Internal shell annuli yield inaccurate growth estimates in the freshwater mussels Elliptio complanata and Lampsilis radiata

1. The objective of this study was to compare, in freshwater bivalves, growth rates inferred from the spacing of internal annuli to those obtained directly by measuring annual changes in shell length.
2. The unionid mussels studied were Elliptio complanata and Lampsilis radiata from a pond in the north-eastern U.S.A. Age was inferred from internal lines of 157 E. complanata and twenty-five L. radiata . Actual length change was determined from comparisons of annual remeasurements of marked E. complanata ( n = 520) in 1992–95 and L. radiata ( n = 81) in 1993–95.
3. High retrieval rates of painted mussels demonstrated the efficacy of the marking method.
4. Annual length changes determined by remeasurement were significantly lower than annual length changes predicted by length-at-age data from internal annuli. Should this be a common occurrence, past estimates of annual growth based on annuli are probably too large, and unionid mussels may be much older than previously assumed.     

Non-annual external annuli in the freshwater mussels Anodonta grandis grandis and Lampsilis radiata siliquoidea*

1. Two species of freshwater mussels, Lampsilis radiata siliquoidea and Anodonta grandis grandis were measured and permanently marked with pointed, plastic tape positioned at the postero-lateral edge of the shell. Mussels were returned to original conditions at two sites in an oligotrophic lake, retrieved at yearly intervals, re-measured, and external annuli that had been added since marking were counted. 2. External annuli were formed much less frequently than annually; the overall median number of annuli formed each year was 0.5. In one of the four populations studied, the rate of annulus formation was >1 in small animals and <1 in large ones. Many mussels showed no new external annuli at all, even several years after marking. 3. Ford-Walford plots of shell annuli did not yield consistent indices of shell growth. Repeated measurement of mussels in successive years showed that estimates of growth based on shell annuli consistently overestimated real shell growth.     

Substratum patch selection in the lacustrine mussels Elliptio complanata and Pyganodon grandis grandis

1. Sediment selection was investigated under controlled conditions in two common lake-dwelling species of freshwater mussels (Bivalvia: Unionidae), Elliptio complanata and Pyganodon grandis grandis .
2. Sediment choice was determined in six independent experiments under controlled conditions by distributing mussels randomly or evenly in tanks containing patches of sand and mud, and following their movement among sediment patches in experiments lasting between 30 and 45 days.
3. In all experiments, both species were found most frequently in muddy sediment patches. Movement toward muddy patches occurred rapidly: an average of nearly 80% of Pyganodon grandis grandis were found in mud after 30 days. Elliptio complanata moved rapidly to patches of mud at the start of experiments, but occupation of muddy sediments appeared to decrease after about 30 days.
4. Our results contrast with many field studies that suggest populations of lake-dwelling freshwater mussels infrequently inhabit mud and silt. We therefore postulate that large-scale mussel distribution in lakes is influenced most strongly by factors other than sediment composition.     

Effects of selected pharmaceutical products on phagocytic activity in Elliptio complanata mussels

Municipal wastewaters are recognized as a major source of pharmaceutical and personal care products to the aquatic environment, thereby exposing biota to unknown chronic effects. This study sought to examine the immunotoxic effects of pharmaceutical and urban waste products on the freshwater mussel Elliptio complanata. Hemolymph samples were collected and treated in vitro with increasing concentrations of various drugs (bezafibrate, carbamazepine, fluoxetine, gemfibrozil, morphine, naproxen, novobiocin, oxytetracycline, sulfamethazole, sulfapyridine and trimethoprim) and urban waste related chemicals (coprostanol, caffeine, cotinine) for 24 h at 15 degrees C. In a parallel experiment, mussels were caged and placed in two final aeration lagoons for the treatment of domestic wastewaters. At the end of the exposure period, hemolymphs were tested for phagocytic activity, intracellular esterase activity, cell adherence and lipid peroxidation (LPO). The products that most increased phagocytosis were bezafibrate, gemfibrozil and trimethoprim, while novobiocin and morphine reduced its activity. Intracellular esterase activity was reduced most strongly with sulfamethazole, novobiocin, gemfibrozil, bezafibrate and carbamazepine. Cell adherence was decreased by oxytetracycline, novobiocin and naproxen, and increased by gemfibrozil, bezafibrate and sulfapyridine. Exposure to these products also modulated LPO in hemocytes. Coprostanol and naproxen were more potent to reduce LPO while novobiocin and sulfapyridine were the most potent to induce LPO. The potential to induce LPO was positively correlated with the number of functional groups on the molecule (i.e., its nucleophilicity). Mussels exposed to domestic wastewater treatment plant aeration lagoons had decreased intracellular esterase and phagocytic activity as well, suggesting immunosuppression. PPCPs (pharmaceuticals and personal care products) that are recognized to disrupt cytokine signalling network by the nitric oxide pathway and cell permeability were generally the most potent ones. The data suggest that PPCPs have the potential to cause adverse effects on the immune system of bivalves.     

