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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.

Since the end of the 19th century, the circumpolar fauna of freshwater Mollusca has not been reviewed despite the substantial increase of knowledge on this subject. A review of the freshwater mollusks of the Arctic based on an analysis of published data, own results and examination of museum collections is presented. 104 species of Mollusca have been registered, which constitutes less than 2% of the global diversity. The actual diversity of the Circumpolar freshwater molluscs may lie between 100 and 120 species. No endemic taxa are found in the Arctic. The most species-rich Arctic subregion is Siberia, whereas the North America maintains the poorest fauna. The aquatic malacofauna of Beringia is similar to that of North America, and Beringia as a biogeographic region constitutes a part of the Nearctic. The ways of formation of the Circumpolar malacofauna are discussed, with evaluation of dispersal, adaptation, and environmental filtering as the faunogenesis factors. The number of non-alien species in the Circumpolar freshwater malacofauna remains very low, and only one reliable occurrence of an alien species is known. Among the Arctic molluscs only three have a conservation status other than ‘least concern’ or ‘data deficient’, whereas most resident species are widespread and abundant.

Corals nucleate and grow aragonite crystals, organizing them into intricate skeletal structures that ultimately build the world’s coral reefs. Crystallography and chemistry have profound influence on the material properties of these skeletal building blocks, yet gaps remain in our knowledge about coral aragonite on the atomic scale. Across a broad diversity of shallow-water and deep-sea scleractinian corals from vastly different environments, coral aragonites are remarkably similar to one another, confirming that corals exert control on the carbonate chemistry of the calcifying space relative to the surrounding seawater. Nuances in coral aragonite structures relate most closely to trace element chemistry and aragonite saturation state, suggesting the primary controls on aragonite structure are ionic strength and trace element chemistry, with growth rate playing a secondary role. We also show how coral aragonites are crystallographically indistinguishable from synthetic abiogenic aragonite analogs precipitated from seawater under conditions mimicking coral calcifying fluid. In contrast, coral aragonites are distinct from geologically formed aragonites, a synthetic aragonite precipitated from a freshwater solution, and mollusk aragonites. Crystallographic signatures have future applications in understanding the material properties of coral aragonite and predicting the persistence of coral reefs in a rapidly changing ocean.

While the effects of contemporaneous local environment on species richness have been repeatedly documented, much less is known about historical effects, especially over large temporal scales. Using fen sites in the Western Carpathian Mountains with known radiocarbon-dated ages spanning Late Glacial to modern times (16 975-270 cal years before 2008), we have compiled richness data from the same plots for three groups of taxa with contrasting dispersal modes: (1) vascular plants, which have macroscopic propagules possessing variable, but rather low, dispersal abilities; (2) bryophytes, which have microscopic propagules that are readily transported long distances by air; and (3) terrestrial and freshwater mollusks, which have macroscopic individuals with slow active migration rates, but which also often possess high passive dispersal abilities. Using path analysis we tested the relationships between species richness and habitat age, area, isolation, and altitude for these groups. When only matrix-derived taxa were considered, no significant positive relation was noted between species richness and habitat size or age. When only calcareous-fen specialists were considered, however, habitat age was found to significantly affect vascular plant richness and, marginally, also bryophyte richness, whereas mollusk richness was significantly affected by habitat area. These results suggest that in inland insular systems only habitat specialist (i.e., interpatch disperser and/or relict species) richness is influenced by habitat age and/or area, with habitat age becoming more important as species dispersal ability decreases.

Constrained multivariate analysis is a common tool for linking ecological communities to environment. The follow-up is the development of the double-constrained correspondence analysis (dc-CA), integrating traits as species-related predictors. Further, methods have been proposed to integrate information on phylogenetic relationships and space variability. We expand this framework, proposing a dc-CA-based algorithm for decomposing variation in community structure and testing the simple and conditional effects of four sets of predictors: environment characteristics and space configuration as predictors related to sites, while traits and niche (dis)similarities as species-related predictors. In our approach, ecological niches differ from traits in that the latter are distinguished by and characterize the individual level, while niches are measured on the species level, and when compared, they are characteristics of communities and should be used as separate predictors. The novelties of this approach are the introduction of new niche parameters, niche dissimilarities, synthetic niche-based diversity which we related to environmental features, the development of an algorithm for the full variation decomposition and testing of the community–environment–niche–traits–space (CENTS) space by dc-CAs with and without covariates, and new types of diagrams for the results. Applying these methods to a dataset on freshwater mollusks, we learned that niche predictors may be as important as traits in explaining community structure and are not redundant, overweighting the environmental and spatial predictors. Our algorithm opens new pathways for developing integrative methods linking life, environment, and other predictors, both in theoretical and practical applications, including assessment of human impact on habitats and ecological systems.

Freshwater gastropods are among the most imperiled organisms on Earth. Yet, they are among the most understudied freshwater taxa. Numerous freshwater gastropod species have gone extinct in the last 100 years, but recent rediscoveries indicate that some species were prematurely declared extinct. Such premature extinction declarations remove legal protections, which could facilitate actual extinction. Thus, research and policy recommendations are needed so surveys provide the best information possible for conservation. Here, we examined the case of Lithasia hubrichti, a freshwater gastropod endemic to the Big Black River in Mississippi that was last seen in 1965. In 2022, a freshwater mollusk survey resulted in finding L. hubrichti alive. An additional survey effort in 2023 that prioritized sampling as many sites as possible in a single day clarified the current range of L. hubrichti. Genomic analyses indicated that the species has persisted with a large population size for thousands of years, rather than ever falling below a survey detection limit. When considering the case of L. hubrichti and other recent freshwater gastropod rediscoveries, we conclude that freshwater gastropod surveys should emphasize sampling as many sites as possible under favorable sampling conditions when targeting rare species, rather than expending high sampling effort at a small number of sites or when stream conditions may impact ability to detect target species. We also advocate for policies that encourage partnerships with landowners, which was required to rediscover L. hubrichti.

