Aquatic ecosystems globally have been invaded by molluscs. Tarebia granifera is a highly successful invader, often becoming the dominant aquatic invertebrate species in an invaded ecosystem. Resultingly, it has been suggested that T. granifera may have severe negative impacts on these invaded ecosystems. Limited information is available regarding the population structures and densities of T. granifera, particularly in invaded countries such as South Africa, and information on this could assist in developing management and control strategies for this invasive species. The aim of the present study was to assess the current distribution, densities, and population structures of T. granifera in invaded habitats on the Limpopo and Phongolo River systems, South Africa. This was accomplished by collecting aquatic benthic molluscs from sites across these systems. Water quality parameters were measured at each site and water samples collected for chemical nutrient analyses. The density of snails was determined for each site and the population size and structure as well as birth rate was calculated for T. granifera. Tarebia granifera was found to be the dominant molluscan species in habitats where it was present and all size classes from new-born to mature adults were found throughout at some of the highest densities globally. Worryingly, native molluscan species, were often absent or in much lower densities than reported in literature at sites where T. granifera was present, suggesting a negative effect on the native molluscan density and diversity. Contrary to most previous studies, there were no significant correlations between T. granifera and the selected water quality parameters. Higher densities and new-born recruitment of T. granifera were observed in the spring than autumn, likely in response to shifts in environmental conditions. This study provides crucial insights into the population structure and dynamics of T. granifera in invaded habitats, particularly for relatively newly invaded regions such as southern Africa.

Nineteen sites were selected for sampling in this study based on known distributions of aquatic snails (invasive and native) and accessibility to sites (Figure 1). Four sites were sampled on the lower Phongolo River in northern KwaZulu-Natal (August – September 2017), South Africa, and a total of 15 sites were sampled on the Limpopo River and its tributaries (Limpopo River system), in the Mpumalanga and Limpopo provinces of South Africa (April – May 2021). Two sites in the Limpopo River system on the Olifants River (OLIF-1, OLIF-2) were sampled in consecutive years (2020 and 2021). 

Mollusc sampling and identification

Each of the 19 selected sites was sampled for aquatic benthic molluscs (including snails and bivalves/clams), using a metal-frame square benthic sampler (30 x 30 cm, 2 mm mesh size). The sampler was used to scoop up sediment and sieve through the benthic zone of the river. Random samples, each comprising three replicates consisting of ten scoops per replicate (30 sampler scoops) covering a total area of 2.7 m² were collected. Sampling occurred randomly within the site areas, avoiding deep waters due to the presence of crocodiles (Crocodylus niloticus). Samples were preserved with 70% ethanol in plastic jars and transported back to the laboratory and molluscs were identified to species level according to Appleton (2002) and Fry (2021). Only individuals that were considered “alive” (i.e., shell closed by operculum) at the time of sampling were counted and the density of each species per site (number of individuals sampled/m²) determined.

To determine the population size structures for T. granifera, samples were processed through sieves and snails divided into size classes according to shell length (Table 2). For this study, size classes were used to describe age groups based on the size of sexual maturity in the populations obtained from published literature. Samples were separated into size classes using a King Test (VB 200 300) sieve shaker with Clear Edge Test sieves. Tarebia granifera were divided into mature adults (> 10.39 mm) that are considered able to reproduce, adults (5.28 - 10.39 mm) becoming sexually mature and starting to reproduce, juveniles (1.92 - 5.27 mm) not able to reproduce, and new-born snails (< 1.9 mm) that were recently born (Appleton et al., 2009; Miranda et al., 2011b).

Water quality analysis

Physiochemical water quality variables: pH, temperature (˚C), electrical conductivity EC) (μS/cm), total dissolved solids (TDS) (mg/L), dissolved oxygen (mg/L) and percentage oxygen saturation (%) were measured in situ at each site using Extech EC500 pH/electrical conductivity and Extech DO600 dissolved oxygen water quality meters. Water samples (250 mL) from each site were collected in triplicate for chemical nutrient analyses. Samples were preserved by freezing (-4 ˚C) them on-site using a portable camping fridge/freezer until analysis. In the laboratory, water samples were left to defrost at room temperature and analysed for the following chemical nutrient variables: Turbidity (FAU), SO₄^2- (14791), NH₄⁺ (14752), NO₃^- (14773), NO₂^- (14776), PO₄^3- (14848), and Cl^- (14897) (mg/L) using the appropriate test kits (test kit catalogue numbers in parentheses) and standard protocols with a Merck Spectroquant Pharo 300 Spectrophotometer (Gerber et al., 2015b; de Necker et al., 2022a).