Annotated Checklist and Latitudinal Distribution of Inland-Water Bivalves from Chile

1. Introduction

Freshwater bivalves are important components of inland aquatic ecosystems, contributing to water filtration, sediment bioturbation, nutrient cycling, organic matter processing and benthic habitat structure [1](Vaughn and Hakenkamp 2001). Despite their ecological relevance, non-marine bivalves remain unevenly documented across the Southern Hemisphere. Most global syntheses and conservation assessments have focused on Northern Hemisphere mussel faunas, whereas the diversity, taxonomy and distribution of South American freshwater bivalves are still comparatively less integrated in regional and global biogeographical analyses [2](Graf and Cummings 2007).

In Chile, freshwater bivalves are represented mainly by Hyriidae and Sphaeriidae. Hyriidae is represented by the genus Diplodon, including Diplodon chilensis and Diplodon solidulus, whereas Sphaeriidae includes small-bodied clams assigned to Pisidium, Sphaerium and Musculium [3,4,5](Parada and Peredo 2002, 2006; Ituarte 2007). These families differ markedly in body size, life-history traits, habitat use and probable dispersal mechanisms. Species of Diplodon are comparatively large mussels associated with rivers, lakes and permanent freshwater systems, whereas sphaeriid clams are smaller, often occur in lakes, ponds, wetlands and marginal habitats, and may disperse passively through waterbirds or other vectors.

The Chilean territory provides a natural setting for evaluating the distribution of freshwater bivalves along one of the longest latitudinal gradients in the Southern Hemisphere. Inland waters in Chile extend from arid and semi-arid northern basins to temperate, Patagonian, Magellanic and sub-Antarctic systems [3]. This gradient is particularly useful for identifying northern and southern distributional limits and for comparing the distributional behaviour of large-bodied mussels and small-bodied clams [4].

Previous studies have summarised the taxonomy, ecology and state of knowledge of Chilean freshwater bivalves, including progress and conflicts in their systematics, reproductive biology, karyology, ecology and conservation relevance [3,4,5,6,7,8,9,10,11,12,13](Parada and Peredo 1994, 2002, 2006; Parada et al. 1987, 1989, 1990; Peredo and Parada 1984; Jara-Seguel et al. 2000, 2010; Valdovinos and Pedreros 2007). However, the latitudinal structure of the Chilean fauna and the contrast between Hyriidae and Sphaeriidae have received less explicit attention. In particular, high-latitude records of Pisidium and related sphaeriid taxa are biogeographically important because they reach Patagonia, Magallanes and sub-Antarctic environments [5,14,15](Ituarte 1999, 2007; Rendoll-Cárcamo et al. 2020).

The objective of this study was to provide an annotated and georeferenced synthesis of Chilean inland-water bivalves and to analyse their latitudinal distribution. Specifically, we aimed to: (1) summarise the taxonomic composition of the compiled records; (2) provide an annotated checklist of the species included in the dataset; (3) determine northern and southern latitudinal limits for each species and genus; (4) compare distributional patterns between Hyriidae and Sphaeriidae; and (5) discuss the biogeographical implications of the observed patterns for southern South American inland waters.

2. Materials and Methods

2.1. Dataset and Sources

The analysis was based on a compiled database of Chilean inland-water bivalve occurrence records. The dataset included 90 records and 18 fields containing taxonomic, geographical and bibliographic information. The main variables used in the analysis were family, genus, species, locality, latitude, longitude, altitude and bibliographic reference. The dataset was treated as a literature-derived occurrence compilation rather than as a systematic field survey. Therefore, record frequency should be interpreted as the number of compiled records and not as a direct measure of abundance, population size or sampling effort.

Only records corresponding to inland waters were retained. Marine, strictly estuarine and coastal marine records were excluded when they were not compatible with freshwater or inland-water occurrence. When multiple records corresponded to the same species and locality but came from different bibliographic sources or entries, they were retained as independent literature records unless clear duplication was evident. The database was closed before analysis; no additional occurrence records were incorporated after closure.