Knowledge of factors affecting the diversity of benthic invertebrates is very important for biodiversity conservation, for monitoring, restoring and maintaining aquatic ecosystems. The purpose of this study was to investigate how the diversity and distribution of freshwater mollusks related to environmental factors. Twelve species of freshwater mollusks were identified during a survey of 15 thermokarst lakes in the Gydan Peninsula, Russia. Canonical correspondence analysis (CCA) was used to study the relationships between environmental variables and species abundance. The first CCA axis correlated with the content of macro-ions, mainly Mg2+, Na+, Cl−, substrate types, and plant remnants. Along the second CCA axis stations were ordered by depth and the presence of mosses. We found that the type of substrate, depth, abundance of mosses and hydrochemical parameters play an important role in the distribution of the mollusks in thermokarst lakes. These results expand our understanding of mollusks ecology in thermokarst lakes, and identify environmental stressors important for the performance of abiotic control.

Freshwater mollusks are employed as bioindicators for the assessment of water quality in biomonitoring studies since the water quality of natural resources is crucial for humans. The freshwater snail species known as Viviparus contectus (Viviparidae: Gastropoda) is one that people eat. Here, the levels of heavy metals (Cd, Cr, Pb, As, Zn, and Cu) in water and V. contectus samples were determined. An Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, Spectro Arcos, vertical plasma) was used for the analysing the heavy metal concentrations in water and freshwater snail samples. The results of the current investigation demonstrated that freshwater snails accumulated metals in their water and soft tissues in the following orders: Pb > Cr = Zn = Cu = Cd = As and Zn > Cu > Pb > Cd > As = Cr. Autumn was the time of year when higher amounts of heavy metals (As, Cr, Cu, Zn, and Hg) were found. Pb content in the freshwater snail samples was strongly impacted by seasonal fluctuations (P 0.05). For adults, the EDI (estimated daily intake) values were lower than the TDI (tolerable daily intake) values, and the HI (hazard index) values were below 1. Freshwater snail samples had Zn and Pb levels that were over the FAO/WHO, Turkish Food Codex, JECFA, and EC limit values. Except for Pb, the water study shows mean metal concentrations below the USEPA, Turkish Pollution Control Regulation, and World Health Organisation maximum allowed levels. Aquatic ecosystems were negatively impacted by anthropogenic activities overall, and this study can provide a helpful data set for investigations on metallic contamination in water bodies and biomonitoring in freshwater ecosystems.

The present study reports the reconstruction of paleogeographic conditions of the final late Pleistocene and Holocene in the Middle Yenisei Valley. This research is based on a thorough examination of continuously deposited sediments from the first floodplain terrace of the Berezovka River, employing pedological, malacofaunal, geochemical, and radiocarbon dating analyses. Continuous sedimentation of the first floodplain terrace of the Berezovka River began 13,340+410 calibrated years before present (cal BP), during stabilization of the hydrological regime of the river. Accumulation of basal horizons of gray-blue clay deposits and sandy deposits of channel alluvium occurred 20,830+520 cal BP, and it was caused by floods and freshets on the Yenisei River. During the late Pleistocene, a shift occurred from dry and cold conditions (20,830+520 cal ВР) to a more humid and temperate climate (13,340+410 cal BP). The early Holocene, 8,805+200 cal BP, was marked by a combination of arid and cold climatic conditions. Notably, that era witnessed a rise in the diversity of freshwater mollusk species within the malacofaunal community. During the mid-Holocene (about 5,230+250 cal BP) and the late Holocene period (1,020+220 cal BP), there was an increase in humidity and climate warming, as confirmed by an increase in the number of shells of terrestrial species in the structure of the malacofaunal community. At the end of the late Holocene (220+70 cal BP), the climate became similar to the modern one. Geochemical markers such as Sr/Ba, Cu, Co, Cr, and Zn ratios reveal a transition in climate during the late Pleistocene, from a dry and cold environment to a more humid and warmer one. In the Holocene epoch, the data obtained suggest a change from arid and cold conditions (early Holocene) to the present-day climate, without a distinct indication of the Holocene climatic optimum or the subsequent cooling observed in the neighboring regions.

China is the largest producer and exporter of aquatic products. While earning foreign exchange, its implied water resources output could not be denied. Against the background of promoting water resources and food security, whether aquatic products trade can also achieve the purpose of water-saving has become a topic worthy of attention. Based on the idea of the quality and energy conservation law, this study uses the physical value input–output table and the fish growth model, and calculates the output and input of China's aquatic products virtual water trade indirectly by fishery water coefficient and virtual water of feed crops. The results show that while China's aquatic products trade is water-saving in general, it is water-wasting in some parts, and its environmental function needs to be improved. In particular, the aquatic products trade with South Asia, Central Asia, Northern Europe, North America, South America and Oceania is virtual water net import, while that with other regions is virtual water net export. The trade of shellfish and marine fish is water-saving, while that of mollusk, crustacean and freshwater fish is water-wasting. China, as a globally responsible power, should try to dynamically optimize the trade structure of aquatic products.