David J. Berg

Mantle biopsy: a technique for nondestructive tissue-sampling of freshwater mussels

Mantle biopsy is a means of obtaining tissue samples for genetic, physiological, and contaminant studies of bivalves; but the effects of this biopsy on survival have not been determined. We describe a simple technique for obtaining such samples from unionacean bivalves and how we compared survival among biopsied and control organisms in field experiments. Survival was not significantly different between treatment and control groups. Power estimates for these results were between 0.42 and 0.73. Results were similar among species and among habitats. Mantle biopsy is a technique that allows genetic, biochemical, and contaminant studies of mussel populations when destruction of individuals should be avoided.

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.

Drosophila (Diptera: Drosophilidae) Response to Changes in Ecological Parameters Across an Urban Gradient

Abstract Understanding the changes in biodiversity correlated with urbanization is essential for monitoring the complex effects of human activity on native ecosystems. We hypothesized that the Drosophila community native to temperate woodland forests would change along a gradient of urbanization, and could therefore serve as a model system in studies on urbanization. We used an urbanization gradient we had previously characterized in Southwest Ohio. Community composition gradually changed along the gradient, although community diversity did not. Abundance varied significantly among sites, with one species, Drosophila melanogaster, increasing in abundance from the least to the most urbanized sites. We used 28 parameters from three sets of environmental data—land cover, vegetation, and temperature and humidity—to build a model with Canonical Correspondence Analysis and characterize the species-environment relationship. The most predictive variables explaining the distribution of the Drosophila community were maximum temperature, maximum saturation deficit, percent lawn cover, average diameter at breast height (dbh) of shrubs and trees, and number of tree species. We conclude that the presence of individual, easily identifiable Drosophila species may serve as robust indicators of the habitat degradation brought about by urbanization, and as ideal models for exploring animal response to urbanization.

Phylogeographic and population genetic analyses reveal Pleistocene isolation followed by high gene flow in a wide ranging, but endangered, freshwater mussel

Freshwater organisms of North America have had their contemporary genetic structure shaped by vicariant events, especially Pleistocene glaciations. Life history traits promoting dispersal and gene flow continue to shape population genetic structure. Cumberlandia monodonta, a widespread but imperiled (IUCN listed as endangered) freshwater mussel, was examined to determine genetic diversity and population genetic structure throughout its range. Mitochondrial DNA sequences and microsatellite loci were used to measure genetic diversity and simulate demographic events during the Pleistocene using approximate Bayesian computation (ABC) to test explicit hypotheses explaining the evolutionary history of current populations. A phylogeny and molecular clock suggested past isolation created two mtDNA lineages during the Pleistocene that are now widespread. Two distinct groups were also detected with microsatellites. ABC simulations indicated the presence of two glacial refugia and post-glacial admixture of them followed by simultaneous dispersal throughout the current range of the species. The Ouachita population is distinct from others and has the lowest genetic diversity, indicating that this is a peripheral population of the species. Gene flow within this species has maintained high levels of genetic diversity in most populations; however, all populations have experienced fragmentation. Extirpation from the center of its range likely has isolated remaining populations due to the geographic distances among them.

A conceptual model linking demography and population genetics of freshwater mussels

Abstract Population viability analysis (PVA) provides a mechanism for analyzing extinction risk by considering processes, such as random fluctuation in demographic features, loss of genetic variation, environmental stochasticity, and the occurrence of catastrophes. Freshwater mussels (Unionoidea) are candidates for PVA because of elevated risk of extinction from anthropogenic activities. We designed a stage-based conceptual model summarizing demographic and genetic changes throughout the mussel life cycle that are associated with changes in population size. We discuss what is known about these stages and the processes that affect transitions between stages. Considerably more information is known about adults than other life stages. Much more work must be done on nonadult life stages because they are potentially vulnerable to disruption via environmental degradation, habitat fragmentation, and loss of vertebrate hosts. New approaches, such as development of molecular identification keys, use of microsatellite markers, and assignment tests to measure dispersal, promise to increase our understanding of nonadult life stages, breeding systems, and linkages among populations. Few studies have attempted to use theory from population biology and conservation genetics to gain insight into strategies for effective conservation. We suggest that more work must be done with species that are not yet critically imperiled because study of such species is likely to yield useful data for PVAs and insight into the mechanisms regulating freshwater mussel populations.

