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Invading the invaders: Reproductive and other mechanisms mediating the displacement of zebra mussels by quagga mussels
Abstract
Dreissenids are invasive bivalves, native to water bodies of the Ponto-Caspian region of southwestern Asia. Following dispersion in Europe in the early nineteenth century, they were accidentally introduced into the Great Lakes region of North America in the 1980s and 1990s. Recently, they were discovered in the southwestern USA. Initially, Dreissena polymorpha (zebra mussel, ZM) spread more rapidly than Dreissena rostriformis bugensis (quagga mussel, QM); however, QM is becoming predominant in many areas of the Great Lakes and was the first to appear west of the Continental Divide, in Lake Mead. In Europe, as well, ZM was the first species to spread widely in western Europe from its endemic range; however, QM have recently been found in areas previously colonized only by ZM. This article reviews the dynamics of this double dreissenid invasion and considers the question: what mechanisms mediate the displacement of ZM by QM? Despite their similar appearance, QM differ from ZM in temperature and salinity tolerance, byssal thread attachment, growth, respiration rates, assimilation efficiency, enzymes such as thiaminase, depth of occurrence, and reproduction. Differences in reproduction include the depth at which reproductively active animals are found, the temperature at which spawning can be initiated, number of gametes produced, and length and timing of their annual reproductive cycle. A hypothetical role of hybrids between the species mediating species change is suggested. Future investigations of the displacement of ZM by QM should consider the role of reproductive differences (hybrids, responses to environmental chemicals, etc.) in mediating the change.
... Studies comparing physiological and ecological optima of the two dreissenids (Ram et al., 2012) reveal that the quagga mussel spends less energy on defence against predators Rudstam & Gandino, 2020), and detoxification of environmental contaminants (Kerambrun et al., 2018;Louis et al., 2019Louis et al., , 2021Potet et al., 2016Potet et al., , 2018Schäfer et al., 2012). Hence, the quagga mussel can allocate more energy into growth and reproduction, which has been suggested as the major difference in life strategy between the two dreissenids (Ram et al., 2012). ...
... Studies comparing physiological and ecological optima of the two dreissenids (Ram et al., 2012) reveal that the quagga mussel spends less energy on defence against predators Rudstam & Gandino, 2020), and detoxification of environmental contaminants (Kerambrun et al., 2018;Louis et al., 2019Louis et al., , 2021Potet et al., 2016Potet et al., , 2018Schäfer et al., 2012). Hence, the quagga mussel can allocate more energy into growth and reproduction, which has been suggested as the major difference in life strategy between the two dreissenids (Ram et al., 2012). While different allocation of energy budget in both species seems to contribute substantially to the shift of dominance, only Makhutova et al. (2012) compared the nutrient range (by analytical detection of lipids) between the two dreissenids to determine their energy budget. ...
Dreissena rostriformis bugensis (quagga mussel, QM) has spread into areas occupied by an earlier invader, Dreissena polymorpha (zebra mussel, ZM) in Europe and North America. Usually QM displaces ZM within a few years or both species coexist, although the mechanisms driving these outcomes have not been uncovered clearly.
In Lake Balaton (central‐eastern Europe), QM displaced ZM in the oligotrophic (food‐limited) basin, whereas they coexist in the eutrophic (food‐rich) basin. Searching for the drivers of interactions in dreissenid assemblages, we compared survival, growth, allometry, shell hardness, biomacromolecule content and superoxide dismutase (SOD) expression (indicating nutrition stress) of dreissenids collected in both basins in a field survey, and in individuals collected from the food‐rich basin and experimentally transplanted (10 weeks) to the food‐limited or food‐rich (i.e. the same) basin.
In the field survey, QM from the food‐rich basin showed the greater height increment per unit length than coexisting ZM and food‐limited conspecifics. ZM had the hardest shells of all the mussel populations. In the food‐rich basin, ZM did not differ from QM in weight, protein, and carbohydrate contents, but had higher lipid content and SOD expression. Food‐limited QM, compared to conspecifics from the food‐rich basin, had weaker shells, but their protein, carbohydrate, and lipid contents showed faster increments per unit size, thus adults made up for the initial advantage of the food‐rich population.
QM survived better than ZM after transplantation irrespective of the basin. Shells were harder in ZM versus QM and in the food‐rich versus food‐limited conditions. QM grew at both locations, whereas ZM only in the food‐rich basin. The protein and carbohydrate contents were greater in the food‐rich versus food‐limited basin, with no interspecific differences. Lipid content in QM was higher in the food‐limited versus food‐rich basin, whereas the opposite held for ZM.
