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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.

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.

Invasive bivalves may cause great ecological, evolutionary, and economic impacts in freshwater ecosystems. Species such as Corbicula fluminea , Dreissena bugensis , Dreissena polymorpha , Limnoperna fortunei, and Sinanodonta woodiana are widely distributed hyper-successful invaders, but several others not yet invasive (or at least not considered as such) may become so in the near future. These species can affect hydrology, biogeochemical cycling, and biotic interactions through several mechanisms, with impacts ranging from individuals to ecosystems. Freshwater invasive bivalves can create no-analog ecosystems, posing serious difficulties for management, but new techniques are becoming available which may enhance options to detect early introductions and mitigate impacts. Although knowledge about the biology of these bivalves has increased considerably in the last two decades, several fundamental gaps still persist; we suggest new research directions that are worth exploring in the near future.

Recent research efforts have significantly advanced our knowledge on Asian freshwater mussel (Bivalvia: Unionidae) diversity and distribution. Here we provide a modern consensus of the diversity, biogeography and conservation of Unionida in the region comprising East and Southeast Asia (excluding Wallacea) and Asian Russia. A data review confirmed the presence of 228 native and 3 non-native Unionida (98% Unionidae, 2% Margaritiferidae), rendering the region a global hotspot of freshwater mussel diversity. Species richness was highest in China (particularly Yangtze basin) in absolute numbers and Cambodia when correcting for country area, and decreased gradually towards the south and steeply towards the north and east. Six of the seven unionid subfamilies are native to the region, with species richness peaking in Southeast Asia for Rectidentinae, Gonideinae, Parreysiinae and Modellnaiinae, China for Anodontinae and Unioninae, and Asian Russia for Margaritiferidae. Conservation status and data collected after 1980 were not available for 61 and 24% of species, respectively. Dams, deforestation and pollution are likely the major threats to mussels in the region, though data in this respect are scarce. The Philippines, Laos, Indonesia, Myanmar and Malaysia are among the countries with the poorest data availability and urgently require research.

The North America freshwater mussel fauna has suffered an inordinately high recent extinction rate, and the small size and isolation of many remaining populations portends a continued diminishment of this fauna. Causes of extinction and imperilment are varied but revolve around massive habitat loss, deterioration, and fragmentation. The National Strategy for the Conservation of Native Mussels, published in 1997, has guided efforts to address this crisis. Considerable progress has been made toward several of the Strategies’ goals, particularly increasing our knowledge of mussel biology, promoting mussel conservation, and development of techniques for captive mussel propagation. However, mussel conservation should focus more directly on reducing fragmentation through bold and aggressive habitat restoration. In addition to dam removal, improvement in dam tailwater flows, and restoration of channelized streams, identification of factors that eliminated mussels from many otherwise intact streams is critical. Translocation and captive propagation will be key elements in reestablishing mussel assemblages in restored habitats, but these techniques should be used with caution and primarily to increase the occurrence of a species throughout its historical range. Conserving mussel diversity in an ever-changing world is dependent on promoting the natural, long-term sustainability and evolutionary potential of mussel populations.

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.

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.

Nutrient loads and nutrient cycling, especially of phosphorus and nitrogen, are among the most important controls on the character of freshwater ecosystems and have been greatly affected by human actions. Despite the widespread importance of nutrients in freshwater ecosystems, the varied linkages between nutrient cycling and freshwater mussel populations have not been thoroughly described. Here, I explore three of these linkages. First, I suggest that nutrient loads are related to the well-being of mussel populations through several mechanisms, probably producing a nonlinear and non-monotonic relationship between nutrient loads and mussel populations. Second, I discuss the ability of mussels to spatially focus nutrients from the overlying water onto the sediments, which has not been fully appreciated, perhaps because nutrient cycling has been viewed chiefly from the viewpoint of the well-mixed water column rather than the patchy sediments. Third, I discuss the ability of mussel populations to accumulate and release nutrients, introducing time lags into nutrient dynamics and stoichiometry (“nutrient capacitance”). Finally, I propose a speculative analysis of the role of freshwater mussels in the nutrient cycles of pristine river systems, which must have been much greater than in modern rivers, with their high nutrient loads and depleted mussel populations.

Based on literature review and malacological collections, 168 native freshwater bivalve and five invasive species have been recorded for 52 hydrographic regions in South America. The higher species richness has been detected in the South Atlantic, Uruguay, Paraguay, and Amazon Brazilian hydrographic regions. Presence or absence data were analysed by Principal Coordinate for Phylogeny-Weighted. The lineage Veneroida was more representative in hydrographic regions that are poorer in species and located West of South America. The Mycetopodidae and Hyriidae lineages were predominant in regions that are richest in species toward the East of the continent. The distribution of invasive species Limnoperna fortunei is not related to species richness in different hydrographic regions there. The species richness and its distribution patterns are closely associated with the geological history of the continent. The hydrographic regions present distinct phylogenetic and species composition regardless of the level of richness. Therefore, not only should the richness be considered to be a criterion for prioritizing areas for conservation, but also the phylogenetic diversity of communities engaged in services and functional aspects relevant to ecosystem maintenance. A plan to the management of this fauna according to particular ecological characteristics and human uses of hydrographic regions is needed.

Freshwater ecosystems are the most vulnerable worldwide and freshwater bivalves rank amongst the most threatened animals in the world. Surveying and monitoring freshwater bivalves are difficult tasks: they are difficult to find, hard to identify (taxonomic expertise is needed), and working underwater is technically challenging. It is therefore crucial to find more efficient methods to survey and monitor these species. Here, we present the first metabarcoding approach for freshwater bivalves and compare environmental DNA (eDNA) and traditional surveys. We describe two sets of primers (for Unionida and Venerida) developed for freshwater bivalves eDNA metabarcoding. These primers have been tested in the field, with about 300 studied sites. Results were compared to freshwater bivalves’ surveys using traditional methods, with eDNA always detecting more species than traditional surveys, especially when Sphaerids were taken into account. While our study initially focused on Western Palearctic freshwater bivalve species, our primers were confronted in silico with available sequences and have proven to be effective at a global scale. The results show that eDNA metabarcoding, with our developed primers, is a remarkable tool allowing for non-invasive surveys, detection of rare and inconspicuous species, absence data and overall freshwater bivalves routine monitoring.