Alterations in DNA metabolism in Elliptio complanata mussels after exposure to municipal effluents

This study sought to examine the genotoxic potential in Elliptio complanata freshwater mussels exposed to a physically and chemically treated municipal effluent before and after ozone treatment. Mussels were continuously exposed to increasing concentrations of the effluents for 14 days. Genotoxicity was determined by tracking changes in key enzymes for purine and pyrimidine synthesis (dehydrofolate reductase and aspartate transcarbamoylase), catabolism of purines (xanthine oxido-reductase) and DNA strand-break levels as determined by the alkaline precipitation assay. Other biomarkers related to xenobiotic biotransformation (cytochrome P4503A and glutathione S-transferase activities), metal metabolism (labile zinc and redox state of metathioneins) and oxidative stress (superoxide dismutase activity) were also determined in the mussels. The data revealed that dehydrofolate reductase activity was reduced by the initial effluent and increased by the ozonated effluent. Aspartate transcarbamoylase activity was significantly induced only with the ozonated effluent. The levels of DNA strand breaks responded in a biphasic manner in mussels exposed to the physically and chemically treated effluent where an initial decrease was observed at a low effluent concentration (3% v/v) followed by an increase in DNA strand breaks at a higher effluent concentration (20%). This response pattern was lost in the ozonated effluent, where only a decrease in DNA breaks was found. Xanthine oxidoreductase activity was not significantly affected but did correlate significantly with dehydrofolate reductase activity. Multivariate factorial and canonical analyses revealed that oxidative stress and metal/xenobiotic metabolism markers were strongly correlated with DNA strand breaks in mussels, suggesting that the presence of metals (zinc) and planar hydroxylated hydrocarbons present in these effluents were strong contributors to the observed response. We conclude that municipal effluents contain a complex mixture of pollutants that could modulate DNA synthesis and repair mechanisms in mussels.     

Toxicological effects of primary-treated urban wastewaters, before and after ozone treatment, on freshwater mussels (Elliptio complanata)

This study examined the toxic potential of a primary-treated municipal effluent, before and after ozonation, in freshwater mussels. Animals were exposed to various concentrations (0, 1, 3, 10 and 20% v/v) of a primary-treated effluent and also after a treatment with ozone at 10 mg/L in continuous flow-through mode for seven weeks. A suite of biomarkers was used to assess the potential toxic effects of various contaminants typically present in municipal wastewaters: heavy metal metabolism (metallothioneins and labile zinc), cytochrome P4501A1 and 3A4, glutathione S-transferase activities (biotransformation of organic compounds), lipid peroxidation and xanthine oxidoreductase (oxygen radical scavenging), DNA damage, mitochondrial electron transport activity at various temperatures and gonad lipid levels (cellular energy allocation) and aspartate transcarbamoylase and dihydrofolate reductase (gonad activity). On the one hand, some biomarkers, including metallothioneins, labile zinc, glutathione S-transferase, cytochrome P4503A4 activity, dehydrofolate reductase and aspartate transcarbamoylase, were readily decreased. In contrast, these biomarkers, cytochrome P4501A1, gill lipid peroxidation, DNA strand breaks in gills and digestive gland, mitochondrial electron transport at high and low temperatures (temperature-dependent activity) and total gonad lipids, were readily increased. In general, ozone treatment reduced adverse effects by either decreasing the intensity of the toxic responses or increasing the threshold concentration. For gill lipid peroxidation, however, intensity was greater at a higher threshold concentration. Ozone treatment eliminated the temperature sensitivity of the mitochondrial electron transport system, indicating a loss of interaction between temperature and urban pollution in terms of energy expenditure in mussels. Ozone treatment could significantly decrease either the toxic potency or intensity of urban pollutants at the expense of increased oxidative stress in gills of freshwater mussels.     