We demonstrated that the dreissenid species could coexist in food‐rich conditions, despite the higher level of stress in ZM (as shown by weaker survival, higher SOD expression), whereas QM displaced ZM under food‐limiting conditions, probably due to the ability to replace missing storage carbohydrates with accumulated lipids. Nevertheless, QM from the food‐limited basin also showed symptoms of nutritional stress (changes in biomacromolecule content, lower shell hardness). Results suggest that the ability to show a rapid change in metabolism could be an important advantage of QM over ZM in their competition.
... The morphology of the new shells was congruent with the illustrations and descriptions reported for D. bugensis/D. rostriformis bugensis in Rosenberg and Ludyanskiy (1994), Claxton et al. (1997), Ram et al. (2012), andTeubner et al. (2016). Specifically, the new organisms were characterized by pale colour near the hinge, a rounded transition of the ventral and dorsal shell surfaces, and convex ventral shell surface. ...
... The introduction of quagga mussel is often accompanied by a decline in zebra mussel populations (Orlova et al. 2005b;Ram et al. 2012). In the littoral zone of Lake Constance, the replacement of zebra mussels by quagga mussels was very fast, occurring in around four years, from 2016 to 2019 (Haltiner et al. 2022). ...
... In the present study, we assessed the behavioral response to chronic motorboat noise of one of the most successful freshwater invasive species worldwide, the quagga mussel, Dreissena rostriformis (Karatayev et al., 2021;Mei et al., 2016). Native to the Dnieper-Boh estuary in Ukraine (Mills et al., 1996), this bivalve spread to Eastern Europe in the 1940s, to Western Europe at the end of 2000s, and to North America in the 1980s, gradually replacing its relative, the zebra mussel, Dreissena polymorpha (Matthews et al., 2014;Nalepa et al., 2010;Ram et al., 2012). In France, it was reported for the first time in the Moselle in 2011 (bij de Vaate and Beisel, 2011), from where it moved south to reach the French perialpine lakes region in 2015 (Haltiner et al., 2022). ...
The response of invasive species to noise and how it can modulate their behavior and ecological impact have received scant attention. We conducted a two-phase laboratory experiment to investigate the effect of motorboat noise on the quagga mussel, Dreissena rostriformis bugensis. We first measured aggregation during a prolonged rearing phase under laboratory background noise supplemented or not with motorboat sounds. We then monitored valve gaping and estimated the filtration rate of mussels in the presence or absence of motorboat noise. Prolonged noise exposure increased aggregation and valve gaping. The relationship between valve gaping and filtration was significantly positive for control mussels and not significant for the mussels that experienced motorboat noise. Further research is needed to understand the physiological origins of the response to noise and the consequences on life-history traits and mussel-based ecosystem processes such as phytoplanktonic primary production, benthification and biofouling.
... Zebra and Quagga Mussel are typically found in lakes and rivers attached to a wide variety of hard substrates such as rocks, shellfish, and aquatic plants (Garton et al. 2013). Despite their similar appearance, Quagga and Zebra Mussel differ in temperature and salinity tolerances, growth, depth of occurrence, and life history traits (Ram et al. 2011, Karatayev et al. 2015. Both species have short life spans but are prolific breeders producing 1 × 10 4 to 1 × 10 6 eggs per female per year depending on size and environmental conditions (Pollux et al. 2010). ...
Zebra Mussel (Dreissena polymorpha) and Quagga Mussel (D. rostriformis bugensis) are aquatic invaders with substantial economic and ecological impacts that continue to spread in Canada. A new ecological risk assessment (that differs from the 2012 assessment) was conducted by Fisheries and Oceans Canada (DFO) Science for freshwater ecosystems across Canada incorporating updated and improved data with greater resolution (9,260 × 9,260 m grid cell). This risk assessment characterized the potential for mussels to be introduced (propagule pressure) and establish (habitat suitability), along with their potential ecological impacts to derive a metric of Ecological Risk for two separate scenarios of establishment using either a calcium-based model or a maximum entropy (MaxEnt) habitat suitability model. Ecological Risk values are not absolute, and areas of Low risk do not necessarily indicate that Zebra and Quagga Mussel cannot be introduced, establish, or impact those Canadian ecosystems, but rather indicates that the risks are lower relative to areas at higher risk. As such, both scenarios identified Low to High risk areas with sub-drainages with the highest risk in proximity to the current distribution of these species, particularly the Laurentian Great Lakes system (both Zebra Mussel and Quagga Mussel) and Manitoba (Zebra Mussel). Outside of the current distribution, calcium-based models for both species identified Moderate risk areas throughout the southern portions of most provinces. In the Maritime provinces for which data was not available in the previous assessment, most habitat suitability models identified the area as Moderate risk for both Quagga and Zebra Mussel, particularly New Brunswick which also exhibited some discrete areas of High risk. For the rest of Canada, including Newfoundland and Labrador and the Territories for which data were also unavailable in the previous assessment, the risk for both species across most habitat suitability models was predominantly Low, with most of the Arctic Archipelago being below the thermal tolerance for both species. To facilitate Aquatic Invasive Species (AIS) management decision-making, Ecological Risk is summarized at the sub-drainage level for all of Canada which was the spatial scale used in the 2012 assessment.
