80% or better confidence. Our large river diver-assisted sampling methodology has been shown to be a useful and appropriate methodology for obtaining large geographic scale baseline distribution (bed and species), species richness, density and population and community numerical standing crop estimates information where tradeoffs are required in order to complete a project within time and budget constraints.

Long‐lived organisms provide an integrative footprint of agricultural land use

Nitrogen (N) fertilizer runoff into rivers is linked to nutrient enrichment, hydrologic alteration, habitat degradation and loss, and declines in biotic integrity in streams. Nitrogen runoff from agriculture is expected to increase with population growth, so tracking these sources is vital to enhancing biomonitoring and management actions. Unionid mussels are large, long-lived, sedentary, primary consumers that transfer particulate material and nutrients from the water column to the sediments through their filter feeding. Because of these traits, mussels may provide a temporal integration of nitrogen inputs into watersheds. Our goals were to (1) establish a baseline delta15N signature for unionid mussels in watersheds not heavily influenced by agriculture for use in comparative analyses and (2) determine if mussels provide an integrative measure of N sources in watersheds with varying percentages of agriculture across large spatial scales. We compiled tissue delta15N data for 20 species of mussels from seven geographic areas, including 23 watersheds and 42 sample sites that spanned varying degrees of agricultural intensification across the eastern United States and Canada. We used GIS to determine land cover within the study basins, and we estimated net anthropogenic nitrogen inputs (NANI) entering these systems. We then determined the relationship between mussel tissue delta15N and percentage of land in agriculture (%AG) and net anthropogenic N loading. The delta15N of mussel tissue could be predicted from both %AG and net anthropogenic N loading, and one component of NANI, the amount of N fertilizer applied, was strongly related to the delta15N of mussel tissue. Based on our results, mussels occupying a system not affected by agricultural land use would have a baseline delta15N signature of approximately 2.0 pe thousand, whereas mussels in basins with heavy agriculture had delta15N signatures of 13.6 per thousand. Our results demonstrate that mussels integrate anthropogenic N input into rivers at a watershed scale and could be a good bioassessment tool for tracking agriculture N sources.

Freshwater Mussel (Bivalvia: Unionidae) Assemblages of the Lower Cache River, Arkansas

Abstract Freshwater mussel beds of the lower 68 km of the Cache River, AR, were delineated and sampled using diving and stratified random sampling methodology to determine species richness, density, size structure, and population and community numerical standing crop (CNSC). A total of 38 mussel beds were delineated, including 14 major beds (Mbeds) and 24 minor beds (mbeds). Twenty six species were collected, four of which were previously unknown from the Cache River. Amblema plicata, Megalonaias nervosa, and Plectomerus dombeyanus were the most abundant. Estimates of CNSC ranged from 3705 ± 1908 to 122,115 ± 24,194 individuals in Mbeds with mean densities ranging from 6.2 to 44.1 mussels/m2. Nine of 16 species with > 10 individuals had a unimodal size frequency distribution and the other seven had multi-modal distributions. This study found impressive mussel assemblages in the lower Cache River, previously thought to contain only refugial pockets of mussel assemblages. Further monitoring of some species is recommended based on lack of recruitment.

An introduction to directions in freshwater mollusk conservation: molecules to ecosystems