The growth of juvenile native mussels (Elliptio complanata) in lakes varies with sediment characteristics and site exposure

1. Unionid mussels are among the largest and longest-lived freshwater invertebrates and can play an important role in these ecosystems. They are also one of the most endangered groups of organisms. The juvenile stage is a particularly vulnerable part of mussel life history and is one of the most poorly known.
2. I compared the growth of young Elliptio complanata at 17 nearshore sites, in shallow (polymictic) and stratified lake basins, along gradients of sediment characteristics and site exposure (effective fetch). At each site, the growth of six to 14 small (20–68 mm) mussels collected on the sediment surface was measured, using internal growth lines. The growth of very young endobenthic mussels (22–40 mm) was also measured on four to six mussels at each of two sites.
3. Juveniles spend several years in the sediments, and I found that during this period growth is not constant but declines rapidly with age. Shifts in δ¹⁵N signatures suggest that juveniles change their habitat use towards a more planktonic baseline around the time of maturation, when they reach a length of 30–50 mm. Identical δ¹³C signatures in juveniles and adults suggest that both rely on food of planktonic origin, whether deposited or suspended.
4. The growth of juvenile mussels varies in a complex but predictable manner with sediment characteristics and wind-driven physical forces. Growth was highest in fine sediments with low organic content, in highly organic but coarse sediments, and in protected nearshore areas with low effective fetch. Interestingly, I also found high growth rates at exposed nearshore areas with fine sediments, suggesting that areas where bottom topography creates a refuge from currents and waves may provide particularly good conditions for the early growth of juvenile mussels.
5. Some parts of the shoreline may be more important than others for native mussel populations, and if we can identify those, they may warrant additional protection.

     

Evaluation of estrogenic effects of municipal effluents to the freshwater mussel Elliptio complanata

Municipal effluents are an important source of estrogens to the aquatic environment. The purpose of this study was to examine the estrogenicity of municipal effluents to the indigenous freshwater mussel, Elliptio complanata. First, estradiol-binding sites in gonad homogenates were characterized to determine the binding affinity and specificity of estrogens. Mussels were exposed to increasing concentrations of a municipal effluent for 96 h at 15 degrees C. In another experiment, mussels were placed in cages and submerged for 62 days at 1.5 km upstream and 5 km downstream of a municipal effluent plume in the St. Lawrence River. Mussels were harvested for assessment of vitellogenin-like proteins in the hemolymph and determination of total lipid, carbohydrate and protein in the gonad. The presence of specific estrogen-binding sites was found in both male and female gonads. Binding of estradiol to cytosol proteins reached saturation, yielding a dissociation constant of 0.4 nM. Vitellogenin (Vg) levels increased significantly in both the hemolymph and the gonad after exposure to the effluent. Moreover, females appeared to be more sensitive than males to producing Vg. Mussels exposed in situ to contaminated surface waters had higher levels of Vg at the downstream site, again, females had higher levels of Vg than did males. On the other hand, lipid and sugar levels in male gonads were significantly increased at the downstream site. Moreover, mussels at the downstream site had decreased shell growth length and increased total and soft tissue weights. We conclude that municipal effluents contain bio-available xenoestrogens at levels sufficient to elicit effects in freshwater mussels.     

Within- and among-lake variation in shell morphology of the freshwater clam Elliptio complanata (Bivalvia: Unionidae) from south-central Ontario lakes

To compare the relative magnitude of variation in shell morphology within and among lakes, Elliptio complanata were collected from low and high exposure areas in each of four small lakes in south-central Ontario. Nested ANOVA's on shell length, height, width and weight revealed that shell morphology varied much more between sites of differing exposure within a lake than among lakes of differing alkalinity. Canonical variates analysis showed that clams from high exposure areas had larger and proportionately taller and heavier shells than those from low exposure areas. There was no relationship between alkalinity of lakes and shell morphology. These results suggest that the use of unionid shell morphology to predict long-term whole lake water chemistry (e.g. alkalinity) requires sampling designs which take into account within-lake variation in shell morphology.     