... Both species have similar life cycles, involving a free-swimming veliger larval stage followed by attachment to hard substrate (Glyshaw et al., 2015). However, the species differ in terms of salinity and thermal tolerance, growth, respiration rates, and the depth at which reproduction typically occurs (Ram et al., 2012). Several studies have used both quagga and zebra mussels interchangeably for contaminant monitoring within the Great Lakes and its tributaries (Evariste et al., 2018;Mills et al., 1993a;Richman & Somers, 2005, 2010, although potential differences in contaminant accumulation and sensitivity have been observed (Evariste et al., 2018;Nowicki & Kashian, 2018). ...
Environmental metabolomics has emerged as a promising technique in the field of biomonitoring and as an indicator of aquatic ecosystem health. In the Milwaukee Estuary (Wisconsin, USA), previous studies have used a non‐targeted metabolomic approach to distinguish between zebra mussels ( Dreissena polymorpha ) collected from sites of varying contamination. To further elucidate the potential effects of contaminants on bivalve health in the Milwaukee Estuary, the present study adopted a caging approach to study the metabolome of quagga mussels ( Dreissena bugensis rostriformis ) deployed in six sites of varying contamination for 2, 5 or 55 days. Caged mussels were co‐deployed with two types of passive sampler (POCIS and SPMDs) and data loggers. In conjunction, in‐situ quagga mussels were collected from the four sites studied previously and analyzed for residues of contaminants and metabolomics using a targeted approach. For the caging study, temporal differences in the metabolomic response were observed with few significant changes observed after 2 and 5 days, but larger differences (up to 97 significantly different metabolites) to the metabolome in all sites after 55 days. A suite of metabolic pathways were altered including biosynthesis and metabolism of amino acids, and up‐modulation of phospholipids at all sites, suggesting a potential biological influence such as gametogenesis. In the caging study, average temperatures appeared to have a greater effect on the metabolome than contaminants, despite a large concentration gradient in PAH residues measured in passive samplers and mussel tissue. Conversely, significant differences between the metabolome of mussels collected in situ from all three contaminated sites and the offshore reference site were observed. Overall, these findings highlight the importance of contextualizing the effects of environmental conditions and reproductive processes on the metabolome of model organisms to facilitate the wider use of this technique for biomonitoring and environmental health assessments.
... had colonized a total of 772 water bodies in the United States and Canada by 2010, with zebra mussels occupying 17 times more water bodies than quagga mussels (Benson, 2014). In the early 2000s, a shift was observed and quagga mussels became the predominant first colonizers, favored by small differences in physiology and depth of reproduction (Ram et al., 2012). Quagga mussels first appeared in the western U.S. in Lake Mead (AZ-NV USA) in early 2007 (Stokstad, 2007) and rapidly spread across the Colorado River system colonizing 30 lakes and reservoirs in Arizona, California, and Nevada by the end of 2008 (Nalepa, 2010). ...
Background
In recent decades, invasive quagga mussels have expanded to the Western United States from the Great Lakes region of North America. Most studies that evaluate the invasion potential of quagga mussels in western water bodies have utilized physiological and life history information from zebra mussels, a related taxon. Few studies have assessed the potential for invasion using specific information from quagga mussel life history or experiments that test for their survival in the fresh and saline waters of the western United States.
Methods
We investigated quagga mussel survival, growth, and reproduction using semi-natural experiments under temperature and light controlled conditions across a gradient of water salinity (fresh to brackish) and pH (8.4–11). Water from Lake Mead was used as a positive control in our experiment, and water from Pyramid Lake and the Truckee River was used as brackish and freshwater treatments, respectively. The mussels used in the experiments were collected from Lake Mead.