Past climate change drives current genetic structure of an endangered freshwater mussel species

Historical‐to‐recent climate change and anthropogenic disturbance affect species distributions and genetic structure. The Rio Grande watershed of the United States and Mexico encompasses ecosystems that are intensively exploited, resulting in substantial degradation of aquatic habitats. While significant anthropogenic disturbances in the Rio Grande are recent, inhospitable conditions for freshwater organisms likely existed prior to such disturbances. A combination of anthropogenic and past climate factors may contribute to current distributions of aquatic fauna in the Rio Grande basin. We used mitochondrial DNA and 18 microsatellite loci to infer evolutionary history and genetic structure of an endangered freshwater mussel, Popenaias popeii, throughout the Rio Grande drainage. We estimated spatial connectivity and gene flow across extant populations of P. popeii and used ecological niche models (ENMs) and approximate Bayesian computation (ABC) to infer its evolutionary history during the Pleistocene. structure results recovered regional and local population clusters in the Rio Grande. ENMs predicted drastic reductions in suitable habitat during the last glacial maximum. ABC analyses suggested that regional population structure likely arose in this species during the mid‐to‐late Pleistocene and was followed by a late Pleistocene population bottleneck in New Mexico populations. The local population structure arose relatively recently, perhaps due to anthropogenic factors. Popenaias popeii, one of the few freshwater mussel species native to the Rio Grande basin, is a case study for understanding how both geological and anthropogenic factors shape current population genetic structure. Conservation strategies for this species should account for the fragmented nature of contemporary populations.

Salinity tolerance as a potential driver of ecological speciation in amphipods (Gammarus spp.) from the northern Chihuahuan Desert

Abstract Ecological speciation is the process by which barriers to gene flow arise between populations as a result of ecologically based divergent selection. Environmental salinity has been identified as one of the most important ecological drivers influencing distribution, abundance, and species richness of aquatic organisms. Springs of the northern Chihuahuan Desert vary in salinity and are home to the Gammarus pecos (Crustacea:Amphipoda) species complex. We used field experiments to compare salinity tolerance among 9 amphipod populations from geographically discrete habitats differing in ambient salinities and to calculate salinity response distances among populations. Cluster analysis placed populations into 3 groups corresponding to low, medium, and high ambient salinities. Partial Mantel tests revealed significant positive correlations between salinity tolerance and habitat salinity after controlling for other variables, such as geographic distance and neutral genetic differences. Our results provide evidence that ecological speciation could be occurring among amphipod populations at different springs, as indicated by dissimilar physiological responses that are correlated with differences in ambient spring salinities. Gene flow is restricted among populations, the restriction is reinforced by dispersal barriers between springs, and selection might preserve variants that most effectively tolerate local salinity levels. Gammarus diversification in the northern Chihuahuan Desert is driven by vicariance and isolation, along with local selection and adaptation.

Heritability of Heat Tolerance in Zebra Mussel Veligers

Abstract Previous work has suggested that zebra mussel populations differ in temperature tolerance and researchers have hypothesized that these differences have been caused by natural selection. For selection to act on a phenotypic character such as heat tolerance, there must be additive genetic variation for the character within populations. Individuals must have different alleles at loci that contribute to measurable differences in the phenotypic character of interest. We tested the hypothesis that heat tolerance of zebra mussel veligers has an additive genetic component and is therefore heritable. We used a full-sib, half-sib design with a large number of full-sib families (20), half-sib families (5), and offspring (averaging 41) per family (estimated power, β = 0.75). We exposed family groups to a lethal temperature of 34°C and determined individual time-to-death. Our best estimate of heritability (which can range from 0 to 1) was −0.125 (±0.095), which implies that heritability of heat tolerance among veligers was extremely low. These results are surprising given the high genetic diversity reported for North American zebra mussel populations. A lack of additive genetic variation for heat tolerance within the studied population means that heat tolerance of the veliger stage is not likely to increase further via natural selection. Further studies examining both the veliger and adult stages, and examining populations in different watersheds are required to show whether this lack of variation is widespread among North American zebra mussel populations.