2.2. Taxonomic Treatment

The dataset included species belonging to two freshwater bivalve families: Hyriidae and Sphaeriidae. Hyriidae was represented by Diplodon, whereas Sphaeriidae was represented by Musculium, Pisidium and Sphaerium. Species names were retained as recorded in the compiled sources and harmonised following the taxonomic treatments available for Chilean and southern South American freshwater bivalves [3,4,5,14] (Parada and Peredo 2002, 2006; Ituarte 1999, 2007). This treatment allows the database to preserve the nomenclature used in the primary literature while providing a standardised working list for the present analysis.

2.3. Data Cleaning and Georeferencing

The database was imported into R using the package readxl. Latitude values were inspected before analysis and corrected when evident formatting errors were detected, particularly records in which southern latitudes were written with an initial dot instead of a minus sign. Longitude values were retained in decimal degrees. The coordinates were treated as approximate locality coordinates when they came from bibliographic information rather than from direct GPS measurements. Records with uncertain or potentially inconsistent coordinates should be flagged in the final submitted dataset.

2.4. Descriptive and Spatial Analyses

Species-level summaries were calculated using dplyr. For each species, the number of records, northernmost latitude, southernmost latitude and latitudinal range were estimated. Southernmost records were identified by arranging the dataset by latitude, with more negative values representing higher southern latitudes. Genus-level summaries were also calculated to compare the distributional ranges of Diplodon, Musculium, Pisidium and Sphaerium.

Occurrence records were converted to spatial objects using sf, with coordinates defined in the WGS84 geographic reference system. A base map of Chile was obtained using rnaturalearth and rnaturalearthdata, and distribution maps were produced using ggplot2 and ggrepel. All analyses were conducted in R [16](R Core Team 2025) using readxl [17] (Wickham and Bryan 2023), dplyr [18] (Wickham et al. 2023), ggplot2 [19] (Wickham 2016), sf [20] (Pebesma 2018), rnaturalearth and rnaturalearthdata [21] (South 2017), and ggrepel [22] (Slowikowski 2024).

3. Results

3.1. Taxonomic Composition

A total of 90 occurrence records corresponding to 12 inland-water bivalve species were compiled for Chile. The records belonged to two families, Hyriidae and Sphaeriidae, and four genera: Diplodon, Musculium, Pisidium and Sphaerium. Sphaeriidae accounted for most of the species richness, with ten of the twelve species, whereas Hyriidae was represented by two species. By contrast, Hyriidae accounted for most compiled occurrence records because Diplodon chilensis alone represented 52 records, equivalent to 57.8% of the database (Table 1).

3. 2- Annotated Checklist

Diplodon chilensis (Gray, 1828) - Hyriidae

The most frequently recorded species in the dataset. It occurs mainly in central-southern Chile and northern Patagonia, with a compiled latitudinal range of approximately 30.5-46.6°S. Its high frequency reflects both broad distribution and greater historical research attention compared with other Chilean inland-water bivalves.

Diplodon (Australis) solidulus (R. A. Philippi, 1869) - Hyriidae

Represented by a single compiled record near 41.5°S. This taxon requires careful nomenclatural verification before submission because subgeneric usage and synonymy in South American Hyriidae have changed across the literature.

Musculium argentinum (d’Orbigny, 1835) - Sphaeriidae

Represented by eight records between approximately 38.5°S and 41.2°S. The species has been reported from Chilean continental waters and contributes to the regional diversity of small-bodied sphaeriid clams.

Musculium patagonicum Pilsbry, 1911 - Sphaeriidae

Represented by one high-latitude record from Puerto Natales, approximately 51.7°S. This record contributes to the evidence that Sphaeriidae reach Magellanic freshwater environments.

Pisidium chilense (d’Orbigny, 1846) - Sphaeriidae

The most frequently recorded sphaeriid in the dataset, with 15 records and a latitudinal range of approximately 36.6-44.6°S. It represents the widest species-level range among the Chilean sphaeriids included here, excluding the single northern outlier records of other species.

Pisidium huillichum Ituarte, 1999- Sphaeriidae

Represented by four records from southern Chile, between approximately 40.6°S and 43.3°S. Its distribution reinforces the importance of temperate and northern Patagonian freshwater systems for sphaeriid diversity.

