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Impact of habitat engineering by invasive Corbicula clams on native European unionid mussels
Abstract
Biological invasions cause biodiversity erosion on a global scale. Invasive species spreading beyond their natural range compete with native fauna for food and space, push native species to suboptimal habitats, impairing their behaviour and thus limiting their occurrence. Freshwater ecosystems are especially vulnerable to biological invasions and their ecological and economic impacts. The invasive Asian clams (Corbicula spp.), due to their opportunistic life style, can occur at densities of thousands ind. m−2. They act as ecosystem engineers transforming bottom substrata through accumulation of shells. Our goal was to determine the effect of substratum modification by living Corbicula and their shells on substratum choice and behaviour of Unio tumidus and Anodonta anatina, two European freshwater mussel species of the highly imperilled Unionidae family. We assessed their substratum selection in pairwise choice tests (pure sand vs. sand modified by living Corbicula or their shells, sand modified by shells vs. living Corbicula). Next, we tested locomotion and burrowing of unionids on pure substratum and substrata modified by Corbicula. Unionids avoided sand modified by living Corbicula and their empty shells, not distinguishing between these two types of substratum modification. In the presence of Corbicula, their burrowing was shallower or it took them longer to obtain the same depth as in the pure sand. Additionally, on sand modified by Corbicula shells, we observed a locomotion increase (U. tumidus) or slowing down (A. anatina). Our research showed a novel mechanism of negative impact of Corbicula on unionids, consisting in pushing them away from their optimal habitats. This may contribute to their habitat loss and future declines in invaded ecosystems.
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Among species of Corbicula, only the hermaphroditic androgenetic lineages are invasive while sexual species are restricted to the native range (mainly Asia). Four androgenetic lineages of Corbicula spp. have been identified in American and European freshwater systems, with the Corbicula sp. form A/R lineage being abundant on both continents. This lineage is considered an important invasive species because of its impact on aquatic ecosystems and industrial cooling systems. The present study identified the invasive lineages of Corbicula spp. in freshwater environments of South America based on morphometric data and used genetic marker (mtDNA COI gene) to investigate their genetic relatedness to the invasive lineages from Europe and North America. Four important results arise: (1) there are two invasive lineages of Corbicula in South America, Corbicula sp. form C/S and Corbicula sp. form A/R, and they are clonally expanding their distribution in continental aquatic systems of South America; (2) populations with intermediate morphotype were detected in three distinct sites of Brazil; (3) one population with individuals presenting a new COI haplotype was recovered with an intermediate morphotype; (4) the morphotype of the invasive lineages of Corbicula spp. in South America presents high correlation with their COI haplotypes.
Freshwater mussels are one of the most threatened animal groups in the world. In the European Union, threatened and protected mussel species are not adequately monitored, while species considered to be common and widespread receive even less attention. This is particularly worrying in the Mediterranean region, where species endemism is high and freshwater habitats are severely affected by water scarcity. In the absence of hard data on population trends, we report here a long-term comparison of freshwater mussel assemblages at 132 sites covering 15 different hydrological basins in Portugal. This study reveals a widespread decline of 60 % in the number of sites and 67 % in the overall abundance of freshwater mussels across Portugal over the last 20 years, indicating that all species are rapidly declining and threatened with extinction. These results show that current legislation and conservation measures are largely ineffective and highlight the importance of updating the Habitats Directive to enforce standard monitoring protocols for threatened species in the European Union and to extend monitoring to other freshwater species thought to be common and widespread. Efficient water management, restrictions on irrigation expansion in important biodiversity areas, mitigation of hydrological changes and loss of aquatic habitat connectivity caused by physical alterations are urgently needed to reverse these declining population trends. For the severely endangered species Margaritifera margaritifera, Potomida littoralis, and Unio tumidiformis, where populations are now critically low, more urgent action is needed, such as ex-situ conservation, protection of remaining populations and large-scale habitat restoration.
The introduction of freshwater mussels in France was unintentional and relatively recent. Two species were introduced in the second half of the nineteenth century, and a further eight after World War II. This note summarises the available information on introduced freshwater bivalves in France, focusing on the main pathways and vectors of dispersal, based on the data provided by the French National Database (INPN) and environmental DNA data. The introductions were either by sea (mainly for brackish water species, but also for Asiatic Clams), or by the European river system and its canals connecting the Danube to the Rhine (Dreissenidae, Sphaeriidae). A third mean of introduction was the transport of fish, for the Chinese pound mussel, which first established in southern France. Canals connect the major French basins (Rhine, Rhône, Seine, Loire and Garonne) and the route of dispersal of introduced species through natural hydrosystems and canals can be traced by looking at their progression, decade by decade. Fish transport within France is probably an important dispersal vector for the Chinese pound mussel, as occasional introductions appear to have contributed to its spread in the north-east and west of France. The use of the eDNA was an effective tool in completing the species distribution maps.
Corbicula fluminea is a widespread and problematic invasive bivalve species in many freshwater ecosystems. Here, a systematic literature review was performed to synthesise what is presently known about C. fluminea in relation to its biology, ecology, and management. Of the total 1156 studies analysed, most were conducted in North America and Europe, with a primary focus on toxicology and ecology. In the native range, most studies assessed the role of C. fluminea in human nutrition and health. We have also identified a series of knowledge gaps that should be considered by future research. These include, for example, the need to better assess basic biological and ecological features (e.g. distribution, abundance, biomass, population structure, modes of reproduction, natural enemies) of C. fluminea, while the resolution of severe taxonomic problems that persist within the Corbicula species complex should be addressed with novel molecular tools. Further, better understanding of the mechanisms that underpin the impacts of C. fluminea in the invaded range is required. Given the high invasiveness of C. fluminea, novel methodologies and techniques to improve early detection, prediction of spread, and effective management for successful containment and eradication are urgently needed.
Freshwater biodiversity is under threat, but long-term quantitative studies showing major demographic declines in invertebrate species are still scarce. Here we focus on a long-term study (2004 to 2019) using four native freshwater mussel species (Order Unionida) colonizing two canals of the Ebro River (Spain). Special attention was given to Pseudunio auricularius (Spengler, 1793), a critically endangered species. Generalized linear mixed models results showed significant temporal effects on mussel densities, with a continuous decline in all species present, being Anodonta anatina, Potomida littoralis and Unio mancus now considered locally extinct. Pseudunio auricularius is still present in the studied canals, but at very low densities (0.01 ind/m2). Capture-recapture data on P. auricularius show a progressive decline in its survival probability, down to 0.15 in 2020 in the Canal Imperial de Aragón, although in the Canal de Tauste it remains close to 1. Based on these results, we discuss several hypotheses that may explain this rapid collapse of unionid populations. Given the precarious conservation status of freshwater mussels in both canals, effective management measures should be urgently applied, including habitat restoration and captive breeding.
We identified 14 emerging and poorly understood threats and opportunities for addressing the global conservation of freshwater mussels over the next decade. A panel of 17 researchers and stakeholders from six continents submitted a total of 56 topics that were ranked and prioritized using a consensus-building Delphi technique. Our 14 priority topics fell into five broad themes (autecology, population dynamics, global stressors, global diversity, and ecosystem services) and included understanding diets throughout mussel life history; identifying the drivers of population declines; defining metrics for quantifying mussel health; assessing the role of predators, parasites, and disease; informed guidance on the risks and opportunities for captive breeding and translocations; the loss of mussel-fish co-evolutionary relationships; assessing the effects of increasing surface water changes; understanding the effects of sand and aggregate mining; understanding the effects of drug pollution and other emerging contaminants such as nanomaterials; appreciating the threats and opportunities arising from river restoration; conserving understudied hotspots by building local capacity through the principles of decolonization; identifying appropriate taxonomic units for conservation; improved quantification of the ecosystem services provided by mussels; and understanding how many mussels are enough to provide these services. Solutions for addressing the topics ranged from ecological studies to technological advances and socio-political engagement. Prioritization of our topics can help to drive a proactive approach to the conservation of this declining group which provides a multitude of important ecosystem services.
