Overworked and underpaid: the ecology and function of a vulnerable freshwater mussel, Westralunio carteri

Jake R Daviot

doi:10.60867/00000095

Freshwater mussels (Bivalvia: Unionida) play vital roles in freshwater ecosystems, contributing to overall ecosystem functioning. However, sensitivity to environmental changes, obligate parasitic reproduction, late maturity, and limited mobility make them highly vulnerable to extinction. It is therefore crucial to understand the ecological contribution of freshwater mussels, as declines may result in cascading impacts. Australia has low species diversity in comparison to well-studied Northern Hemisphere regions, however, Australian species are almost twice as likely to be at risk compared to non-Mediterranean regions. Westralunio carteri is the sole species of freshwater mussel found on the west coast of southwestern Australia, a global biodiversity hotspot, and has experienced substantial range declines over the last 50 years. Past work on W. carteri has focused on distribution, habitat preferences, range decline, and factors driving these declines, however we are yet to understand the ecological importance of the species. Drying and salinisation are the primary drivers of declines for a suite of south-western Australia’s freshwater species, including W, carteri, with further increases in aridity anticipated in coming decades. Given these increasing pressures, it is essential to understand the ecosystem role performed by W. carteri, not only to forecast how these processes may be impacted with further decline, but to bolster conservation efforts for this flagship species. The overarching aim of this thesis was to quantify the ecosystem function and services provided by W. carteri, and to determine the extent to which artificial habitats provide refuge for the species. First, I assessed the biofiltration capacity of W. carteri under three water temperatures (15, 20 and 25 °C) under laboratory conditions, with estimates confirmed with field studies. Westralunio carteri exhibited a mean clearance rate of 177.5 mL g.tissue-1 h-1 in the laboratory, with increased rates at higher temperatures. Clearance rates of W. carteri aligned with other Hyriidae mussel species, and more broadly with clearance rates of other Unionida species. Biofiltration provided by W. carteri is most influential under conditions of low flow and high water residence time, such as in drought refuge pools. The volume of water filtered by W. carteri under these conditions is proportionally high, providing increased water clarity. I then assessed the ability of W. carteri to clear E. coli from inoculated river water and the persistence of E. coli in mussel biodeposits post-exposure. Mussels were exposed to river water inoculated with E. coli and bacterial concentrations were measured over 24 h. Westralunio carteri demonstrated significant reduction of E. coli concentrations with a maximum 2.73 log (99.8 %) reduction at 12 h post exposure and an estimated clearance rate of 544.0 mL ind.-1 h- 1. Westralunio carteri concentrated E. coli in biodeposits, depositing bacteria to sediments. This aggregation of E. coli may result in increased secondary consumption, as dense mussel beds are often associated with higher abundance of other invertebrates. Thirdly, I assessed the role of W. carteri in consumer mediated nutrient recycling in two different rivers. The organic matter (OM), nitrogen (N) and phosphorus (P) content of mussel excretion and deposition (faeces + pseudofaeces) was quantified, along with storage in mussel shells and soft tissues. Mussels excreted waste high in N, providing labile inorganic N for primary production. Biodeposits had low N:P in a P limited system, suggesting biodeposits may be a rich source of P for other invertebrates. Storage rates of N:P were dependent on mussel density, but in high densities, W. carteri may provide nutrient storage at similar rates to aquatic macrophytes. These results align with literature from the Northern Hemisphere, showing that W. carteri contributes to consumer driven nutrient dynamics, with more pronounced effects during periods of low flow and extended water residence. Finally, I analysed field survey data to assess the abundance and demographic structure of W. carteri across artificial and natural habitats. Results showed a subset of artificial habitats supported high abundance and demographically viable populations, while some habitats, showed impaired populations at risk of extirpation. Mussel demographics differed between sites overall, with a broader size range observed in natural populations. Where artificial habitats are stable and analogous to natural conditions, they may provide valuable conservations arks for W. carteri amidst natural habitat loss. This study provides the first experimental quantification of the vital ecosystem functions and services performed by W. carteri. This research has further shown artificial habitats can be a viable conservation ark for W. carteri amidst broader patterns of habitat loss. These results emphasise that without urgent and unified conservation effort, we risk irreversible losses of the ecosystem function and service provided by W. carteri. Freshwater mussels globally are experiencing an unprecedented decline, often going unnoticed. By highlighting the critical ecological role of mussels, we can broaden public and scientific engagement in conservation efforts. Protecting mussels means safeguarding not just a single species, but also the essential ecosystem functions and services that support other freshwater organisms and human communities alike.

Overworked and underpaid: the ecology and function of a vulnerable freshwater mussel, Westralunio carteri

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