Background & Summary

The West Palearctic is one of the most anthropogenically altered regions on Earth, and its freshwater habitats in particular have been drained, polluted and physically degraded by human activities over the past millennia1,2. As a result, freshwater species, especially those that are more sensitive to human disturbance, have dramatically declined or even disappeared from this region3,4. This is the case of freshwater bivalves, one of the most imperilled groups of animals on Earth5. Although the ecological importance of these animals is increasingly recognised, information on their distribution and population trends is highly fragmented, making effective conservation a challenge6,7.

In the West Palearctic, there are only 93 species of native freshwater bivalves, compared to almost 10,000 recognized extant species of bivalves worldwide, of which approximately 86% are marine8. The remaining species inhabit freshwater and correspond mainly to two speciations in this environment: the freshwater mussels (also known as naiads) of the Unionida order with about 1,000 species, and the pea or fingernail bivalves of the Sphaeriida order, which includes roughly 250 species7,8. These groups account for almost 90% of all freshwater bivalve species worldwide. The few remaining species are found scattered across other bivalve groups5. In the West Palearctic, the native diversity of freshwater bivalves (see Supplementary Table 1 for the complete list) is mainly composed of the two major groups already mentioned: the freshwater mussels with 43 species and the fingernail/pea bivalves with 39 species. The remaining species include Corbicula fluminalis from the Cyrenidae family and 10 species of the family Dreissenidae (which generally attach to stones or any other hard surface using a byssus thread).

Eight non-native species also occur in the region. This includes: one East Palearctic Asian freshwater mussel (the Chinese pond mussel Sinanodonta woodiana); two Nearctic North American pea bivalves (the Long peaclam Sphaerium transversum and the Ridgebeak peaclam Euglesa compressa); three East Palearctic Asian cyrenid clams (Corbicula fluminea, C. leana, and C. largillierti); and finally, two species which are primarily found inhabiting brackish environments, although they can also be found in freshwater habitats (the Conrad’s false mussel Mytilopsis leucophaeata and the gulf wedge clam Rangia cuneata). Except for S. tranversum and E. compressa, which do not appear to have spread widely9, all other non-native species have traits that can result in competition with native species10 and significant ecological and economic damage in the areas they invade11. This includes C. fluminalis and two of the dreissenid species, the zebra (Dreissena polymorpha) and the quagga (D. bugensis) mussels that, despite being native to the Ponto Caspian region, have now expanded extensively to other regions of the West Palearctic12.

Conservation status and Distribution related issues

The conservation status of freshwater bivalve groups in the study area varies considerably. Freshwater mussels (Unionida) are highly imperilled, with 77.3% of the species assessed as Threatened or Near Threatened13, while only 8.3% of the pea bivalves (Sphaeriida) fall into these categories13. Precise and dependable conservation status assessments demand accurate distribution and trend analyses, given that almost all freshwater bivalve assessments rely on distribution-related traits. Criterion C of the IUCN Red List, which estimates population size and trends using the number of individuals, is notably arduous to apply in conservation assessments of elusive underwater invertebrates like freshwater bivalves. Therefore, all conservation assessments of freshwater bivalves use criteria A (population size reduction) and B (limited distribution range) from the IUCN Red List. Moreover, most (81%) assessments using Criterion A generally estimate population declines based on distribution parameters such as Extent of Occurrence (EOO) and Area of Occupancy (AOO)13.

Recent studies in systematic conservation planning reveal that extensive protected area networks, mainly intended for conserving terrestrial biodiversity, such as the Natura 2000 network in Europe, do not provide sufficient protection for freshwater biodiversity14. It is therefore essential to improve the representation of freshwater biodiversity in these networks and enhance their capacity to address threats and specific ecological needs15. Accurate and comprehensive distributions of freshwater species are crucial in identifying important conservation areas, as these exercises rely heavily on species distributions and patterns of species aggregation and composition.

Several sources containing documented distributions of freshwater bivalves are readily available, such as GBIF (https://www.gbif.org/), particularly for the larger and more conspicuous freshwater mussels. However, species identifications in these databases are based on shell morphology since the majority of records are derived from shell collections16. Identification of freshwater bivalves using shell morphology demands significant expertise, particularly of the minute pea bivalves with some species exhibiting noteworthy shell similarity17. In addition, various genera display cryptic diversity where morphologically indistinguishable species are distinct only at the molecular level18,19. Conversely, certain species that were previously thought to be separate exhibit introgression and were subsequently considered as a single species20. Therefore, the accurate identification of freshwater bivalves presents a considerable challenge, resulting in distribution databases of these animals that are highly unreliable and biased due to frequent misidentifications at the species level. Our current knowledge of the occurrence and spatial distribution of freshwater bivalve species in the West Palearctic is far from complete. New species have recently been described (e.g. in Sphaeriida: Euglesa interstitialis21; Euglesa moroccana and Odhneripisidium сaucasus17; and in Unionida: Anodonta seddonae and Leguminaia anatolica19) and others may yet be undescribed in the less explored areas, such as most of the Caucasus, the Middle East, and parts of North Africa (Fig. 1). In addition, the dataset compiled here highlights major gaps in knowledge of the distribution of vast areas that are still largely unrepresented.

Fig. 1
figure 1

Distribution maps of: (A) all records, (B) all freshwater mussels (Unionida), (C) all fingernail/pea bivalves (Sphaeriida), and (D) all non-native species from the dataset.