A pioneering, quantitative study published in Journal of Animal Ecology in 1966 on freshwater mussel populations in the River Thames, UK, continues to be cited extensively as evidence of the major contribution that mussels make to benthic biomass and ecosystem functioning in global river ecosystems.
Ecological alteration, as well as declines in freshwater mussel populations elsewhere, suggest that changes to mussel populations in the River Thames are likely to have occurred over the half century since this study.
We resurveyed the site reported in Negus (1966) and quantified the changes in mussel population density, species composition, growth patterns and productivity.
We found large declines in population density for all unionid species. The duck mussel Anodonta anatina decreased to 1.1% of 1964 density. The painter's mussel Unio pictorum fell to 3.2% of 1964 density. The swollen river mussel Unio tumidus showed statistically nonsignificant declines. In contrast to 1964, in 2020 we found no living specimens of the depressed river mussel Pseudanodonta complanata (classified as Vulnerable by the IUCN Red List) but found new records of the invasive, nonnative zebra mussel Dreissena polymorpha and Asian clam Corbicula fluminea. Additionally, we found strong decreases in size‐at‐age for all species, which now grow to 65–90% of maximum lengths in 1964. As a result of reduced density and size, estimated annual biomass production fell to 7.5% of 1964 levels.
Since mussels can be important to ecosystem functioning, providing key regulating and provisioning services, the declines we found imply substantial degradation of freshwater ecosystem services in the River Thames, one of the UK's largest rivers. Our study also highlights the importance to conservationists and ecologists of updating and validating assumptions and data about wild populations, which in the present era of anthropogenic ecosystem alteration are undergoing significant and rapid changes. Regular population surveys of key species are essential to maintain an accurate picture of ecosystem health and to guide management.
Europe has a long history of human pressure on freshwater ecosystems. As pressure continues to grow and new threats emerge, there is an urgent need for conservation of freshwater biodiversity and its ecosystem services. However, whilst some taxonomic groups, mainly vertebrates, have received a disproportionate amount of attention and funds, other groups remain largely off the public and scientific radar. Freshwater mussels (Bivalvia, Unionida) are an alarming example of this conservation bias and here we point out six conceptual areas that need immediate and long‐term attention: knowledge, threats, socioeconomics, conservation, governance and education. The proposed roadmap aims to advance research, policy and education by identifying the most pressing priorities for the short‐ and long‐term conservation of freshwater mussels across Europe.
Animal-mediated nutrient cycling research tends to emphasize either native or invasive fauna, yet communities comprising both groups are common, and biogeochemical control may shift from native to invasive species, altering local nutrient regimes. In North American rivers, co-occurring native mussels (Unionidae) and the invasive clam, Corbicula fluminea, have strong nutrient cycling effects through filter-feeding and bioturbation. When these two groups co-occur, the degree to which their nutrient cycling effects differ remains unclear. We quantified bivalve density, biomass, and nutrient excretion rates at four reaches in each of two rivers once during the same year to test whether differences in density and biomass led to different spatial and temporal nutrient cycling and stoichiometry patterns for co-occurring mussels and Corbicula. We hypothesized high densities, coupled with small body size would elevate Corbicula population-level nutrient cycling rates above those of less dense assemblages of larger-bodied mussels. Corbicula occurred at all mussel beds and their densities generally exceeded mussel densities, but Corbicula biomass was consistently lower. High densities and greater mass-specific excretion rates led to Corbicula population-level excretion rates that were greater than or equal to mussel aggregate rates at half the reaches. Abiotic conditions limited bivalve nutrient supply relative to ambient concentrations, but their contributions increased during low flows and are likely concentrated at finer spatial scales. Our results suggest spatial variation in invasive and native trait distribution associated with phylogenetic tribes influences the potential for animal-mediated nutrient cycling to shift from native to invasive species control. Overall, our study highlights the need for new management paradigms that account for nutrient cycling by invasive species.
The Chinese pond mussel, Sinanodonta woodiana, is non-native to European freshwater systems. Originating from Asia, it was brought to Europe in the 1970s and is now spreading across many countries. Management of the species is currently limited by a lack of information on the actual distribution and population characterisation of S. woodiana as well as on the co-occurrence of native mussels potentially resulting in competition. For this paper, we examined nine water systems in the German federal state of Bavaria with randomly observed and anecdotally reported S. woodiana occurrences. We recorded the density and biomass of S. woodiana and of the co-existing mussel species as well as the habitat characteristics. We found S. woodiana in eight water systems, co-existing with other native and invasive mussel species in seven of them. The distribution of S. woodiana was geographically widespread throughout Bavaria, indicating great invasion potential. In one fish pond, S. woodiana was the only mussel species occurring with the highest mean biomass (1286.0 ± 1067.7 g m−2) and with the biggest specimen (25.0 cm and 1616.0 g). The wide range of habitat variables matched the expectation of a wide ecological niche for the species. This study provides evidence that S. woodiana has already become established in many water systems in Bavaria, with fish ponds and fisheries management practises being a likely vector. To counteract this species and to conserve native mussel species, it is important to implement effective legislation, to take measures to eradicate this invasive mussel transnationally, and to raise public awareness.
Infaunal freshwater mussels are highly threatened and declining worldwide. One of the potential threats to mussels consists of biological invasions. We intended to investigate the habitat overlap and behavioural differences between native (Unio pictorum, Unio tumidus, Anodonta anatina, Anodonta cygnea) and invasive (Asian Sinanodonta woodiana) unionid bivalves to determine potential sources of competition. Furthermore, we investigated differences between S. woodiana from the established population in artificially heated waters and from the recent population in a natural thermal regime. We used pairwise choice tests on mud, medium, coarse and very coarse sand, mixture of medium and coarse sand, fine, medium and coarse gravel, and observed mussel locomotion and burrowing in preferred and non-preferred substrata. All species generally preferred fine-grained materials. The widest preference range was exhibited by S. woodiana (both populations), whereas A. cygnea was the most selective. The preferences of the cold-water population of S. woodiana were shifted towards coarser materials compared to conspecifics from the heated waters, and highly overlapped with the preferences of the native species. Anodonta cygnea most often moved horizontally and spent the shortest time deeply burrowed. Both Unio species were deeply burrowed for the largest amount of time and the horizontal locomotion of U. tumidus was the lowest among the test species. Sinanodonta woodiana, especially from the heated water population, exhibited relatively weak locomotion (compared to A. cygnea) and burrowing (compared to Unio spp. and A. anatina). Deep burrowing was more common on fine-grained materials. Our results suggest that the native mussels can be threatened by S. woodiana due to their overlapping habitat preferences, potentially hindering habitat separation. However, mobile native mussels may be capable of migrating and avoiding competition. Accumulating knowledge of the biology and ecology of freshwater mussels could contribute to the creation and improvement of conservation plans to protect these threatened animals.
Molluscs are among the most diverse and widespread animal groups in freshwater habitats. Unfortunately, like most freshwater taxa, they are decreasing dramatically and are now among the most threatened animals on Earth, with many species already extinct or on the brink of extinction. Here, we review our current knowledge on the biodiversity and conservation of freshwater molluscs using the concept of knowledge shortfalls. We focus on seven previously proposed key shortfalls to review and analyse existing knowledge gaps relating to (1) taxonomy, the Linnean Shortfall; (2) distribution, the Wallacean Shortfall; (3) abundance and population dynamics, the Prestonian Shortfall: (4) evolution, the Darwinian Shortfall; (5) abiotic tolerances, the Hutchinsonian Shortfall; (6) traits, the Raunkiaeran Shortfall; and (7) biotic interactions, the Eltonian Shortfall. In addition, we address a new shortfall, which relates to the application and effectiveness of conservation measures, including assessments, methods, funding, and policies, the Ostromian Shortfall. Based on our review, we provide recommendations and suggest pathways to overcome these existing shortfalls. This work also introduces the articles in this special issue of Hydrobiologia, which represent key contributions to the First International Freshwater Mollusk Conservation Society Meeting held in Verbania, Italy, in 2018.