Effects of municipal effluents on serotonin and dopamine levels in the freshwater mussel Elliptio complanata

Sex differentiation and gametogenesis represent critical steps in the reproductive process and are subject to hormonal control by serotonin, dopamine and steroids such as estradiol-17beta and testosterone. The purpose of this study sought to examine the endocrine-disrupting activity that a primary-treated municipal effluent might have on the metabolism of biogenic amine levels. First, serotonin receptors transfected in Chinese hamster ovary (CHO) cells were used to screen for the presence of serotonin receptor agonist or antagonist. Second, one group of Elliptio complanata mussels were exposed to single compounds likely to be found in municipal wastewaters and another group was exposed in situ to the municipal effluent plume for 90 days in experimental cages. Results showed that solid phase C-8 extracts of surface water downstream a municipal effluent could activate the transport of serotonin by receptors at a distance of at least 5 km from its outfall thereby indicating the presence of serotonin mimics in the effluent dispersion plume. Levels of serotonin and monoamine oxidase (MAO) activity in nerve ganglia of mussels exposed for 90 days to the municipal effluent were, respectively, reduced and increased at a distance 10-km downstream. Injections of estradiol-17beta and nonylphenol in mussels decreased the levels of serotonin and dopamine, but increased MAO activity in the gonad and nerve ganglia. Exposure to estrogenic chemicals present in municipal effluents may therefore alter the normal metabolism of serotonin and dopamine, both of which are involved in sexual differentiation in bivalves and fish. Chemicals acting through E2 receptor-mediated pathways and serotonin receptors are likely to cause the observed effects.     

Biomass and annual production of the freshwater mussel Elliptio complanata in an oligotrophic softwater lake

SUMMARY. An extensive survey of Mirror Lake, New Hampshire, was carried out by divers with SCUBA to assess the importance of the freshwater mussel Elliptio complanata in this softwater lake ecosystem. Density (0.032 adults m⁻²), biomass (52 mg m⁻² as dry organic matter) and annual production (6.4 mg m⁻² as dry organic matter) of the mussel population are low when compared with results from other studies, corresponding with the general observation that mussels are scarce in soft, oligotrophic waters. We reject the traditional view that the low mussel density is a result of low calcium concentrations in Mirror Lake, and propose that mussel populations may be regulated by a scarcity of appropriate fish hosts in unproductive lakes. Elliptio complanata is probably not important in the metabolism or biochemistry of the Mirror Lake ecosystem.     

Effects of pharmaceutical products and municipal wastewaters on temperature-dependent mitochondrial electron transport activity in Elliptio complanata mussels

The advent of global warming has given rise to the issue on how temperature impacts the susceptibility of ectothermic organisms to pollution. The purpose of this study was to examine the effects of pharmaceutical products and domestic wastewaters on temperature-dependent mitochondrial electron transport activity in the freshwater mussel Elliptio complanata. Mitochondria from mussels were freshly prepared and exposed to increasing concentrations of various pharmaceutical products known to be found in municipal effluents for 30 min at 4, 12 and 20 degrees C. Electron transport activity as well as lipid peroxidation and DNA strand breaks were determined in the mitochondria. Next, mussels were placed in the aeration lagoons of two municipal wastewater treatment plants for 30 days. Mitochondrial electron transport (MET), temperature-dependent MET (MET(T)) and lipid peroxidation in gonad were then determined. The results show that all products were able to increase MET activity, but at two different ranges of threshold concentration. Certain pharmaceutical products (i.e., ibuprofen, cotinine, fluoxetine, coprostanol and trimethoprim) increased MET(T) at a lower threshold concentration than observed at 20 degrees C. Products of lesser potency in reducing lipid peroxidation were those that produced more DNA strand breaks in mitochondria. Both MET and MET(T) were significantly increased in mussels exposed to aeration lagoon effluents. Lipid peroxidation was also increased in the gonad and was significantly correlated with MET and MET(T) activities. The data indicate that pharmaceutical products and municipal effluents increase respiration rates in isolated mitochondria, such that interaction with temperature could enhance the susceptibility of mitochondrial energy production and oxidative stress in environments contaminated by domestic wastewater.     

Handling effects on survival and growth of plain pocketbook Lampsilis cardium (Rafinesque, 1820) freshwater mussels

Hydrobiologia - Handling freshwater mussels may have negative impacts on their survival, growth, or reproduction, and this may affect our interpretation of how mussels respond to research and...     

