Assessment of past and present distribution of freshwater molluscs in KwaZulu-Natal, South Africa, particularly at River Eco-Status Monitoring Programme sites.

Abstract

Globally, freshwater biodiversity is declining in major river basins. As a whole, South Africa is not exempt from this trend; the rivers of KwaZulu-Natal Province are being impacted by various anthropogenic activities that threaten their sustainability, leading to the loss of natural land cover and the ultimate decline in environmental functionality and connectivity in KwaZulu-Natal. Across South Africa and KwaZulu-Natal, nearly 71% of freshwater ecosystems are considered either endangered or critically endangered because of anthropogenic activities. Previous work on freshwater Mollusca in South Africa, especially in KwaZulu-Natal, is well-documented, including recent distribution range and taxonomic studies. Historically, ca. 69 freshwater mollusc species have been recorded for KwaZulu-Natal, supported by 124 years of historical collection data. However, most recent and ongoing surveys in the province have recorded fewer freshwater mollusc species than previously documented, suggesting a decline of mollusc diversity in KwaZulu-Natal. Additionally, ever since the River Eco-Status Monitoring Programme (REMP) was established in the last decade, aquatic macroinvertebrates, including 11 mollusc families, have been used for biomonitoring of freshwater ecosystems in South Africa, while historical records have reported 16 mollusc families, and no higher resolution has been used to assess mollusc diversity collected at the REMP sites in KwaZulu-Natal. Thus, updated information on the diversity of freshwater molluscs across major freshwater ecosystems of KwaZulu-Natal is needed to support conservation efforts and decision-making. The aim of this study was to analyse past and present changes in the diversity and distribution of freshwater molluscs in KwaZulu-Natal, particularly at REMP sites. Historical records of 124 years were sourced from four databases of South African natural history collections housed at the Albany Museum in Makanda, Eastern Cape Province (AMM), East London Museum in East London, Eastern Cape Province (ELM), KwaZulu-Natal Museum in Pietermaritzburg, KwaZulu-Natal (KZNM) and the National Freshwater Snail Collection (NFSC) of South Africa in Potchefstroom, North West Province. Specimen records were curated and validated against two independent online open data infrastructures. The number of collected individuals and diversity amongst collections were compared, and trends in the database were analysed. Additionally, historical mollusc taxa were mapped to the nearest 10 km radius, based on their locality descriptions and coordinates, to match the REMP sites in KwaZulu-Natal. A total of 52 sites were sampled four times between February 2024 and September 2025, and were part of the REMP surveys conducted. The sampling sites belonged to the three water management areas, namely ‘U’, V’, and ‘W’, of KwaZulu-Natal, and some of them overlapped with historical localities. Freshwater mollusc specimens were collected using the kick-stir-sweep methodology outlined in the South African Scoring System 5 (SASS5) and active search through hand-picking during each sampling event. The identification keys developed by Brown (1994), and Appleton (2002) were used to identify collected molluscs morphologically to species level. Water quality variables (pH, water temperature, dissolved oxygen (DO), total dissolved solids (TDS), electrical conductivity (EC) and salinity) were recorded in situ at each sampling site, and biotope availability was assessed. This study found that 69 freshwater mollusc species in KwaZulu-Natal are presently preserved in natural history collections in South Africa (55 gastropods and 14 bivalves), belonging to 16 families (11 gastropods and five bivalves). The list of 69 mollusc species included historical records, online sources, and our latest sampling surveys from 2024 and 2025. The greatest increase in specimen collection and deposition occurred during the 1960s and 1970s, peaking at 77% before declining thereafter. A large proportion of specimens collected during this period were deposited at the National Freshwater Snail Collection of South Africa (NFSC) in Potchefstroom, North West Province. Family Planorbidae amongst the gastropods was the most diverse (17 species), while Sphaeriidae was the most diverse within the bivalves (seven species). The ‘U’ WMA harbours the most diverse mollusc community, with a total of 54 species (43 gastropods and 11 bivalves). The gastropods Tomichia natalensis (Connolly, 1939) and Tomichia tristis (Morelet, 1889) are listed as Critically Endangered (CR), and Tomichia ventricosa (Reeve, 1842) Endangered (EN) on the International Union for Conservation of Nature (IUCN) Red List; while the vast majority of freshwater molluscs are listed as either Least Concern (LC) or Data Deficient (DD). In addition, this study found that there was a huge disconnection between museum specimens in natural history collections in South Africa and online data repositories such as the Global Biodiversity Information Facility (GBIF) and Freshwater Biodiversity Information System (FBIS), as most of the data used were not available on these two online data repositories. The results of this study also revealed a higher number of invasive freshwater mollusc species than endemic species. Among biotopes and water quality variables measured, it was reported that the availability of vegetation and total dissolved solids (TDS), were the primary variables determining the number of collected individuals and the distribution of molluscs. Compared with recent studies and the ongoing freshwater mollusc surveys by FBIS, this study recorded 24 species during field samplings of February 2024 and September 2025, belonging to 11 families (seven gastropods and four bivalves). Although more species were recorded in this study than in recent studies and in the FBIS’s ongoing surveys over the past five years (2020 –2025), this study recorded 45 fewer species than historical data. Compared with historical records mapped to the nearest 10 km radius at the 52 REMP sites, 13 species were recorded, whereas this study recorded 24 species, with high or low diversity at some sites.