1. Post-maturation growth leading to indeterminate growth patterns is widespread in nature. However, its adaptive value is unclear. Life history theory suggests this allocation strategy may be favored by temporal pulses in the intensity of mortality and/or the capacity to produce new tissues.
2. Addressing the origin of indeterminate growth and the variability of growth patterns , we studied the growth of duck mussels, Anodonta anatina, a pan-European unionid, in 18 Polish lakes. For each population, the sex, size, and age of collected mussels were measured to estimate Bertalanffy's growth curve parameters. We integrated information on A. anatina mortality rates, lake trophy, biofouling by zebra mussels, Dreissena polymorpha, and the prevalence of parasitic trematode larvae to identify selective conditions in lakes.
3. We found two sources of mortality in A. anatina populations, pertaining to adverse effects of zebra mussel biofouling and trophy state on mussel survival. Additionally, populations with heavier biofouling presented a smaller abundance of parasites, indicative of a relationship between filtering intensity and contraction of water-borne trematode larvae by filtering A. anatina.
4. Consistently for each sex, populations with a greater trophy-related mortality were characterized in A. anatina by a smaller asymptotic size L max , indicative of a life history response to mortality risk involving early maturation at a smaller body size. In all populations, females featured higher mortality and larger asymptotic size versus males.
5. Our findings support a theoretical view that adaptive responses to selection involve adjustments in the lifetime resource allocation patterns. These adjustments should be considered drivers of the origin of indeterminate growth strategy in species taking parental care by offspring brooding in body cavities.
The coypu ( Myocastor coypus ) is a semi-aquatic herbivorous rodent that has been suspected to prey on freshwater unionid mussels in Japan. Fecal DNA metabarcoding that targeted bivalves and terrestrial plants was performed to examine the diet of exotic coypu in the Kiso River, central Japan. Thirty-two fecal samples from four floodplain waterbodies were collected seasonally. In one of the waterbodies, live and dead mussels were investigated to examine the possible effects of coypu predation on mussel population structures. Common coypu predation on unionid mussels was confirmed in two waterbodies. Midden surveys showed that medium- to large-sized mussels tended to be consumed by coypu; possibly resulting in population structures with few large-sized individuals. Seasonal diet selection of terrestrial plants was also detected: two perennial plants were consumed throughout the year, whereas some perennial plants were consumed seasonally. The number of terrestrial plant species/taxa as primary food was higher in summer than in winter, which was attributed to the high consumption of annual plants in the summer. Future studies need to examine the exhaustive diet of coypu including other animals and plants, as well as its effects on freshwater ecosystems in invaded regions.
Mass mortality events, the rapid, catastrophic die-off of organisms, have recently been recognized as important events in controlling population size, but are difficult to quantify given their infrequency. These events can lead to large inputs of animal carcasses into aquatic ecosystems, which can have ecosystem scale impacts. Invasive freshwater bivalves such as the Asian clam Corbicula fluminea, the zebra mussel Dreissena polymorpha, the golden mussel Limnoperna fortunei, and the Chinese pond mussel Sinanodonta woodiana can attain high densities and biomass and play important roles in aquatic ecosystems through filtration, bioturbation, and excretion. Invasive bivalve species can best be described as R-selected species and appear not to have the same tolerance to abiotic stressors as native species, causing them to be prone to mass mortality events in their invasive range. In contrast to their ecological effects while alive, the frequency and impacts of mass mortality events of invasive freshwater bivalves are not well-understood. Here we review the causes and impacts of mass mortality events, as well as identify important questions for future research. Extreme abiotic conditions, including both drought and flooding, as well as high and low temperatures were the primary drivers of mass mortality events. Short-term impacts of mass mortality events include large pulses of nitrogen and increased oxygen stress due to large amounts of soft tissue decomposition, while shells can impact habitat availability and nutrient cycling for decades. Impacts on biological communities (bacteria, fungi, and macroinvertebrates) are less studied but some examples exist concerning C. fluminea. Better documentation of mass mortality events, particularly their magnitude and frequency, is needed to fully understand the impacts invasive bivalve species have on ecosystems, especially as climate change may make mass mortality events more frequent and/or have a larger magnitude.
The Asian clam, Corbicula fluminea, was introduced into North America in the 1920s—first observed in the Columbia River—and has expanded its range across the continent and into South America and Europe, yet little is known about its ecology and potential to impact food webs. To evaluate prey selectivity and feeding rates of C. fluminea, we conducted laboratory feeding experiments using water from two distinct Columbia River environments (unimpounded river and reservoir) during July and October 2016. The mean clearance rate on microplankton was 270 (± 53.6 SE) ml water clam⁻¹ h⁻¹ and mean ingestion rate was 2.45 (± 0.83 SE) µg C clam⁻¹ h⁻¹, although rates varied with season and location. In the reservoir in July, clams preferred diatoms and showed an avoidance of dinoflagellates and flagellates; during October in the unimpounded river, clams preferred flagellates while showing a significant avoidance of cyanobacteria. Diatoms were dominant at both sites, and were ingested by clams; however, clams ingested cyanobacteria at very low rates. Substantial consumption of microplankton such as diatoms and rejection of cyanobacteria by C. fluminea may provide competitive advantages to cyanobacteria, leading to microplankton community composition shifts and other changes to food webs in the Columbia River.
A new large population of the Asian clam Corbicula fluminea (O.F.Müller, 1774) was found within a reach (ca 210 km long) of one of the largest European rivers, the Vistula.The largest population and the largest individuals were found at the outlet of a channel collecting heated water from the cooling process at the Połaniec power station and adjacent parts of the river.In the northern part of the study area, bivalves occurred at the channel margins, in groyne fields, and in the shallows of sand banks or sandbars.The clams were less numerous in places where the river was regulated with straightened banks and stone ripraps.Twenty-five other taxa of mollusks were found altogether at the sampling stations where Corbicula was observed, including other non-indigenous mollusk species. © 2018 Faculty of Oceanography and Geography, University of Gdańsk, Poland 2018.
“One hundred worst” lists of alien species of the greatest concern proved useful for raising awareness of the risks and impacts of biological invasions amongst the general public, politicians and stakeholders. All lists so far have been based on expert opinion and primarily aimed at representativeness of the taxonomic and habitat diversity rather than at quantifying the harm the alien species cause. We used the generic impact scoring system (GISS) to rank 486 alien species established in Europe from a wide range of taxonomic groups to identify those with the highest environmental and socioeconomic impact. GISS assigns 12 categories of impact, each quantified on a scale from 0 (no impact detectable) to 5 (the highest impact possible). We ranked species by their total sum of scores and by the number of the highest impact scores. We also compared the listing based on GISS with other expert-based lists of the “worst” invaders. We propose a list of 149 alien species, comprising 54 plants, 49 invertebrates, 40 vertebrates and 6 fungi. Among the highest ranking species are one bird (Branta canadensis), four mammals (Rattus norvegicus, Ondatra zibethicus, Cervus nippon, Muntiacus reevesi), one crayfish (Procambarus clarkii), one mite (Varroa destructor), and four plants (Acacia dealbata, Lantana camara, Pueraria lobata, Eichhornia crassipes). In contrast to other existing expert-based “worst” lists, the GISS-based list given here highlights some alien species with high impacts that are not represented on any other list. The GISS provides an objective and transparent method to aid prioritization of alien species for management according to their impacts, applicable across taxa and habitats. Our ranking can also be used for justifying inclusion on lists such as the alien species of Union concern of the European Commission, and to fulfil Aichi target 9.