Pisidium llanquihuense Ituarte, 1999- Sphaeriidae

Represented by three records, with a compiled range of approximately 40.6-42.6°S. The species appears associated with southern Chilean freshwater systems.

Pisidium magellanicum (Dall 1908)- Sphaeriidae

Represented by two records, including the southernmost occurrence in the dataset: Laguna El Salto, Isla Navarino, approximately 55.0°S. This taxon is central to the biogeographical interpretation of high-latitude sphaeriid distribution.

Pisidium lebruni Mabille, 1884 - Sphaeriidae

Represented by one record from Punta Arenas, approximately 53.1°S. Together with P. magellanicum and M. patagonicum, this species documents the extension of Sphaeriidae into Magellanic inland waters.

Pisidium meierbrooki Kuiper & Hinz, 1983- Sphaeriidae

Represented by a single northern Chilean record at approximately 18.2°S. This record strongly influences the genus-level northern limit of Pisidium in the compiled dataset and should be verified in the final database.

Sphaerium forbesi (Philippi, 1869)- Sphaeriidae

Represented by a single northern record at approximately 18.2°S. The record defines the northern limit of Sphaerium in the analysed dataset and requires coordinate and taxonomic verification before submission.

Sphaerium lauricochae (Philippi, 1869)- Sphaeriidae

Represented by a single record from northern Chile, approximately 21.6°S. Its inclusion indicates the occurrence of Sphaerium in arid or high-altitude northern inland-water systems, but final taxonomic validation is recommended.

3.3. Latitudinal Distribution

The analysed records covered a broad latitudinal gradient from approximately 18.2°S in northern Chile to 55.0°S in the Magallanes Region. The northernmost occurrences corresponded to Pisidium meierbrooki and Sphaerium forbesi in northern Chile, whereas the southernmost record corresponded to Pisidium magellanicum from Laguna El Salto, Isla Navarino, at approximately 55.0°S. Diplodon chilensis exhibited the broadest species-level distribution among frequently recorded taxa, extending from approximately 30.5°S to 46.6°S, equivalent to a range of 16.1 degrees of latitude (Table 2; Figure 1).

3.4. Latitudinal Distribution

The analysed records covered a broad latitudinal gradient from approximately 18.2°S in northern Chile to 55.0°S in the Magallanes Region. The northernmost occurrences corresponded to Pisidium meierbrooki and Sphaerium forbesi in northern Chile, whereas the southernmost record corresponded to Pisidium magellanicum from Laguna El Salto, Isla Navarino, at approximately 55.0°S. Diplodon chilensis exhibited the broadest species-level distribution among frequently recorded taxa, extending from approximately 30.5°S to 46.6°S, equivalent to a range of 16.1 degrees of latitude (Table 2; Figure 1).

3.5. Genus-Level Patterns

At the genus level, Diplodon accounted for 53 records and two species, whereas Pisidium comprised five species and 25 records. Pisidium exhibited the broadest genus-level latitudinal distribution, extending from 18.2°S to 55.0°S, although this broad range is strongly influenced by single northern and southern records. Diplodon ranged from 30.5°S to 46.6°S, Musculium reached 51.7°S and Sphaerium was restricted to northern Chile in the compiled dataset (Table 3).

4. Discussion

4.1. Diversity and Composition of Chilean Inland-Water Bivalves

This study provides a georeferenced synthesis and annotated checklist of Chilean inland-water bivalves based on compiled literature records. Although the analysed fauna is species-poor compared with that of larger South American countries, it is biogeographically distinctive because it combines large-bodied hyriid mussels and several small-bodied sphaeriid clams. The dataset also illustrates an important contrast between species richness and record frequency: Sphaeriidae contributed most species, whereas Diplodon chilensis dominated the compiled occurrence records.