The goal of conservation biology is to protect Earth’s biological diversity. Biodiversity includes genetic diversity (the number and frequency of genes within species), species diversity (i.e., plants, animals, and microorganisms on Earth), and assemblage/ecosystem diversity (McNeely et al. 1990). Thus, biodiversity can be studied from a multitude of disciplinary perspectives, ranging from molecular genetics to evolutionary biology, and from autecology (e.g., life history) through population, community, and ecosystem ecology. Moreover, studies of biodiversity often address biotic and abiotic factors that influence biodiversity at spatial scales that range from microhabitat to global. The more that is known about biodiversity, the more likely it is that scientists and conservationists will be able to protect biodiversity. Thus, scientific knowledge about organisms drives conservation efforts. However, we know far more about some groups of organisms than we do about others. For example, Strayer (2006) reported that the best-studied freshwater invertebrate groups have roughly the same number of described species as do freshwater fishes, but the number of published papers about those freshwater invertebrates is 1/10 the number of published papers about freshwater fishes. Freshwater mollusks belong to one of the more intensively studied invertebrate groups. Freshwater mollusks are among the most imperiled taxonomic groups in the world, and they constitute 708 of the ;7000 species included in the 2002 International Union for the Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species (Lydeard et al. 2004). Furthermore, 42% of the 693 recorded animal species extinctions are mollusks, and 99% of the mollusks that have become extinct were nonmarine taxa. Freshwater bivalves of the superfamily Unionoidea are distributed worldwide, but they are most diverse in North America. Between 850 and 900 freshwater bivalve species are recognized, 200 of these species are on the IUCN Red List, and 189 of the listed species are located in the USA (Lydeard et al. 2004). Thirty-seven of the 297 known North American taxa are presumed extinct, and another 165 are considered possibly extinct, critically imperiled, imperiled, or vulnerable (Master et al. 2000). The scorecard is even more dismal for freshwater gastropods in the USA. Approximately 60 freshwater gastropod species in the USA are presumed extinct, 20 are on the US federal endangered or threatened species list, and another 290 species are of conservation concern (Johnson 2003). In other words, 9% of all freshwater gastropod species in the USA are extinct, and 48% are conservation targets. This rate of imperilment exceeds that of every other major animal group in North America (Perez and Minton 2008). Because of the growing concern about the conservation of freshwater mussels in North America, stakeholders met in 1995 to discuss freshwater mussel declines and to gather information on trends, research needs, and recovery activities, and they drafted a National Strategy for the Conservation of Native Freshwater Mussels. The 2007 Symposium of the Freshwater Mollusk Conservation Society (FMCS; http://ellipse.inhs.uiuc.edu/FMCS/) was held in Little Rock, Arkansas, from 13 to 15 March 2007. The theme of the symposium was Directions in Freshwater Mollusk Conservation: Molecules to Ecosystems. The plenary session of the symposium was structured to provide information to support revision of the 1 E-mail addresses: achristian@astate.edu 2 john.harris@arkansashighways.com

Niche Partitioning of the Sympatric Yellowcheek Darter Etheostoma moorei and Rainbow Darter Etheostoma caeruleum in the Little Red River, Arkansas

Abstract We studied how habitat selection influences the distribution and densities of a stenotypic yellowcheek darter Etheostoma moorei (Raney and Suttkus) and eurytypic rainbow darter E. caeruleum (Storer) in a headwater stream subjected to periodic drying. We spatially sampled in habitats within riffle substrates and at four sites four to six times per year in the Middle Fork of the Little Red River. Yellowcheek darters were associated with cobble and gravel more than rainbow darters. Rainbow darters moved along the substrate surface and positioned themselves further downstream in riffles, whereas yellowcheek darters occurred further upstream in or near crevices between gravel and cobble. Neither species was found in the hyporheic zone during riffle drying. The rainbow darters used pools during riffle drying but not yellowcheek darters, which could explain recolonization of previously dry upstream sites by rainbow darters but not yellowcheek darters. Rainbow darters were at their greatest densities in upstream riffles where yellowcheek darters had not recolonized, suggesting niche partitioning. These temporal differences in microhabitat selection appear to enable the coexistence of these two sympatric and ecologically similar darters.

Distribution, Population Characteristics, and Physical Habitat Associations of Black Bass (Micropterus) in the Lower Eleven Point River, Arkansas

Abstract We report the longitudinal distribution and population characteristics of Micropterus dolomieu (Smallmouth Bass), M. salmoides (Largemouth Bass), and M. punctulatus (Spotted Bass) of the lower Eleven Point River of Arkansas. Smallmouth Bass were the most abundant species collected, followed by Largemouth Bass and Spotted Bass. Abundance of Smallmouth Bass was greatest upstream and declined significantly downstream; abundance of Spotted Bass was significantly greater downstream. Largemouth Bass were evenly distributed throughout the river. Associated with these species distribution trends were a downstream decline in stream slope and velocity, and an increase in the proportion of pools relative to riffles and runs. Proportional stock structures were similar and relatively low for each species (range = 22.9–32.3). Growth rates of Smallmouth Bass and Spotted Bass were high relative to other study populations, with age-3 fish reaching ≈300 mm total length. Diets of adult Smallmouth Bass and Spotted Bass were similar. Diets of Smallmouth Bass varied by season (increased feeding on insects during the summer months) and length group (transition of feeding on insects to fishes to crayfishes). Relative weight was greater for Largemouth Bass (Wr = 96) and Spotted Bass (95) than for Smallmouth Bass (89). Based on habitat, abundance, diet and condition factors, the Arkansas portion of the Eleven Point River contains a viable and balanced population of black bass.