Ecosystem services are the benefits that humans derive from ecosystems. Freshwater mussels perform many important functions in aquatic ecosystems, which can in turn be framed as the ecosystem services that they contribute to or provide. These include supporting services such as nutrient recycling and storage, structural habitat, substrate and food web modification, and use as environmental monitors; regulating services such as water purification (biofiltration); and provisioning and cultural services including use as a food source, as tools and jewelry, and for spiritual enhancement. Mussel-provided ecosystem services are declining because of large declines in mussel abundance. Mussel propagation could be used to restore populations of common mussel species and their ecosystem services. We need much more quantification of the economic, social, and ecological value and magnitude of ecosystem services provided by mussels, across species, habitats, and environmental conditions, and scaled up to whole watersheds. In addition, we need tools that will allow us to value mussel ecosystem services in a way that is understandable to both the public and to policy makers.
The global decline of freshwater mussels is related with a great variety of factors, including the introduction of invasive species. However, the possible effects of other invasive bivalves, such as the Asian clam Corbicula fluminea (Müller, 1774), remain mainly unknown and highly speculative with very few manipulative experiments addressing this issue. In this study, field and laboratory experiments were conducted to assess the possible negative effects of C. fluminea on the native freshwater mussel Unio delphinus Spengler, 1783. Growth, physiological condition, and the locomotor activity were assessed in U. delphinus at increasing C. fluminea density. U. delphinus exhibited lower growth, lower physiological condition, and higher locomotor activity at higher C. fluminea density, which may suggest that this unionid is negatively affected by C. fluminea and may be displaced to less favorable habitats. Although we were not able to establish the main mechanism(s) responsible for these results, possibilities include competition for food resources, competition for space which may result in mussel displacements and/or changes in microhabitat features as a result of bioturbation activities, and production of feces and pseudofeces by C. fluminea.
The first record of the Asian clam Corbicula fluminea in Bulgaria was from the Danube River at Vetren in 2001. In 11 years the species has established in the entire Bulgarian stretch of the Danube, from Vrav (rkm 836) to Vetren (rkm 395), where it reached densities up to 16 560 ind./m2. The species extended its range rapidly upstream of the Danube tributaries reaching 80 km upstream from the confluence with the Danube (Iskar River). Recently, the species also established in standing waters, two reservoirs and one sand-pit lake, located at altitudes of up to 525 m a.s.l. The most probable dispersal vector of C. fluminea in the inland waters of Bulgaria is passive upstream transport which is probably facilitated by some human activities, such as fishing, fish stocking, recreational activities, sand and gravel extraction.
Freshwater mussels of the Order Unionida provide important ecosystem functions and services, yet many of their populations are in decline. We comprehensively review the status of the 16 currently recognized species in Europe, collating for the first time their life-history traits, distribution, conservation status, habitat preferences, and main threats in order to suggest future management actions. In northern, central, and eastern Europe, a relatively homogeneous species composition is found in most basins. In southern Europe, despite the lower species richness, spatially restricted species make these basins a high conservation priority. Information on freshwatermussels in Europe is unevenly distributed with considerable differences in data quality and quantity among countries and species. To make conservation more effective in the future, we suggest greater international cooperation using standardized protocols and methods to monitor and manage European freshwater mussel diversity. Such an approach will not only help conserve this vulnerable group but also, through the protection of these important organisms, will offer wider benefits to freshwater ecosystems.
Alien species (those carried outside their original ranges by human activities) have strongly affected the distribution and abundance of mollusks in many North American fresh waters. The best known of these aliens in North America is the zebra mussel (Dreissena polymorpha), which has nearly extirpated native unionid clams from infested lakes and rivers by fouling their shells and outcompeting them for food. Zebra mussels also have reduced populations of native sphaeriid clams, and both increased and reduced populations of snails. The effects of the other well-known alien bivalve in North America, Corbicula fluminea, are surprisingly poorly known. Corbicula probably caused some populations of native bivalves to decline, but other native populations seem to coexist with Corbicula. Several plausible mechanisms of interaction between Corbicula and the native biota have been proposed, but not demonstrated. Other aliens, including the recently arrived snail Potamopyrgus antipodarum, probably compete strongly with native freshwater mollusks under some circumstances. Several alien species, such as round goby and some sunfishes and crayfishes introduced outside their native ranges in North America, are effective predators on native mollusks and have strong effects on their distribution and abundance. Other aliens (particularly aquatic plants) affect mollusks by altering the food base or the physicochemical environment. Alien species can affect water quality, cycling of contaminants, and performance of biological indices of water quality. Because of ineffective control of aliens in North America, they may be an increasingly important factor in molluscan distribution as new species arrive from other continents and established species spread throughout the continent.
The effects of algal density, water flow and substrate type on the survival and growth of newly metamorphosed juveniles of the rainbow mussel (Villosa iris) were investigated in 40 l recirculating culture systems. A split-plot design with a whole plot factor combining algal density and water flow, and a split-plot factor of substrate were applied in this study to test the three factors. Juveniles reared in 4 juvenile culture units (550 ml) with fine sediment (<200 mu m), sand (300-500 mu m), limestone gravel (800-1000 mu m) and without substrate in each recirculating system receiving water flow at levels of 3.75 ml s(-1) or 7.5 ml s(-1); algae concentrations of 35,000 cells ml(-1), 87,500 cells ml(-1), or 175,000 cells ml(-1) were sampled every 10 days during a 30-day experiment. Substrate type significantly affected survival and growth of juveniles (p < 0.0001), while algal density and water flow had no significant influence on their survival or growth (p > 0.05). Juveniles had significantly higher survival and faster growth in a substrate of fine sediment when compared to other substrate types (p < 0.05). Escapement rate of juveniles exceeded 39% overall in culture units. Water quality was consistent throughout the experimental duration and within a suitable range for juvenile mussels. Results confirmed that fine sediment is the best substrate for juvenile culture in a laboratory recirculating system. The split-plot design was effective for conducting this multi-factor experiment and saved significant time and resources. Published by Elsevier B.V.