The prominence of Diplodon chilensis in the dataset probably reflects a combination of true ecological distribution, conspicuous body size and stronger historical research attention. Chilean Diplodon populations have been studied in relation to reproduction, population dynamics, feeding behaviour, karyology, growth, trophic interactions and conservation [7,8,10,13,23,24,25,26,27,28,29,30,31,32] (Peredo and Parada 1984; Parada et al. 1987, 1990; Lara and Parada 1988, 1991; Lara and Moreno 1995; Lara et al. 2002; Peredo et al. 2003; Valdovinos and Pedreros 2007). In contrast, sphaeriids are small, less conspicuous and more easily overlooked in routine surveys, which may partly explain their lower frequency in compiled records despite higher taxonomic richness.

4.2. Contrasting Latitudinal Patterns

The most relevant biogeographical result is the contrast between the dominant but less austral distribution of Diplodon and the higher-latitude reach of Sphaeriidae [3]. Diplodon chilensis showed the broadest range among individual species with many records, but its southern limit in the dataset was approximately 46.6°S [4]. By contrast, sphaeriid clams reached Magellanic and sub-Antarctic environments, including Pisidium magellanicum at Isla Navarino, approximately 55.0°S [15].

This pattern suggests that southern distributional limits differ substantially among Chilean inland-water bivalve lineages [3,4]. Large freshwater mussels appear mainly associated with continental temperate and Patagonian freshwater systems, where permanent habitats and hydrographic connectivity are more favourable [3]. Small-bodied sphaeriids, in contrast, can occur in isolated, cold and high-latitude freshwater habitats. Their small size, short generation time, ecological flexibility and potential for passive dispersal may facilitate persistence in environments where large unionoid mussels are absent [4].

4.3. Biogeographical Implications for Southern South America

Patagonia, Tierra del Fuego and Isla Navarino are key areas for understanding the historical biogeography of freshwater organisms in southern South America [33,34]. These regions were strongly affected by Quaternary glaciations and subsequent post-glacial landscape reorganisation. The occurrence of Pisidium magellanicum, Pisidium lebruni and Musculium patagonicum in Magellanic and sub-Antarctic freshwater environments suggests that sphaeriid clams may be useful model organisms for studying high-latitude persistence, post-glacial colonisation and dispersal in southern inland waters [15].

The interpretation proposed here should be treated as a working biogeographical hypothesis rather than as a definitive reconstruction. The compiled dataset is literature-derived and is affected by uneven sampling effort among regions, taxa and habitat types. Northern and southern range limits based on single records require particular caution and should be validated through taxonomic revision, specimen examination and updated georeferencing [35]. Nevertheless, the observed contrast between Hyriidae and Sphaeriidae is strong enough to justify further research and provides a testable framework for future phylogeographical and ecological studies.

4.4. Conservation and Research Implications

Freshwater bivalves are sensitive to habitat degradation, eutrophication, sediment alteration, hydrological regulation and water pollution. In Chile, Diplodon species have particular conservation relevance because they are large, long-lived and associated with permanent freshwater habitats that are increasingly affected by land-use change and water management [4]. Sphaeriids, although smaller and less conspicuous, may provide important information on isolated wetlands, high-latitude systems and poorly explored inland waters [5].

The present checklist highlights the need for updated field surveys, specimen-based taxonomic confirmation, molecular analyses and public deposition of occurrence data. Future studies should prioritise poorly sampled northern systems, Patagonian waters, Magellanic wetlands and sub-Antarctic freshwater habitats. Integrating morphology, molecular data and standardised georeferenced records will be essential for resolving taxonomic uncertainty and improving conservation assessments of Chilean inland-water bivalves.

5. Conclusions

The inland-water bivalve fauna of Chile comprises a relatively small number of recorded species but exhibits clear biogeographical structure along the country’s extensive latitudinal gradient. Diplodon chilensis was the dominant species in terms of compiled occurrence records and showed the widest range among frequently recorded taxa. However, Sphaeriidae reached the greatest southern distributional limits, with Pisidium magellanicum from Isla Navarino representing the southernmost record in the database. This contrast between Hyriidae and Sphaeriidae suggests that body size, life-history traits, habitat requirements and dispersal capacity may shape freshwater bivalve distributions in southern South America. The revised checklist and georeferenced synthesis provide a baseline for future taxonomic, phylogeographical, ecological and conservation studies of Chilean inland-water bivalves.