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This dataset was created by collecting data from a variety of sources such as published articles, grey literature, field expedition records, regional and national administration report, and online databases dedicated to biodiversity and scientific collections maintained by museums, research institutions and universities. We also incorporated information collected during field expeditions (short-term scientific missions) conducted as part of the CONFREMU project. The project sought to gather the latest and most comprehensive data on the distribution of freshwater bivalve species across Europe and neighbouring countries, as well as to initiate scientific collection expeditions in under-studied regions to address gaps in knowledge.

This dataset includes 270,287 geo-referenced occurrence records covering almost five centuries (1674–2023) and contains information on 101 valid freshwater bivalve species occurring in the West Palearctic region (Fig. 1). To our knowledge, this is the first endeavour to gather distribution records of the complete freshwater bivalve fauna in the West Palearctic or even for Europe alone.

This dataset of Freshwater Bivalves distribution has the potential to function as an invaluable and enduring resource for research into ecological and conservation issues, as well as to draw attention to gaps in taxonomy and sampling.

Methods

Information sources

This dataset is the result of a collaborative effort involving 82 researchers from 29 countries, carried out under the framework of the COST (European Cooperation in Science and Technology) Action CA18239 – Conservation of Freshwater Mussels: A Pan-European Approach (CONFREMU), funded by the European Union. To initiate data compilation, a standardized datasheet template (see Data Records) was distributed to all contributors, who filled it with information based on their own field observations and/or bibliographic sources. These entries were then submitted to the lead author for individual curation, including the correction of errors and taxonomic standardisation using the most up-to-date nomenclature.

Data providers were selected based on their expertise in freshwater bivalves, and most contributed data at the national level. Given the wide geographic scope and depth of regional knowledge, the compilation drew from a highly diverse array of sources. To further enhance the dataset, a comprehensive literature search was performed using Scopus and ISI Web of Knowledge, incorporating both scientific and common names of all target species.

Field data collection by the co-authors employed three complementary methods, wading, snorkelling, and scuba-diving, allowing for thorough investigation across various aquatic habitats and substrate types. To ensure accuracy, only live specimens were recorded; observations of empty shells or fragments were excluded, as these could represent individuals no longer present or transported from other locations.

All co-authors brought extensive taxonomic and ecological expertise, contributing not only original data from their own collections and fieldwork but also facilitating the identification of additional data sources through their professional networks. Ultimately, information from a wide range of origins was consolidated and harmonised into a single, high-quality dataset.

This combined data incorporates six types of source data:

  1. 1.

    Gathered or compiled by the co-authors (e.g. from field expeditions, monitoring)

  2. 2.

    Literature (scientific articles, books, grey literature)

  3. 3.

    Research and conservation projects

  4. 4.

    Online biodiversity databases

  5. 5.

    Museum, research institutions, and university collections

  6. 6.

    National & regional databases on nature conservation

  7. 7.

    Data from environmental agencies

The majority of the data were extracted from the most widely used and frequently updated biodiversity databases (see details in Table 1). These repositories make biological data available under a Creative Commons licence in which the user agrees to acknowledge the source of the data. A total of 44,671 records were derived from field data collected by the co-authors or by them from national colleagues. A total of 1,629 documents (e.g. scientific articles, books, grey literature) containing over 25,000 records on the distribution of freshwater bivalve species were identified by the 82 co-authors. Data (3,540 records) from museum collections were obtained directly from museum curators or online museum databases. Any redundant records with other data sources were eliminated. There is an imbalance in the geographical distribution of records by country, with countries in central and north-western Europe providing a higher number of records (Table 2). The opposite is true for most countries in Asia and Africa, where the number of records is very low (Table 2), highlighting the need for investment in surveys in these countries. It also highlights the failure of the network to attract researchers with data from European countries such as Denmark and Bosnia-and-Herzegovina. For some countries, the limited capacity of the researchers involved meant that it was not possible to import existing records, such as the extensive Sphaeriida records from Sweden and Norway.

Table 1 Sources with the highest contribution of records (>1,000) to the database, with full name, number of records, and country.
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Table 2 Summary of records and species richness per country, ordered by decreasing number of records.
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Data Records

The dataset is available at figshare22. It consists of a spreadsheet with 270,287 records, each represented as a row. The fields on each record, with self-explanatory headers, contain the updated species name, the date when it was collected, the provider and compiler of the information, and then the source of the information, which may be a scientific paper, grey literature, museum or database record, or the own field records of the provider (Table 3). A more detailed data description and access rules for the data of some countries can be found in the supplementary data statement.

Table 3 Fields and full description of the database records.
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Technical Validation

Each record in the dataset comprises 12 fields and was only included in the final compilation if it contained essential information on the species name, year of collection, collector or observer, and geographical ___location (Table 3). The scientific names of all reported species were carefully verified to correct typographical errors or misspellings. Taxonomic information was then standardized and updated using Molluscabase8 to ensure consistency with current nomenclature. Species within the Unio crassus complex were reassigned according their biogeographic context following. To avoid duplication, all records were cross-checked for multiple entries submitted by different data providers and consolidated into single entries where necessary23.

All records in the complete dataset were georeferenced, with an accompanying measure of spatial accuracy. Although the original data were collected with precise geographical coordinates, the accuracy was generalized to a 10 × 10 km grid to protect sensitive species, particularly those of conservation concern or with legal protection, such as Margaritifera margaritifera, to mitigate risks like illegal pearl fishing. The full dataset is available via the Figshare repository22 and online at https://e-mussels.eu.

Usage Notes

Species occurrences can be downloaded at22 with a grid resolution of 10 × 10 km2 and be viewed online (https://e-mussels.eu) with different grid sizes from 10 × 10 km2 to 100 × 100 km2. The dataset comprises the geographical coordinates for each record, its information source, and the species name, according to the Molluscabase8 taxonomy.