Two populations of Corbicula fluminea were found in the Iberian Península; one in Spain and the other in Portugal. A detailed description in terms of ecology shell morphology and microstructure, morphometrics and anatomy is given for the Spanish population from the Mino River. Lectotypes for Tellina fluminea and T. fluminalis, and a neotype for T. fluviatilis are designated and illustrated. Distribuüon and spread of C. fluminea in Europe are revised. Comparisons among some European populations and the populations from Cantón, China, and the Mino River are made. Results suggest that, except for one doubtful population, all records of Corbicula in Europe are attributable to C. fluminea. Corbicula taxonomy begins in 1774 with Müller who described three species in the genus Tellina Linne, 1758: T. fluminalis "in fluvio Asiae Euphrat"; T. fluminea "in arena fluviali Chinae"; T. fluviatilis "in ilumine emporium Can tón Chinae praeterlabente". Since then, many living species of Corbicula Mühlfeldt, 1811, have been described in freshwater and estuarine habitáis from Southeast Asia, the Indian subcontinent, the Pacific islands, and the easternmost part of Europe and África (McMahon, 1983). The fossil record of Corbicula includes Europe, North América and Japan (see Linstow, 1922; Zhadin, 1952; Ellis, 1978; and Britton and Morton, 1979 for a review). The first published record of Corbicula in North América is that of Burch (1944) in 1938. Counts (1981, 1985) cites the presence of the species in 1924 and 1937 in Nanaimo, Vancouver Island, British Columbia, Canadá, and in Ray-mond, Pacific County, Washington, respectively. Since then, it spread widely in most lotic and lentic habitáis, being a pest with very important economic and ecological effects (Sinclair and Isom, 1963; McMahon, 1983). Many papers have been published with records of new localities and biological data using different species ñames, mainly C. fluminea, C. manilensis and C. leana (McMahon, 1983 and references therein). Several hypotheses about the importance of the role of human activities in the spread of Corbicula have been treated (e.g. Thompson and Sparks, 1977; McMahon, 1982). The wide geographical and ecological range of Cor bicula seems to be related to the great variation in shell form and colour. These two features are the most common tax-onomic characters and the only ones used by the early conchologists, suggesting that Corbicula taxonomy probably involves more species ñames than needed. Thus, Talavera and Faustino (1933) {In: Britton and Morton, 1979) placed Corbicula manilensis (Philippi, 1844) into synonymy with C. fluminea, Morton (1977) considered C. leana to be a júnior synonym of C. fluminea, while C. fluviatilis was previously placed into synonymy with C. fluminea by Prashad (1929). Moreover, a thorough review by Britton and Morton (1979) lead the authors to consider that most Asiatic species previously described could be attributed to two taxa: the freshwater species C. fluminea (Müller, 1774) and the estuarine species C. fluminalis (Müller, 1774). Studying North American populations of Corbicula on the basis of ecology, functional morphology and reproduc-tive biology, Britton and Morton (1979) concluded that all belonged to the single species, C. fluminea. The results of this paper, and the conclusions of Morton (1982), seem to provide a good discrimination between C. fluminea and C. fluminalis. In the last decade, Corbicula was also introduced into South América (Ituarte, 1981) and Europe. Mouthon (1981) reports the presence of C. fluminalis in France (La Dordogne) and in Portugal (Tajo River estuary). Nagel (1989) cites the species from the Duero River near Oporto (Portugal) and Girardi (1989-1990) indicates the occurrence of C. fluminalis also in France at the Canal du Midi at Grisolles (Tarn and Garonne). We found two Corbicula populations in the Iberian Península, one in Spain and the other in Portugal, apparent-ly corresponding to C. fluminea. These facts suggest that, as occurred in North América, Europe is being currently invaded by this bivalve and that species discrimination is probably not as clear as previously thought (Morton, 1982), because it still seems to allow the use of various species ñames for morphological variants of the same species concept.
The clam genus Corbicula is an interesting model system to study the evolution of
reproductive modes as it includes both sexual and asexual (androgenetic) lineages.
While the sexual populations are restricted to the native Asian areas, the androgenetic
lineages are widely distributed being also found in America and Europe where they
form a major aquatic invasive pest. We investigated the genetic diversity of native and
invasive Corbicula populations through a worldwide sampling. The use of mitochondrial
and nuclear (microsatellite) markers revealed an extremely low diversity in the
invasive populations with only four, undiversified, genetic lineages distributed across
Europe and America. On the contrary, in the native populations, both sexual and
androgenetic lineages exhibited much higher genetic diversity. Remarkably, the most
abundant and widely distributed invasive forms, the so-called form A and form R
found in America and Europe respectively, are fixed for the same single COI (cytochrome
c oxydase subunit I) haplotype and same multilocus genotype. This suggests
that form R, observed in Europe since the 1980s, derived directly from form A found
in America since the 1920s. In addition, this form shares alleles with some Japanese
populations, indicating a Japanese origin for this invasive lineage. Finally, our study
suggests that few androgenetic Corbicula individuals successfully invaded the
non-native range and then dispersed clonally. This is one striking case of genetic
paradox raising the issue of invasive and evolutionary success of genetically
undiversified populations
Managers in the U pper M ississippi R iver ( UMR ) are using reductions in the R iver's water levels during summer to mimic historical water regimes and rehabilitate habitats for vegetation and other species. Concerns for the unintended effects of these actions on mussel populations threatened to halt these projects.
Our objective was to characterise the survival and movement of two mussel species in the UMR associated with a water level drawdown. During 2009 (no drawdown) and 2010 (0.3 m summer drawdown), we glued passive integrated transponder tags to 10 A mblema plicata and 10 L ampsilis cardium at each of 11 sites. Five sites were in shallow areas expected to be minimally affected by the drawdown (reference sites), and six sites were in shallow areas expected to be directly affected by the drawdown (treatment sites). About equal numbers of sites within both the reference and treatment areas had low and high slopes. Tagged mussels were randomly allocated across sites (within years).
Recovery of tagged mussels was >88% in 2009 and 2010. Mortality was similar and low (mean, c . 5% in both species) among reference sites but was variable and relatively high (means, c . 27% in L . cardium and c . 52% in A . plicata ) among treatment sites; variation in mortality among treatment sites appeared related to slope.
The study found evidence of drawdown associations with net horizontal movements in A . plicata but not L . cardium . Weekly horizontal movements in both species were significantly correlated with changes in water elevation. We observed significant slope associations related to the drawdown for mortality and net horizontal movement in A . plicata .
There were strong species‐specific differences in the effects of the drawdown on mortality, vertical movement and horizontal movement. These results suggest that A . plicata responded to the drawdown by vertical movement into the substratum, whereas L . cardium responded by horizontal movement to deeper water. No directionality of movement was observed in either species.
Collectively, these data suggest that drawdowns can influence the mortality, movement and behaviour of mussels in the UMR . However, more information on spatial and temporal distributions of mussels is needed to better understand the magnitude of these effects. Results from this study are being used by resource managers to better evaluate the effects of this management tool on native mussel assemblages.
The European bitterling Rhodeus sericeus Pallas is a freshwater fish belonging to the Acheilognathinae, a subfamily that has an unusual spawning symbiosis with freshwater mussels. Female bitterling possess long ovipositors that they use to place their eggs onto the gills of a mussel through the mussels' exhalant siphon. Males fertilize the eggs by releasing sperm into the inhalant siphon of the mussel. The embryos develop inside the mussel for approximately a month, eventually leaving the mussel as actively swimming larvae. Because they use a discrete spawning site that can be readily manipulated they have recently been used in studies to understand oviposition choice with respect to mate and spawning site quality, host–parasite coevolution, alternative male mating tactics, and linking behavioural decisions with population dynamics. The current state of knowledge of bitterling reproductive ecology is reviewed and synthesized, and future directions for research on this species are proposed.
Predation is a dominant structuring force in ecological communities. In aquatic environments, predation on bivalves has long been an important focal interaction for ecological study because bivalves have central roles as ecosystem engineers, basal components of food webs, and commercial commodities. Studies of bivalves are common, not only because of bivalves' central roles, but also due to the relative ease of studying predatory effects on this taxonomic group. To understand patterns in the interactions of bivalves and their predators we synthesised data from 52 years of peer-reviewed studies on bivalve predation. Using a systematic search, we compiled 1334 studies from 75 countries, comprising 61 bivalve families (N = 2259), dominated by Mytilidae (29% of bivalves), Veneridae (14%), Ostreidae (8%), Unionidae (7%), and Dreissenidae and Tellinidae (6% each). A total of 2036 predators were studied, with crustaceans the most studied predator group (34% of predators), followed by fishes (24%), molluscs (17%), echinoderms (10%) and birds (6%). The majority of studies (86%) were conducted in marine systems, in part driven by the high commercial value of marine bivalves. Studies in freshwater ecosystems were dominated by non-native bivalves and non-native predator species, which probably reflects the important role of biological invasions affecting freshwater biodiversity. In fact, while 81% of the studied marine bivalve species were native, only 50% of the freshwater species were native to the system.
