James A. Stoeckel

Larval Dynamics of a Riverine Metapopulation: Implications for Zebra Mussel Recruitment, Dispersal, and Control in a Large-River System

Zebra mussels (Dreissena polymorpha) in the Illinois River are likely to function as a metapopulation (a system of local populations connected by dispersal). River currents reduce or eliminate internal recruitment by local populations, making them dependent upon external recruitment from propagules (larvae) produced by upriver populations. Because of the importance of external recruitment, understanding the population dynamics of riverine zebra mussels requires knowledge of larval flux. In 1994 and 1995, larval drift past a fixed site on the Illinois River was characterized by pulses of similar-sized individuals. These pulses were composed primarily of D-stage (straight hinge) larvae, not yet competent to settle. The continual passage of pulses of young larvae suggests that the upriver source population(s) spawned in frequent distinct bursts throughout the summer months rather than spawning only once or twice. Overall larval abundance was a poor predictor of availability of competent larvae. Although larvae were frequently present in high numbers, larvae >200 μm in shell height (competent to settle) were present only sporadically at our fixed study site from May to November in 1994, and present only from June to early July in 1995. Settlement at a given site on the Illinois River is likely to be sporadic rather than constant. Colonization and population growth at a given site will occur only if suitable habitat and environmental conditions are available during the windows when competent larvae are available. Changes in mean size as a larval cohort drifted downriver suggested a growth rate of 6.72 μm/d. At this growth rate, most larvae probably travel a minimum of 304.6 km (190.4 miles) before settling. Thus, larvae spawned in the main channel of the lower 70% of the Illinois River are most likely to settle in the Mississippi River. We suggest that colonization and population growth in the upper 70% of the Illinois River are strongly dependent upon larvae produced at upriver source sites in Lake Michigan and the Chicago area waterways. Control of zebra mussels in a given stretch of the river will depend upon control of the upriver source populations.

Seston quality controls zebra mussel (Dreissena polymorpha ) energetics in turbid rivers

Abstract Feeding processes and energetic balance of zebra mussels were both related to the quantity and quality of natural seston. Filtration rate and pseudofeces production increased while clearance rate remained constant with increasing seston concentration. Ingestion rate, assimilation efficiency, and assimilation rate all increased with increasing food quality, measured as the ratio of organic to inorganic material in the seston. Respiration rate did not change with either food quantity or quality. As a result, scope for growth declined with decreasing food quality, and fell below 0 cal mg−1 h−1 at an organic:inorganic ratio of 0.5. The association between feeding processes and food quality appears related to a breakdown in the ability of zebra mussels to selectively ingest high-quality organic particles when the organic content of the seston is low. Ingestion, assimilation efficiency, assimilation rate and scope for growth were all higher when seston was amended with an addition of a natural assemblage of algae. Food quality may be a better indicator of environmental conditions suitable for growth than food quantity. These results suggest that the conditions of high suspended inorganic sediment concentrations in large turbid rivers represent a difficult growth environment for the zebra mussel.

Rapid development of molecular resources for a freshwater mussel, Villosa lienosa (Bivalvia:Unionidae), using an RNA-seq-based approach

Abstract.  Freshwater mussels (Unionidae) are among the most endangered groups of organisms in the world, and their conservation and recovery are priorities throughout North America, especially the southeastern USA. We used a ribonucleic acid sequencing (RNA-seq)-based approach to develop molecular resources for Villosa lienosa, the little spectaclecase. We sequenced barcoded samples (Illumina HiSeq 2000 platform) and assembled (Trans-ABySS) 778,234 contigs (average length  =  707.5 base pairs [bp]) from 162 million filtered reads. We identified 23,742 unigene hits against the National Center for Biotechnology Information nonredundant database and 36,582 microsatellites with sufficient flanking sequence for primer design. Microsatellite validation indicated a 36% polymorphic rate (16/44 tested markers) in the tested population (26 individuals; mean  =  5 alleles/marker). Analysis of differentially expressed genes between heat-stressed and untreated controls enabled us to identify 604 genes involved in stress-response pathways. Real-time polymerase chain reaction validation of gene-expression results using individual samples confirmed RNA-seq patterns (r  =  0.847, p < 0.001). RNA-seq is a powerful tool for rapid development of molecular resources in nonmodel species, and our study is the first large-scale transcriptome project in freshwater mussels. The validated microsatellite set and stress-associated genes are being used in parentage analysis and health-assessment surveys to support mussel conservation.

Establishment of Daphnia lumholtzi (an Exotic Zooplankter) in the Illinois River

Abstract Daphnia lumholtzi, an exotic zooplankter, recently has become established in the Illinois River. This species was first detected at Illinois River Mile (IRM) 121.1 near Havana, Illinois in June of 1995, and exhibited a peak abundance of 22.5/L in August. Population trends and densities in the Illinois River resemble those reported in lakes and reservoirs of the southeastern United States, suggesting D. lumholtzi is adaptable to lotic as well as lentic systems. Individuals were found at sampling sites as far upriver as IRM 195.9.