Results
After 12 h in brackish water (4 ppt, pH 9.3), we observed 100% mortality of adult mussels. The swelling and disintegration of body tissues and high mortality rates indicated that high potassium, sodium, and chloride concentrations were the likely causes of death in brackish water treatments. In contrast, mussels were able to survive, grow, and reach sexual maturity in freshwater (0.1 ppt) with a low calcium concentration (17 mg L ⁻¹ ) after 57 days. Mussels died after 2 days at pH 11 and after 12 days at pH 10; during the 14-day monitoring period, no mortality was detected at pH 9.0, 9.3, or 9.5 and mussels did not exhibit any visual indications of stress. Understanding quagga mussel physiological and environmental tolerances appears to be essential for assessing their invasion potential in aquatic habitats.
... By 2008, they were found in the upper Colorado and Arkansas drainages in Colorado, as well as a lake in Utah and a reservoir in the Monterey Bay watershed, California. The spread of zebra mussels between river basins was facilitated by canals and barges in the eastern and midwestern regions, while isolated rivers and lakes were infested through the transport of mussels on trailered boats or fishing gear [118][119][120][121]. According to our study results, C. fluminea dominate the lower Danube sector and the arms of the Danube with flowing waters while D. polymorpha were identified along the Danube course and in the lakes of the Danube Delta. ...
The objective of this study is to provide an updated account of the distribution history of two invasive molluscs, Corbicula fluminea and Dreissena polymorpha, both in Europe and worldwide. In addition to this, the study also intends to review their ecological requirements to gain a better understanding of their invasive potential and distribution dynamics. Specifically, the study focuses on updating the distribution and ecological characteristics of these freshwater bivalves in the lower sector of the Danube River and the lakes of the Danube Delta. The purpose is to better understand their invasive and distribution dynamics and to develop effective measures to limit their spread in the future. To achieve this, environmental proxies such as sediment particle size and Total Organic Carbon (TOC) concentrations were used to assess their tolerances. However, the results did not show a significant correlation between the densities of these bivalves and the analyzed environmental parameters. Despite this, the species were found in high densities and formed well-developed benthic communities in some stations. The study contributes to the understanding of the invasiveness of these bivalve species and their distribution range dynamics. Nonetheless, further investigation is required to fully comprehend the role of environmental parameters in their distribution. The study covers the period between 2010 and 2020 and focuses on the lower Danube River sector and Danube Delta.
One of the top ecological priorities is to find sensitive indicators for pollution monitoring. This study focuses on the bioconcentration and responses (condition index, survival, oxygen consumption, heart rates, and oxidative stress and neurotoxic effect biomarkers) of mussels from the Volga River basin, Dreissena polymorpha and Dreissena bugensis, to long-term exposure to toxic chemicals such as tributyltin (TBT, 25 and 100 ng/L) and copper (Cu, 100 and 1000 μg/L). We found that TBT was present in the tissues of zebra and quagga mussels in comparable amounts, whereas the bioconcentration factor of Cu varied depending on its concentration in water. Differences in responses between the two species were revealed. When exposed to high Cu concentrations or a Cu-TBT mixture, quagga mussels had a lower survival rate and a longer heart rate recovery time than zebra mussels. TBT treatment caused neurotoxicity (decreased acetylcholinesterase activity) and oxidative stress (increased levels of thiobarbituric acid reactive substances) in both species. TBT and Cu levels in mussel tissues correlated positively with the condition index, but correlated with the level of acetylcholinesterase in the mussel gills. The principal component analysis revealed three main components: the first consists of linear combinations of 14 variables reflecting TBT water pollution, TBT and Cu levels in mussel tissues, and biochemical indicators; the second includes Cu water concentration, cardiac tolerance, and mussel size; and the third combines weight, metabolic rate, and heart rates. Quagga mussels are less tolerable to contaminants than zebra mussels, so they may be used as a sensitive indicator.