In terms of approach, most studies used predation trials, visual analysis of digested contents and exclusion experiments to assess the effects of predation. These studies reflect that many factors influence bivalve predation depending on the species studied, including (i) species traits (e.g. behaviour, morphology, defence mechanisms), (ii) other biotic interactions (e.g. presence of competitors, parasites or diseases), and (iii) environmental context (e.g. temperature, current velocity, beach exposure, habitat complexity). There is a lack of research on the effects of bivalve predation at the population and community and ecosystem levels (only 7% and 0.5% of studies respectively examined impacts at these levels). At the population level, the available studies demonstrate that predation can decrease bivalve density through consumption or the reduction of recruitment. At the community and ecosystem level, predation can trigger effects that cascade through trophic levels or effects that alter the ecological functions bivalves perform. Given the conservation and commercial importance of many bivalve species, studies of predation should be pursued in the context of global change, particularly climate change, acidification and biological invasions.
1. Clams from the Corbicula species complex of Asian origin are invasive all over the world, currently extending their distribution to waters of the natural thermal regime in Central European areas. Features linking the clams to their habitats in newly invaded areas have not been fully examined. These include substratum preferences and mobility, which may be key to determining potential niche overlap with native bivalves and gaining a better knowledge of their potential active dispersal and impact. Therefore, our aim was to determine the substratum preferences, horizontal locomotion and burrowing behaviour of three Corbicula morphospecies from Poland, Central Europe: two populations of undetermined Corbicula sp. from an artificially heated habitat and water of the natural thermal regime (cold water), as well as Corbicula leana, and Corbicula fluminea (from artificially heated and cold water, respectively).
2. To determine the preferred substratum range, we used pairwise choice tests with a range of muddy, sandy, and gravelly substrata, as well as zebra mussel shells. Then, we conducted behavioural observations (locomotion and burrowing) by CCTV cameras on substrata found as preferred and non-preferred by clams.
3. All morphospecies preferred fine-grained over coarser substrata. C. fluminea from cold water had the widest range of substratum preferences. By contrast, C. leana from artificially heated water and Corbicula sp. from cold water showed the narrowest substratum preferences and spent the longest time deeply burrowed. The longest duration of burrowing activity was noted on non-preferred coarser substrata, which may indicate difficulties in burrowing. On non-preferred coarse substrata, C. fluminea (cold water) crawled more than the other morphospecies.
4. Our results show that sediment preferences and behaviour (burrowing and loco-motion) differ among various Corbicula morphospecies in Europe, probably also varying their invasive potential.
5. This knowledge is important for determining the overlap of habitat preferences of non-native Corbicula clams with native bivalves, which is critical for determining potential competition strength, impact, and/or displacement. Our results also indicate that habitats with fine sediments could be more threatened by Corbicula invasions.
Here, we investigate the shell shape variation of some closely related freshwater species of the bivalve genus Corbicula using descriptive (qualitative), geometric morphometric and traditional conchometric approaches. The combination of these different approaches allows for an effective discrimination between the species C. fluminalis, C. fluminea and C. leana, as well as an unidentified Corbicula sp. The roundness of the shell hinge is an important diagnostic feature, as are shell sculpture (ribs), symmetry of the apertural margin, and both position and extension of the umbo. We also identify possible hybrids between C. fluminalis and C. leana, with these showing features intermediate to those of the parent species. We examine variability of shell features of C. leana in selected areas in Europe and compare these results with material from the native range of Japan and Korea. For C. leana, we identify two geographic morphotypes from the native area; within Europe, there is a high morphological diversity of this species with several new forms arising, most probably as a result of hybridization.
• Freshwater mussels (Order Unionida) are highly threatened. Interspecific competition for food sources with invasive alien species is considered to be one of the factors responsible for their decline because successful invaders are expected to have wider trophic niches and more flexible feeding strategies than their native counterparts.
• In this study, carbon (δ¹³C: ¹³C/¹²C) and nitrogen (δ¹⁵N: ¹⁵N/¹⁴N) stable isotopes were used to investigate the trophic niche overlap between the native freshwater mussel species, Anodonta anatina, Potomida littoralis, and Unio delphinus, and the invasive bivalve Corbicula fluminea living in sympatry in the Tua basin (south-west Europe).
• The species presenting the widest trophic niches were C. fluminea and A. anatina, which indicate that they have broader diets than U. delphinus and P. littoralis. Nonetheless, all the species assimilated microphytobenthos, sediment organic matter, and detritus derived from vascular plants, although with interspecific variability in the assimilated proportions of each source. The trophic niche of the invasive species overlapped with the trophic niche of all the native species, with the extent varying between sites and according to the species.
• From the three native species analysed, Potomida littoralis may be at a higher risk for competition for food with C. fluminea in the Tua basin, if food sources become limited, because this native mussel presented the narrowest trophic niche across sites and the highest probability of overlapping with the trophic niche of C. fluminea.
• Given the global widespread distribution of C. fluminea, the implementation of management measures devoted to the control or even eradication of this invasive alien species should be a conservation priority given its potential for competition with highly threatened native freshwater mussels.
1. Conservation efforts have increased in response to global mussel declines, and effective surveys are a crucial step in assessing and monitoring mussel populations and in determining their conservation status. The burrowing behaviour of mussels can affect their detectability, and a better understanding of these behaviours would help to improve survey design and guidelines.
2. The burrowing depth of mussels may differ between seasons, habitat conditions, species, and individuals, and little is known about the burrowing behaviour of mussels in subtropical rivers.
3. Burrowing depth variation was examined and compared at three sites in the San Marcos, Guadalupe, and San Antonio river drainages in central Texas. In addition, laboratory experiments were used to determine whether observed differences between field sites and seasons could be linked to differences in substrate type and water temperature and to examine differences between species.
4. Seasonal variation in burrowing depth was found at all field sites, and water temperature was a significant factor for explaining variation in burrowing depth, but there was no clear relationship between burrowing depth and temperature in shorter term laboratory experiments, where individual variation was high and burrowing behaviour seemed to be solely a function of time.
5. Mussels burrowed significantly deeper in finer substrate (sand vs. gravel) in both field and laboratory experiments. Few significant differences between species were found in the field, but no differences were found in the laboratory experiments.
6.The results suggest that surveys may need to follow different guidelines depending on local conditions, such as substrate and water temperature. Surveys will be less efficient and may fail to detect larger proportions of populations in colder water temperatures. In addition, a larger proportion of burrowed mussels can be expected at sites with finer substrate, such as sand. Under these conditions, visual searches will not suffice, as a large part of the population or specific species may be overlooked.
Bivalve shells can persist over a geological time, acting as important physical resources to the associated fauna. However, few studies have investigated their relevance as persistent long-term ecological attributes to the ecosystem. As such, it is relevant to investigate the shell decays in riverine systems subjected to different environmental conditions. Towards this end, shells of four bivalve species (Anodonta anatina, Corbicula fluminea, Potomida littoralis and Unio delphinus) were made available individually and in clusters of different sizes. The effects of river flow and seasonality were assessed by recording the decay rates of shells in lentic and lotic habitats throughout the year. Our results evidenced that the decays varied among species and depend on shell size, water flow and season. Thin shelled species (A. anatina and U. delphinus) showed the highest mean percentage of decay per month, 3.17% (lotic) and 2.77% (lotic), respectively, and thick shelled species (C. fluminea and P. littoralis) the lowest, 2.02% (lotic) and 1.83% (lotic), respectively. Size was a relevant variable explaining decays, with the smallest shells presenting the highest values, 1.2–2.0 times higher compared to the other size classes. Also, robustness showed to be the most relevant feature explaining the decays in thick shelled species. River flow was also a relevant descriptor of the decays, with higher decays observed in the lotic compared to the lentic habitats. Furthermore, lower decays were observed mainly during summer (lentic site), and autumn (lotic site) associated to the burial effect of leaves. In summary, shells of the native species A. anatina and U. delphinus are expected to persist and contribute less as habitat engineering species, than shells of the native P. littoralis and invasive C. fluminea species. This is especially valid to lotic habitats where the decays were up to 2.13 times higher than in lentic habitats.