Novel Polyfluorinated Compounds Identified Using High Resolution Mass Spectrometry Downstream of Manufacturing Facilities near Decatur, Alabama

Concern over persistence, bioaccumulation, and toxicity has led to international regulation and phase-outs of certain perfluorinated compounds and little is known about their replacement products. High resolution mass spectrometry was used to investigate the occurrence and identity of replacement fluorinated compounds in surface water and sediment of the Tennessee River near Decatur, Alabama. Analysis of legacy Per- and polyfluoroalkyl substances (PFASs) revealed a marked increase in concentrations downstream of manufacturing facilities, with the most abundant compounds being perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate (PFBS), and perfluorooctanoic acid (PFOA) as high as 220 ng L-1, 160 ng L-1, and 120 ng L-1, respectively. A series of nine polyfluorinated carboxylic acids was discovered, each differing by CF2CH2. These acids are likely products or byproducts of a manufacturing process that uses 1,1-difluoroethene, which is registered to a manufacturing facility in the area. Two other predominant compounds discovered have structures consistent with perfluorobutanesulfonate and perfluoroheptanoic acid but have a single hydrogen substituted for a fluorine someplace in their structure. A polyfluoroalkyl sulfate with differing mixes of hydrogen and fluorine substitution was also observed. N-methyl perfluorobutane sulfonamidoacetic acid (MeFBSAA) was observed at high concentrations and several other perfluorobutane sulfonamido substances were present as well.

High suspended solids as a factor in reproductive failure of a freshwater mussel

Abstract. Elevated suspended solids are a widespread stressor of aquatic ecosystems, but their effects on growth and reproduction in freshwater mussels are largely unknown. We fertilized experimental ponds to create a gradient in total suspended solids (TSS) and examined the effects of TSS on growth, nutritional status, reproduction, and clearance rate in Ligumia subrostrata. The number of females that became gravid declined sharply with increasing TSS, and no gravid females were found in the highest TSS treatments. The proportion of gravid females was not related to the TSS organic∶inorganic ratio. Fertilization was an all-or-nothing phenomenon. In all females that did become gravid, 98 to 99% of eggs were fertilized regardless of TSS, and total fecundity was unrelated to TSS. Clearance rates declined sharply as TSS increased but showed a threshold relationship in which clearance was uniformly low at TSS > ∼8 mg/L. Reproductive failure probably was not caused by poor body condition or nutritional status because growth (length and mass) and energetic status (measured as caloric density) were not related to TSS. We propose 2 mechanisms that implicate interference of TSS with fertilization as the cause of reproductive failure. Reduced clearance rate could decrease the chance of females encountering suspended sperm during filter feeding, or an increase in pseudofeces production could bind sperm in mucus and lead to its egestion before fertilization. Interruption of fertilization coincident with high TSS is a potential mechanism to explain the lack of mussel recruitment in many locations. Monitoring and reduction of TSS, especially during the spawning season, may help create conditions necessary for maintenance and recovery of mussel populations. More research is needed to explore the generality of this pattern across a broad range of mussel species including those adapted to lotic environments or that use different brooding strategies.

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.

Transcriptomic Profiling of Differential Responses to Drought in Two Freshwater Mussel Species, the Giant Floater Pyganodon grandis and the Pondhorn Uniomerus tetralasmus

The southeastern US has experienced recurrent drought during recent decades. Increasing demand for water, as precipitation decreases, exacerbates stress on the aquatic biota of the Southeast: a global hotspot for freshwater mussel, crayfish, and fish diversity. Freshwater unionid mussels are ideal candidates to study linkages between ecophysiological and behavioral responses to drought. Previous work on co-occurring mussel species suggests a coupling of physiology and behavior along a gradient ranging from intolerant species such as Pyganodon grandis (giant floater) that track receding waters and rarely burrow in the substrates to tolerant species such as Uniomerus tetralasmus (pondhorn) that rarely track receding waters, but readily burrow into the drying sediments. We utilized a next-generation sequencing-based RNA-Seq approach to examine heat/desiccation-induced transcriptomic profiles of these two species in order to identify linkages between patterns of gene expression, physiology and behavior. Sequencing produced over 425 million 100 bp reads. Using the de novo assembly package Trinity, we assembled the short reads into 321,250 contigs from giant floater (average length 835 bp) and 385,735 contigs from pondhorn (average length 929 bp). BLAST-based annotation and gene expression analysis revealed 2,832 differentially expressed genes in giant floater and 2,758 differentially expressed genes in pondhorn. Trancriptomic responses included changes in molecular chaperones, oxidative stress profiles, cell cycling, energy metabolism, immunity, and cytoskeletal rearrangements. Comparative analyses between species indicated significantly higher induction of molecular chaperones and cytoskeletal elements in the intolerant P. grandis as well as important differences in genes regulating apoptosis and immunity.

Evaluation of a Crayfish Burrowing Chamber Design With Simulated Groundwater Flow

Abstract Crayfish are a highly diverse group of freshwater crustaceans with great ecological and economic importance. Many species construct terrestrial burrows within which they reproduce, overwinter, and/or avoid desiccation. Basic life-history and environmental tolerance information is still lacking for many species, particularly in regards to terrestrial portion(s) of their life cycle. In this paper we present and evaluate a design for a crayfish burrowing chamber that allows for experimental control of burrowing conditions, including substrate, groundwater height and flow, as well as direct observation of burrow construction and measurement of groundwater quality. Results of evaluation assays with Cambarus striatus, a widespread secondary burrower native to the southeastern United States show that chambers are amenable to studies examining burrowing behavior, survivorship, growth and water quality. Water table height was easily manipulated in chambers. Seepage rates were strongly affected by substrate type: burrows in coarse substrate filled rapidly but collapsed quickly whereas burrows in fine substrates filled very slowly and did not collapse. Cambarus striatus burrowed readily in fine substrate, produced complex burrows with chimneys, and exhibited high survivorship and positive growth over a 4 week period. Ammonia levels were higher in burrows than in the groundwater flowing through the chambers, but did not significantly affect growth or survivorship of C. striatus. Maximum burrow depth was significantly affected by substrate type, but not presence/absence of chelipeds. Suggested improvements include larger chamber size and the use of mesh barriers to keep crayfish from burrowing against the inner chamber walls.

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.