Biological invasions and anthropogenic noise represent two major threats to fresh water ecosystems but the response of invasive species to noise and how it can modulate their behavior and ecological impact have received scant attention. In this study, we conducted a two-phase laboratory experiment to investigate the effect of motorboat noise on the quagga mussel Dreissena bugensis, one of the world’s most invasive species causing detrimental ecological and economic impacts. We first measured aggregation patterns during a 14 to 18-day rearing phase where the mussels experienced laboratory background noise supplemented or not with motorboat sounds that mimic nautical activity during summmer. Afterwards, we monitored the valve activity and estimated the filtration rate of mussels from both rearing conditions in the presence or absence of motorboat noise for 12 hours. Our results showed that aggregation rate and mean aggregate size were higher with motorboat noise. Conversely, valve activity did not differ between the two noise conditions but was significantly increased by the previous repeated exposure to noise during the rearing phase. The relationship between valve activity and filtration rate was positive for the mussels not exposed to boat noise, positive but weaker for the mussels that experienced boat noise during one of the two experiment and not significant for those under boat noise during the whole investigation. Further research is needed to understand the physiological origins of the response to noise and the consequences on life-history traits and mussel-based ecosystem processes such as phytoplanktonic primary production, benthification and biofouling.
SYNOPSIS. The prolific reproductive capabilities of the zebra mussel, Dreissena polymorpha, have facilitated the rapid spread and high densities of this biofouling organism since its accidental introduction into North America less than 10 years ago. Research on its reproductive mechanisms and capabilities may be valuable not only in predicting its further spread, but also in investigating basic mechanisms of reproduction and development and in developing new strategies to mitigate its impact. Since zebra mussels are dioecious and fertilization occurs externally, coordinated maturation, spawning, and other mechanisms have evolved to increase the probability of successful fertilization. The zebra mussel undergoes an annual cycle of gonadal growth and gamete maturation, culminating in one or more spawning events in late spring or early summer. Temperature, rates of temperature change, food availability, and effects of neighboring mussels seem to be critical variables that determine reproductive responses. Serotonin is a biogenic amine which is implicated in spawning behavior and can reliably trigger spawning. Serotonin is present in the gonad in neural varicosities that encircle groups of gametes, and specific serotonergic ligands can mimic or block spawning caused by serotonin. In females, serotonin reinitiates meiosis causing maturation from prophase I to metaphase I prior to spawning. Spawned oocytes contain substances that are species specific sperm chemoattractants. The sequence of binding, entry, and subsequent nuclear movements have been observed with fluorescence and scanning microscopy. Despite their negative ecological and economic impacts, zebra mussels have also provided a new and easily obtainable resource for studies of reproductive mechanisms.
The discovery of a morphologically distinct dreissenid mussel in the profundal zone of Lake Erie suggests the presence of either a third dreissenid mussel species in the Great Lakes or a previously unknown morphological phenotype of an existing dreissenid species. We examined the morphometrics and molecular systematics of the zebra mussel (Dreissena polymorpha) and the profundal and epilimnetic forms of the quagga mussel (Dreissena bugensis) from Lakes Erie and Ontario. In an attempt to resolve the taxonomic status of the profundal form of the quagga mussel, we sequenced a 710 base pair fragment of the cytochrome oxidase subunit I mitochondrial gene of the two forms of the quagga mussel. No nucleotide differences were found, supporting the hypothesis that the profundal form of the quagga mussel is a phenotype of D. bugensis, not a separate species. In contrast, the second and third principal component scores from an analysis of the morphological variables shell length, shell width, shell height, and shell mass separated the epilimnetic and profundal forms of the quagga mussel into two groups, but grouped zebra mussels from all depths together. The most parsimonious explanation for our results is that D. bugensis shows plasticity in shell morphology with respect to depth, whereas D. polymorpha does not.
Here the westernmost record in Europe of the invasive quagga mussel Dreissena rostriformis bugensis is discussed, i.e. in the Haringvliet, an enclosed freshwater Rhine-Meuse estuary in The Netherlands. In 2006 and 2007 small numbers of quagga mussels were found in between large numbers of zebra mussels, Dreissena polymorpha. They were recorded during the period-ical analyses of settlement plates of the international biofouling monitoring project SETL. Identifications were done on the basis of morphology and confirmed by DNA-barcoding tech-niques. The quagga mussels show a patchy distribution in the Dutch Haringvliet and Hollands diep. No settlement of quagga and zebra mussels was recorded between the second half of March to the first half of June both in 2006 and 2007. The main settlement period of quagga and zebra mussels in the research area is concluded to be from the second half of June until the end of December. The calculated maximum shell growth rates of both invasive mussel species are similar to somewhat higher than those recorded in literature, i.e. 0.077 mm/day to 0.141 mm/day.
