Detritus is an important energy source of stream food webs. Being a mix of allochthonous and autochthonous sources, it is often unknown, which components contribute to the growth of stream organisms. This study focussed on the comparison of two different detritus types (riparian detritus and stream detritus) with respect to food quality and effects on growth as a fitness parameter of juvenile freshwater pearl mussels (FPM). We performed feeding experiments with juvenile FPM under laboratory conditions using the two detritus types from four different natural sources each. Food quality was determined by analysing the fatty acid composition. Stream detritus (conditioned to stream environment including autochthonous microbes) resulted in significantly higher growth rates of juvenile FPM than predominately terrestrial-based riparian detritus indicating higher food quality. Significantly positive correlations were found between mussel growth and different groups of polyunsaturated fatty acids (PUFA). This suggests that especially trace substances such as long-chained n-3 PUFAs and a high ratio of n-3 to n-6 PUFAs enhance the food quality of stream detritus for juvenile FPM. These results highlight the importance of instream conditioning of detritus for the food mix in headwater streams and the importance of PUFAs for the development of juvenile FPM.

Freshwater pearl mussels (FPMs, Margaritifera margaritifera, Linnaeus, 1758) are endangered and particularly vulnerable to climate change. To create effective conservation strategies, we studied their thermal tolerance and the impact of elevated water temperatures on growth and survival. Our experiments included field mesocosm studies in five FPM-streams in the Vogtland region (Germany) (2016 to 2020), as well as laboratory experiments at temperatures ranging from 1 to 26 °C. Growth of juvenile FPMs increased significantly within a temperature gradient from 12 to 21 °C. In the streams, maximum growth was 8.9 µm/d in surface water and 6.5 µm/d in the interstitial. The upper thermal tolerance for the mussels ranged from 22.1 to 22.9 °C, resulting in low survival during hot summer periods in 2018 and 2019. Warming during winter (+5 °C) did not significantly affect growth and survival, but survival during winter increased with the pre-overwintering shell length. Exceeding a shell length of about 1100 µm in December indicating gill development corelated to 50% survival. Shell length in December is primarily controlled by growth depending on water temperatures during summer. These findings define the thermal niche of juvenile FPMs (average summer temperatures of 14.5–21 °C) and have implications for water management, conservation strategies, and site selection for releasing captive-breeding mussels.

The freshwater pearl mussel ( Margaritifera margaritifera , FPM) was 200 years ago widespread in the upper Weiße Elster catchment (Vogtland, Germany). Due to population collapse, only a few hundred individuals remain today, primarily from captive breeding activities. This decline significantly reduced the species’ filtering capacity and its contribution to the river’s natural-filtration capacity. To assess this contribution, we measured FPM clearance rates and estimated the historical population sizes based on historical records. We also examined how shell length, water temperature, and food quality affect the clearance rates. At 15–19 °C (typical summer conditions), FPM showed an average clearance rate of 1.95 L h ⁻¹ mussel ⁻¹ . Rates varied depending on shell length and food quality. Based on historical data, the population size around 1800 likely ranged from minimum 250,000 to 3,125,000 individuals. Relating these estimates to the summer discharge of the river catchment, FPM may have filtered 2.7–33.6% of the flow around 1800. In contrast, today’s remaining population filters just 0.027%. It can therefore be concluded that FPM once played a substantial role in maintaining water quality in the Weiße Elster catchment, a role that has become negligible today.

Human impacts have led to dramatic declines of the endangered freshwater pearl mussel Margaritifera margaritifera (FPM) and to a shift of its main distribution areas to more undisturbed but colder headwaters. In this study, we investigated whether feeding on high-quality food (i.e. food rich in n-3 polyunsaturated fatty acids) can compensate for growth limitations due to low water temperatures and whether food quality influences the survival rate of juvenile FPM. Therefore, we conducted feeding experiments with juvenile FPM at three different temperatures (11, 15 and 19°C) and with three different food sources. Afterwards, mussels were reared in cages in a stream to analyse survival during the first winter. Our results showed that food quality can compensate to some extent for growth limitations due to low water temperatures, but sufficiently high growth rates of juvenile FPM can only be achieved by the availability of high-quality food in warmer (> 15°C) environments. The availability of high-quality food during the main growing season also improves the survival of FPM over the first winter. Consequently, restoration efforts must aim to restore habitats in the historical main distribution areas of FPM and to create habitat conditions that allow the development of high-quality food.

A significant part of freshwater mussel populations has strongly declined and many species are severely threatened nowadays. Captive breeding programs often form a central part of conservation strategies. As the life cycles of many mussel species include an obligate parasitic phase, host fish health is a crucial component of successful mussel breeding efforts. However, information about the safe application of fish therapeutants in mussel captive breeding is scarce. This article summarizes information about practical experiences in Europe. In total, eight different therapeutants were used to treat infestations of eight pathogens. Treatment success varied depending on pathogen and prevalence when treatments were initiated. Mussels did not seem to be affected by treatments, especially as long as they remained encysted. In a second step, Virkon S was applied to treat a fungal infection in tanks with brown trout (Salmo trutta) carrying encysted freshwater pearl mussel (Margaritifera margaritifera) glochidia and to disinfect juvenile mussel rearing containers. In both cases, mussels were not harmed and treated fish fully recovered. Results indicate that certain therapeutants can be used safely and successfully at different stages of breeding cycles. Nevertheless, there is still a lack of standard protocols, which would improve efficiency and the safety of treatments.