Freshwater mussels (Bivalvia, Unionida) are one of the most imperilled faunal groups globally, being the introduction of invasive species a possible major mechanism of threat. The Asian clam Corbicula fluminea is a problematic invasive species in aquatic ecosystems and can impair the survival of parasitic larvae (glochidia) of native freshwater mussels. However, this possible mechanism of threat remains speculative and to date very few studies addressed quantitatively this issue. In order to cover this gap, we have performed a series of manipulative laboratory studies to assess how distinct densities of C. fluminea can affect the survival of glochidia after 6, 12, 24 and 48 h of exposure, using larvae of the native freshwater mussel Anodonta anatina. Our results suggest an increase in mortality of A. anatina glochidia with an increase in density of C. fluminea. Two main mechanisms may possibly explain our results: 1) the high filtration capacity of C. fluminea that can contribute to the mortality of glochidia due to the mechanical damage of their fragile shells when passing by siphons and/or digestive tract of C. fluminea and 2) the high excretion capacity of C. fluminea that can lead to mortality of glochidia due to increase in ammonia concentration. Mortality of glochidia was also time dependent with higher values registered after 48 h. This work is one of the first showing the influence of C. fluminea density on the survival of glochidia, being filtration (and consequent passage in the digestive tract) and biodeposition the main potential mechanisms explaining overall mortality. These results also suggest that sites with high densities of C. fluminea may be highly detrimental for the conservation of freshwater mussels, potentially impairing the survival of glochidia and negatively affecting the recruitment of juveniles.
The freshwater pearl mussel Margaritifera margaritifera is a highly threatened species in Europe. Several mechanisms may be responsible for the decline in distribution and abundance of European pearl mussel populations, but almost no quantitative data exists about the possible negative impacts of invasive alien species (IAS). In this study, we clearly demonstrate that the invasive signal crayfish Pacifastacus leniusculus predates pearl mussels, using a laboratorial experiment followed by in situ validation in four rivers in the North of Portugal (Mente, Rabaçal, Tuela and Baceiro Rivers; Douro Basin). In the laboratory, the crayfish had a clear preference for small-sized pearl mussels but no differences in predation were found in mesocosms with and without sediment. In addition, we clearly demonstrated that the signal crayfish predates pearl mussels in natural conditions and detected a significant density dependent effect (i.e., sites with more crayfish presented higher number of pearl mussel shells with marks of predation). Given the recent introduction of the signal crayfish and the potential negative impacts on pearl mussel populations we also investigated its autoecology (distribution, abundance, size structure and sex-ratio) in the four studied rivers. Significant differences in average abundance and size of the crayfish were detected between sites and the sex-ratio was highly skewed to females. In view of the widespread distribution of signal crayfish (and other invasive crayfish species) in Europe, future management actions devoted to the conservation of pearl mussels should take in consideration the possible negative effects of these predators, especially on juveniles.
Bivalve molluscs are abundant in marine and freshwater ecosystems and perform important ecological functions. Bivalves have epifaunal or infaunal lifestyles but are largely filter feeders that couple the water column and benthos. Bivalve ecology is a large field of study, but few comparisons among aquatic ecosystems or lifestyles have been conducted. Bivalves impact nutrient cycling, create and modify habitat, and affect food webs directly (i.e., prey) and indirectly (i.e., movement of nutrients and energy). Materials accumulated in soft tissue and shells are used as environmental monitors. Freshwater mussel and oyster aggregations in rivers and estuaries are hot spots for biodiversity and biogeochemical transformations. Historically, human use includes food, tools, currency, and ornamentation. Bivalves provide direct benefits to modern cultures as food, building materials, and jewelry and provide indirect benefits by stabilizing shorelines and mitigating nutrient pollution. Research on bivalve-mediated ecological processes is diverse, and future synthesis will require collaboration across conventional disciplinary boundaries.
• Macrobiofouling is an important phenomenon in the aquatic environment, resulting in economic losses and environmental changes, including negative impact on hard‐shelled animals. A freshwater invasive byssate bivalve, the zebra mussel Dreissena polymorpha (Dreissenidae), strongly affects bivalves from the family Unionidae by fouling their shells. We tested potential mechanisms explaining variable fouling of different unionids (four native species: Anodonta anatina, A. cygnea, Unio pictorum, U. tumidus and the non‐native Sinanodonta woodiana) by the zebra mussel.
• We conducted a field survey (unionids collected at a sandy and muddy site in a dam reservoir), field experiment (unionids exposed without sediments in suspended baskets to dreissenid settlement) and laboratory experiment (a multiple choice test with and without the possibility of unionid burrowing into sandy sediments).
• In the survey, zebra mussel density was highest on A. anatina and S. woodiana, intermediate on U. pictorum and lowest on A. cygnea and U. tumidus. In the field experiment, A. anatina and S. woodiana were more fouled than Unio spp. In the laboratory, zebra mussels less often attached to U. pictorum and, when unionids could burrow themselves, also to S. woodiana. However, no unionid species was positively selected in the presence of stone substrata.
• The percentage of zebra mussels attached directly to unionids (compared to those attached to conspecifics) in the field survey was negatively related to the overall zebra mussel biomass. Zebra mussel fouling negatively affected the biomass of an allopatric S. woodiana on muddy bottom.
• Dreissenids overgrow different unionid species to variable extent, not only due to the differences in their exposed surface area, but, as our experimental results show, also because of their active substratum selection. Moreover, mussels seem to prefer unionid surface over conspecific shells, the latter being fouled only when overall dreissenid fouling is heavy. As unionids often constitute the main source of hard substratum for zebra mussels in waterbodies, the species composition of a unionid assemblage may affect dreissenid success by offering them variable substratum quality.
Bivalves are ubiquitous members of freshwater ecosystems and responsible for important functions and services. The present paper revises freshwater bivalve diversity, conservation status and threats at the global scale and discusses future research needs and management actions. The diversity patterns are uneven across the globe with hotspots in the interior basin in the United States of America (USA), Central America, Indian subcontinent and Southeast Asia. Freshwater bivalves are affected by multiple threats that vary across the globe; however, pollution and natural system (habitat) modifications being consistently found as the most impacting. Freshwater bivalves are among the most threatened groups in the world with 40% of the species being near threatened, threatened or extinct, and among them the order Unionida is the most endangered. We suggest that global cooperation between scientists, managers, politicians and general public, and application of new technologies (new generation sequencing and remote sensing, among others) will strengthen the quality of studies on the natural history and conservation of freshwater bivalves. Finally, we introduce the articles published in this special issue of Hydrobiologia under the scope of the Second International Meeting on Biology and Conservation of Freshwater Bivalves held in 2015 in Buffalo, New York, USA.
Invasion of North American waters by nonnative Dreissena polymorpha and D. rostriformis bugensis has resulted in declines of the Unionidae family of native North American mussels. Dreissenid mussels biofoul unionid mussels in large numbers and interfere with unionid movement, their acquisition of food, and the native mussels' ability to open and close their shells. Initial expectations for the Great Lakes included extirpation of unionids where they co-occurred with dreissenids, but recently adult and juvenile unionids have been found alive in several apparent refugia. These unionid populations may persist due to reduced dreissenid biofouling in these areas, and/or due to processes that remove biofoulers. For example locations inaccessible to dreissenid veligers may reduce biofouling and habitats with soft substrates may allow unionids to burrow and thus remove dreissenids. We deployed caged unionid mussels (Lampsilis siliquoidea) at 36 sites across the western basin of Lake Erie to assess spatial variation in biofouling and to identify other areas that might promote the persistence or recovery of native unionid mussels. Biofouling ranged from 0.03-26.33 g per mussel, reached a maximum in the immediate vicinity of the mouth of the Maumee River, and appeared to primarily consist of dreissenid mussels. A known mussel refugium in the vicinity of a power plant near the mouth of the Maumee actually exhibited very high biofouling rates, suggesting that low dreissenid colonization did not adequately explain unionid survival in this refugium. In contrast, the southern nearshore area of Lake Erie, near another refugium, had very low biofouling. A large stretch of the western basin appeared to have low biofouling rates and muddy substrates, raising the possibility that these open water areas could support remnant and returning populations of unionid mussels. Previous observations of unionid refugia and the occurrence of low biofouling rates in large areas of the western basin of Lake Erie raise the possibility that unionid and dreissenid coexistence may be possible here and elsewhere.
