A field tool for the rapid measurement of mussel valve closure strength as a proxy for condition

Freshwater mussels (Unionida) are key to the health of many freshwater systems through the ecosystem functions and services they provide, yet their populations are declining across the globe. To address these declines with appropriate conservation actions, it is necessary to apply informative tools for measuring and monitoring mussel health. To do this, a suite of biomarker methods has been applied to mussels. However, baseline biomarker measurements are often not available, no framework exists for the translation of these methods from research into routine mussel health monitoring and they are impractical for giving rapid measurements in the field due to the extensive time, expertise, cost and/or lab processing they require. Here, we developed a tool (the mussel Stress Tester, STer) to measure mussel valve closure strength as a proxy for condition. The STer is a specially designed pair of tongs with prongs that can bend when opening a mussel. The tongs give a numerical reading of this prong displacement (the STer value), which is indicative of the mussel's valve closure strength. During a field trial, we showed that measurements with the mussel STer were repeatable. We demonstrated the tool's applicability for assessing mussel condition by showing that STer value was positively associated with acetylcholinesterase activity, a well-established bioindicator assay for stress. Taking measurements with the mussel STer requires little user training, minimal costs and only a few seconds per mussel. Thus, the STer can be applied rapidly in the field to provide estimations of mussel condition, which could be integrated into wider health assessments to inform appropriate conservation action. Additional work is required to assess whether use of the STer, and associated removal and handling of mussels, causes any long-term harm to individuals. Even so, we envision the widespread application of this tool in rapid mussel health monitoring in the future.