Adaptive responses of freshwater pearl mussels, Margaritifera margaritifera, to managed drawdowns

Abstract

Alterations to water management practices, in response to a growing demand to maximize energy production from renewable sources, threaten to exacerbate anticipated future water shortages caused by severe drought episodes, brought on by climate change. Across Scotland, many highly managed systems are inhabited by some of the last remaining reproductively viable populations of the freshwater pearl mussel, Margaritifera margaritifera. A lack of empirical evidence concerning mussel responses to alterations in flow is inhibiting the development of effective conservation management practices to prolonged drought. This study addressed this knowledge gap by examining the response of M. margaritifera to controlled water level drawdowns in laboratory and field settings. Using a laboratory flume, the responses of M. margaritifera from two different populations (a regulated system versus an unregulated system) were compared across rates of drawdown and density treatments. An analogous field trial was undertaken to examine the responses of M. margaritifera in a regulated system, with a hydroelectric dam facilitating a controlled drawdown. The study showed that M. margaritifera can detect alterations in flow depth, which culminate in the emersion of mussel beds, and respond by undertaking vertical and horizontal movements to mitigate the risk of prolonged aerial exposure. Results from the field trial corroborated findings from the flume experiments, with 80% of mussels shown to avoid emersion successfully by tracking receding water levels. Findings from this study support the role of controlled drawdowns in regulated rivers to reduce mortalities associated with receding water levels during prolonged low‐flow episodes. Differences between populations in response highlight a need to adopt a context‐dependent approach to conservation efforts.

Experimental evidence associates burrowing behavior of Castalia ambigua (Bivalvia: Hyriidae) with shell shape and density

“…May and Pryor (2016) found similar results, with dense mussel beds stabilizing the riverbed in high hydrodynamic energy habitats. Remaining buried for long periods may prevent feeding from the water column (Curley et al, 2021(Curley et al, , 2022. Therefore, a stable riverbed environment, likely created by the aggregation of C. ambigua, may allow the species to be more exposed, facilitating the filtration of food particles.…”

‘Phototaxis’ in the absence of light? Locomotory patterns in unionid mussels

“…For example, vertical movements can mitigate against ectoparasite infestations (Nichols and Wilcox, 1997). Horizontal movements are well-recorded mitigation strategies against desiccation risk when water levels drop (Curley et al, 2022;Johnson et al, 2001;Lymbery et al, 2021) and have reproductive functions, such as aggregation for increased fertilisation success and migration to suitable locations for larval release (Amyot and Downing, 1998;Shelton, 1997;Vicentini, 2005).…”

“…The movements of unionids are influenced by factors such as temperature, seasonality, changes in hydrological conditions, food availability and water quality (Allen and Vaughn, 2009;Block et al, 2013;Bovbjerg, 1957;Curley et al, 2022;Johnson et al, 2001;Levine et al, 2014;Lymbery et al, 2021;Nichols and Wilcox, 1997;Reynolds and Guillaume, 1998;Zapitis et al, 2021a). However, we still have a limited understanding of what drives unionid movement behaviour (Haag, 2012).…”

Long‐term monitoring shows that drought sensitivity and riparian land use change coincide with freshwater mussel declines

“…As a consequence of this long lifespan, and the corresponding time it takes individuals to reach sexual maturity, mussel populations are often slow to rebound reproductively (Haag, 2012). Mussels do show behavioural responses to elevated temperatures and low‐water conditions, such as horizontal or vertical movement (burrowing) in the substrate (Newton, Zigler & Gray, 2015; Lymbery et al, 2021; Curley et al, 2022), but adults move slowly and thus are vulnerable to stream drying (Schwalb & Pusch, 2007; Allen & Vaughn, 2009). Movement behaviour is known to differ interspecifically but has yet to be investigated in many species.…”