Seasonal variation in vertical and horizontal movement of the freshwater bivalve Elliptio complanata (Mollusca: Unionidae)

1. Vertical and horizontal movement were studied in the freshwater bivalve Elliptio complanata at a sandy site in an oligotrophic lake over 3 years. Mussel movement did not vary significantly between day and night. On average, between 2 and 8% of 527 mussels moved each month during the ice-free season and the distance travelled by moving mussels averaged 0.6 cm day^–1.
2. Mussels were endobenthic during the winter, emerged from the sandy substrate in mid-May, peaked in sediment surface abundance in July, and descended into the sediments for the winter in September–October. Vertical displacement of mussels was closely correlated with water temperature although daylength may play a role. Mussels apparently move very little beneath the sediment during the winter.
3. The number of mussels moving horizontally at any given time was linearly correlated with daylength, but the distance travelled during a sampling period was related to daylength in a non-linear fashion. Greatest horizontal displacement of epibenthic mussels was found during spring and early summer, coincident with spawning in E. complanata     

Seasonal variation in vertical and horizontal movement of the freshwater bivalve Elliptio complanata (Mollusca: Unionidae)

1. Vertical and horizontal movement were studied in the freshwater bivalve Elliptio complanata at a sandy site in an oligotrophic lake over 3 years. Mussel movement did not vary significantly between day and night. On average, between 2 and 8% of 527 mussels moved each month during the ice-free season and the distance travelled by moving mussels averaged 0.6 cm day^–1.
2. Mussels were endobenthic during the winter, emerged from the sandy substrate in mid-May, peaked in sediment surface abundance in July, and descended into the sediments for the winter in September–October. Vertical displacement of mussels was closely correlated with water temperature although daylength may play a role. Mussels apparently move very little beneath the sediment during the winter.
3. The number of mussels moving horizontally at any given time was linearly correlated with daylength, but the distance travelled during a sampling period was related to daylength in a non-linear fashion. Greatest horizontal displacement of epibenthic mussels was found during spring and early summer, coincident with spawning in E. complanata     

Evaluation of a nonlethal technique for hemolymph collection in Elliptio complanata, a freshwater bivalve (Mollusca: Unionidae)

Hemolymph, the circulatory fluid of bivalves, transports nutrients, respiratory gases, enzymes, metabolic wastes, and toxicants throughout the body. Hemolymph can provide information pertinent to health assessment of animals or populations, but is not commonly used in freshwater bivalves partly because of the lack of tested, practical techniques for its nonlethal collection. The objective of this study was to evaluate the effect of hemolymph collection on the growth and survival of Elliptio complanata, a freshwater bivalve (Unionidae). We describe a simple technique for the collection of hemolymph from the anterior adductor muscle sinus of E. complanata. To evaluate the effect of hemolymph sampling on mussel survival and growth, 30 mussels sampled using the technique and 30 unsampled controls were followed for 3 mo post collection. Nine animals were sampled 3 times over 7 mo to monitor effects of repeated sampling. No negative impacts on survival or growth were observed in either the singly or repeatedly sampled animals. We also compared the composition of hemolymph collected from the adductor muscle sinus with that collected from the ventricle of the heart. Calcium levels and cell count of hemolymph obtained from the adductor sinus and ventricle were significantly different. There was no significant difference between collection sites for magnesium, phosphorus, ammonia, protein, sodium, potassium, or chloride. We conclude that collection of hemolymph from the adductor sinus is safe for sampled E. complanata and should be explored as a relatively non-invasive, and potentially useful, approach to the evaluation of freshwater mussel health.     

Endocrine disruption and health effects of caged mussels, Elliptio complanata, placed downstream from a primary-treated municipal effluent plume for 1 year

Freshwater mussels, Elliptio complanata, were caged in special benthic pens and were immersed at one upstream (Ups) site and two downstream sites (8 and 11 km) of a primary-treated municipal effluent plume for 1 year. The levels of metallothionein-like proteins (MT), lipid peroxidation, protein-free DNA strands and glutathione S-transferase (GST) activity were assayed in digestive gland, gill and gonad tissues to evaluate biological effects and damage. The levels of monoamines (serotonin and dopamine) in nerve ganglia, ATP-dependent transport activity and monoamine oxidase (MAO) activity were also investigated in the homogenates, synaptosomes and mitochondria, respectively. Results showed that significant amounts of sediment accumulated in cages and 82% of mussels survived the yearlong exposure period at the downstream sites. MT-like proteins were induced in all tissues with the following response intensity: gill (3-fold), digestive gland (1.4-fold) and gonad tissues (1.3-fold). Lipid peroxidation decreased (2.5-fold) in digestive gland but increased in gill (1.6-fold) and in gonad tissues (1.5-fold). GST activity was readily increased in digestive gland (2.5-fold), suggesting the presence of organic contaminants in the plume. Levels of protein-free DNA strands did not vary significantly in digestive gland and gill tissues but were significantly reduced in gonad tissues (2.5-fold) relative to the upstream site. In visceral nerve ganglia, both serotonin and ATP-dependent serotonin transport decreased 1.7-fold with a 4-fold increase of 5-hydroxyindole acetate (5-HIAA, a serotonin metabolite) level relative to the upstream site. However, MAO activity was somewhat reduced at downstream sites (0.7- to 0.9-fold of the activity at the upstream site). Dopamine levels were found to be decreased (1.5-fold), but dopamine ATP-dependent transport activity was increased 1.8-fold, suggesting reduced dopaminergic activity. These results indicate that estrogenic chemicals are likely at play, and the increased dopamine and decreased serotonin ATP-dependent transport suggest that the municipal plume was serotonergic for mussels located at the downstream sites. Mussels exposed for 1 year display a complex but characteristic pattern of responses that could lead to harmful health effects including neuroendocrine disruption of reproduction.     