In 1992, we discovered populations of the nonindigenous quagga mussel Dreissena rostriformis bugensis (Andrusov 1897) in the middle reaches of the Volga River. The same species was found in samples collected between 1994 and 1997 in the Volga delta and in shallow areas of the Northern Caspian Sea. D. r. bugensis always co-occurred with its more widespread congener, the zebra mussel D. polymorpha (Pallas 1771). The quagga mussel's contribution to total Dreissena abundance increased over time in the middle Volga reservoirs and Volga River delta. D. r bugensis was common in the Volga portion of Rybinsk Reservoir during 1997 and, by 2000, it was in Uglich, Rybinsk and Gorky Reservoirs on the Upper Volga River. D. r. bugensis was neither found in Ivankov Reservoir, nor in terminal sections of the Volga-Baltic corridor including the eastern Gulf of Finland. Presently, all but the northern-most regions of the Volga River have been colonized by D. r bugensis. We hypothesize that its introduction into the Volga River and Caspian basin occurred no later than the late 1980s via commercial shipping that utilized the Volga-Don waterway to navigate between the source Black-Azov Sea region and recipient areas on the Volga River. Larval drift likely contributed to establishment of populations at downstream sites, while human-mediated vectors may be responsible for introductions to upstream locations on the Volga River. We anticipate continued northward dispersal in conjunction with shipping activities.
To investigate the occurrence of thermal adaptation in the zebra mussel, Dreissena polymorpha, inhabiting the lower reaches of the Mississippi River, we compared lethal heat-tolerance among three populations (mussels collected at Lake Pepin, MN; Alton, IL; and Baton Rouge, LA). We determined time-to-death at 32°C for 160 individuals per site, for mussels collected at a water temperature of 15°C and then maintained in the laboratory for about 8 wk under uniform conditions. Both shell length and condition index significantly affected survival time and were included as covariates in the analysis for interpopulation differences in heat-tolerance. Zebra mussels from our southernmost location had a higher heat-tolerance than those from the two northern locations. This difference in heat-tolerance among sites may indicate adaptation to local temperature regimes. In addition, in a comparison of heat tolerance within populations, we separated mussels into size classes (where larger mussels have been exposed to local conditions longer) and calculated an adjusted mean time-to-death (TTD). We found a different TTD/size relationship depending on sampling location. Minnesota mussels had decreasing heat tolerance as size increased, where Louisiana mussels had the opposite relationship. These patterns of heat-tolerance within populations indicate a selection pressure for increased heat-tolerance at Louisiana. However, even if the selection pressure is strong at the Louisiana site, it has not (at least not yet) resulted in an adaptation, as high heat-tolerance is not ubiquitous within this population. Zebra mussels may have insufficient genetic variation for heat-tolerance or gene flow may have been too strong for genetic adaptation to occur in the short amount of time that zebra mussels have occurred in the lower Mississippi River.
Dreissenid bivalves, Dreissena polymorpha (zebra mussel) and Dreissena bugensis (quagga mussel) are biofouling species that invaded the Great Lakes region of North America from source populations in Europe in the 1980s. Initially, D. polymorpha spread faster and farther; however, D. bugensis have recently displaced D. polymorpha in many areas of the Great Lakes and was the first to be found west of the Continental Divide. Early detection of dreissenids is important in anticipating and preventing potentially high economic impacts. To study population dynamics and to enhance detection methods, we assessed "spawnability" using a serotonin bioassay and developed a new, sensitive, multiplex PCR method to identify veligers and verify adult species. Contrasting riverine populations were identified in the Saginaw River (100% D. polymorpha) and the Detroit River at Belle Isle (100% D. bugensis in 2010), and mixed populations of mussels (10% to similar to 50% D. polymorpha) were found in Saginaw Bay, Lake Huron. In 1994, when the Detroit River population at Belle Isle was virtually all D. polymorpha (Ram et al. 1996), spawning could not be induced by serotonin until late May, and peak spawnability did not occur until early June. In 2010, D. bugensis at the same site could be induced to spawn in the first week of April, and reached near maximal spawning intensity by mid-May. In 2010, Detroit River veligers were first observed in April and, by PCR species-specific detection, were 100% D. bugensis. Veligers changed to a mixed population of both species later in May and rose to a peak, mixed population in early June. These experiments demonstrate a quantitative, species-specific detection of dreissenid veligers, and lay the groundwork for determining the role of early reproduction and other mechanisms in mediating the displacement of one species by a closely related "cousin."