Unio crassus, an endangered freshwater mussel species, inhabits streams with diversified physical structure, subject to frequent changes in channel morphology. It is usually regarded as a sedentary animal, although individual movement has been observed. The hypothesis that adults can use movement in order to actively select a microhabitat was tested in two experiments simulating dislodgement of mussels by a stream. In the first experiment, begun in 2004, individually tagged Unio crassus individuals were distributed in a regular pattern through ut a 17 m long riffle-pool reach of Cedron River (S Poland). The mussels changed their positions after displacement. The distances travelled differed significantly between parts of the channel: they covered longer distances in the pool (ca 2 in) than in the riffle (ca 0.6 in), with minimal movements on steep banks and dead water areas and large movements in shallows and deep areas. Individuals from shallow places were found to have moved to the steep bank of the pool, built of fine sediments, and to the area of dead water in the lee of boulders. The movements were not related to the water depth gradient: individuals moved to the steep bank even from the deepest parts of the pool, a finding experimentally confirmed with radio-tracked individuals. The second experiment was begun in 2008 in a smaller reach 5 in long. The mussels were put in the deepest part of the reach. Most of them climbed to a steep bank built of fine sediment with minimal flow, and some of them hid beneath the gray 1 armouring the fast-flowing part of the channel. It can be concluded that adult Unio crassus individuals can move to any place in a channel after being disturbed by natural or artificially created fluvial processes. Steep banks of the pool, built of fine sediments, represented the best place of refuge.
Europe's increasing populations of medium-size predators pose a potential threat to freshwater mussels in some regions. Live mussels (Anodonta cygnea) collected from the bottom of Zalew Pinczowski reservoir (S Poland) did not differ in size from shells of individuals predated by otters, but the shells of predated individuals varied in size significantly less, suggesting that only the middle size class is predated. A similar size class was predated in a sample of live mussels equipped with radio transmitters and experimentally distributed near an otter den: 10% were eliminated within a month, indicating substantial predation pressure. Older mussels were not attacked or else the attacks were unsuccessful, suggesting that the otter cannot seriously threaten the reproductive-age part of the population, but predation on middle-size individuals might lead to ageing and eventual extinction of populations, especially small and isolated ones.
The Asian clam Corbicula fluminea (Müller, 1774) has been recognized as one of the most important invasive alien species in aquatic ecosystems and may have significant ecological and economic impacts. Recently, the presence of C. fluminea was associated with changes in benthic and epibenthic fauna. In this study, we aimed to understand the mechanisms underlying the effects of C. fluminea on an estuarine macrozoobenthic assemblage using a manipulative experiment. We used 5 different treatments (control, rock, closed, live, open), which were placed in a low sandy intertidal soft bottom area in the Minho estuary (NW Iberian Peninsula) for 2 months. We found that the presence of live and open empty shells of C. fluminea had positive effects on the density, biomass and species richness of macrozoobenthos, specifically on species belonging to Annelida, Mollusca and Crustacea. Our results may be explained by 2 main mechanisms: (1) the production of feces and pseudofeces by C. fluminea, which increases organic matter content and food resources for some macrozoobenthic species; and (2) ecosystem engineering activities by C. fluminea, which can create conditions for the establishment of other species via shell production and bioturbation in the sediments.
In streams, the creation of nutrient‐transformation hotspots by aggregated organisms may have heterogeneous and strong cumulative influences on stream nutrient dynamics. Here, we examine the potential for aggregations of freshwater mussels to create such hotspots.
We measured nitrogen ( N ) and phosphorus ( P ) excretion rates of six mussel species and body tissue composition of eight. We combined these data with population densities of surveyed mussel beds in the Kiamichi River, OK , to estimate reach‐scale and stream‐scale nutrient recycling and storage. Additionally, we estimated the temporal variability in the magnitude of mussel nutrient recycling combining volumetric excretion at a reach scale with discharge and temperature data.
Mussel beds constituted 1.45% of the area of the Kiamichi River. Mussel nutrient remineralisation varied greatly across beds (11.1–699.5 μmol N m ⁻² h ⁻¹ and 0.8–53.0 μmol P m ⁻² h ⁻¹ ), because of varying mussel densities. The community‐wide average excretion N:P (molar) of the mussel communities was 29.57, with higher excretion N:P significantly associated with higher abundances of Actinonaias ligamentina . Total nutrient storage per bed varied two orders of magnitude (6.3–631.7 kg N and 2.3–227.5 kg P) between mussel beds. Moreover, areal nutrient storage varied among the beds (11.2–133.7 mg N m ⁻² , 4.1–48.9 mg P m ⁻² ) with the majority of nutrient storage in a long‐term store, shell ( c . 87% of total N storage, c . 95% of total P storage).
Freshwater mussels can be important to nutrient dynamics through nutrient regeneration and the creation of storage hotspots. However, the importance of nutrient remineralisation varies dramatically in response to organism patchiness, flow conditions and background nutrient concentrations.
1. Droughts have become prevalent in the south-eastern U.S.A. and are predicted to become more common in the future. Drought conditions have particularly strong negative effects on sessile aquatic organisms with limited dispersal ability. This study explored the linkages between physiological tolerance, behavioural response and survival of three species of freshwater mussels subjected to drought.
2. To assess physiological tolerance, we measured survival under desiccation at 25, 35 and 45 °C in the laboratory. To assess behavioural responses, we tracked horizontal and vertical movement of mussels in a drying stream reach.
3. Uniomerus tetralasmus showed the greatest desiccation tolerance, Lampsilis straminea was intermediate, and Pyganodon grandis had the lowest tolerance at all temperatures.
4. In the drying stream reach, U. tetralasmus showed little horizontal movement and quickly became stranded. The other two species tracked the receding water. It was not until the pool had been reduced to c. 10% of its original size that ≥50% of P. grandis and L. straminea became stranded.
5. Uniomerus tetralasmus and L. straminea burrowed in response to becoming stranded; however, both species burrowed only shallowly (3–4 cm), presumably because burrowing to greater depths did not convey increasing thermal refuge. Pyganodon grandis rarely burrowed. No P. grandis survived the 15-week drought, while 45% of L. straminea and 77% of U. tetralasmus survived.
6. Three strategies emerged for freshwater mussels to cope with drought conditions: tracking (intolerant), track then burrow (semi-tolerant) and burrowing (tolerant). Results suggest that drought poses the greatest threat to intolerant trackers, while tolerant burrowers are the most resistant to drought conditions.
Freshwater bivalves have been highly threatened by human activities, and recently their global decline has been causing conservational and social concern. In this paper, we review the most important research events in freshwater bivalve biology calling attention to the main scientific achievements. A great bias exists in the research effort, with much more information available for bivalve species belonging to the Unionida in comparison to other groups. The same is true for the origin of these studies, since the publishing pattern does not always correspond to the hotspots of biodiversity but is concentrated in the northern hemisphere mainly in North America, Europe and Russia, with regions such as Africa and Southeast Asia being quite understudied. We also summarize information about past, present and future perspectives concerning the most important research topics that include taxonomy, systematics, anatomy, physiology, ecology and conservation of freshwater bivalves. Finally, we introduce the articles published in this Hydrobiologia special issue related with the International Meeting on Biology and Conservation of Freshwater Bivalves held in 2012 in Bragança, Portugal.