Evaluation of potential health risks to Eastern Elliptio (Elliptio complanata) (Mollusca: Bivalvia: Unionida: Unionidae) and implications for sympatric endangered freshwater mussel species

Conservation efforts for freshwater musselspecies require identification and evaluationof potential health risks to populations. Sampling large numbers of individual mussels,however, could damage the small, fragile,extant populations of imperiled mussel species. In this study, a small sample size was designedto reduce lethal sampling, costs, andenvironmental disruption, while still allowing95% confidence in detecting health risks of25% or greater prevalence in the population. Health assessments were conducted on twentyspecimens of the Eastern Elliptio, Elliptio complanata, collected from two NorthCarolina sites as part of a survey to evaluatepotential disease threats to mussels in theregion. Bacteriological sampling of thegastrointestinal tracts yielded 18 aerobicbacterial species, of which Aeromonashydrophila (55.0%), Enterobacter spp.(40.0%), and Bacillus spp.(30.0%) were predominant. Histologicallesions of internal organs included mild tomoderate digestive gland atrophy andinflammation in one mussel, and mild tomoderate parasitism in several individuals. Adistinct difference in parasite prevalence wasevident between infections in E.complanata from the two collection sites. Thetrematode metacercaria of Homalometronarmatum and what appears to be three gillciliate species, the most abundant beingtentatively identified as the scyphidiidperitrich Mantoscyphidia sp., were foundin mussels from one site only. This studydemonstrates a comprehensive diagnosticapproach incorporating multiple modalities toassess the health status of mussel populations,while minimizing the sample size required toobtain valuable information. Furthermore, thisstudy provides baseline health data of E.complanata at two sites in south-central NorthCarolina and suggests the potential usefulnessof E. complanata as an environmentalbioindicator of health risks to sympatricthreatened freshwater mussel populations.     

Physical forces constrain the depth distribution of the abundant native mussel Elliptio complanata in lakes

1. Unionid mussels often account for a large portion of benthic biomass and contribute to nutrient cycling and sediment processes, but are thought to be limited to shallow areas (<2–3 m). 2. The depth distribution and body size of Elliptio complanata were compared in seven Canadian Shield lake basins of different sizes to test what factors determine the upper and lower limit of their depth range. Specifically, I tested whether (i) the upper range of their distribution is limited by exposure to winds and wave action and (ii) the lower range of their distribution is limited by the depth of the thermocline or by the boundary of mud deposition. 3. The average depth distribution of E. complanata shifted to greater depths in larger lake basins. When comparing individual transects, maximum mussel density was found deeper at more exposed sites. Mussel size decreased with increasing depth and was larger, on average, in larger lake basins. These results suggest that physical forces limit the upper range of mussel distribution in lakes. 4. The maximum depth at which mussels were found in different lakes was closely related to thermocline depth. However, mussels were commonly observed below the predicted depth of the mud deposition boundary. The thermocline limits the lower range of mussel distribution in lakes, probably by limiting food availability and by determining water temperature. Substratum type does not limit the lower distribution of mussels. 5. These results suggest that unionid mussels are present in the deeper parts of the littoral zone, especially in large lakes. Therefore, comparisons of mussel populations between sites and between lakes would be biased unless the full depth distribution of these mussels is considered. These results also suggest that long‐term changes in the thermal structure of lakes could affect the range of unionid mussel populations and their functional role in littoral ecosystems.     

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: Phi(ST) = 0.62, P < 0.001; AL: Phi(ST) = 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.