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Abstract
Abstract Environmental DNA (eDNA) from water samples is increasingly used to detect the presence and distribution of species in aquatic ecosystems. However, before implementing eDNA in monitoring programmes, various species‐specific sampling or analytical issues remain to be resolved in order to minimize frequencies of false‐positive and ‐negative results. For example, empty shells from freshwater pearl mussels (Margaritifera margaritifera) contain extractable DNA (chemical extraction from ground‐up shells) suggesting a risk of false‐positive samples at stream sites with extinct populations but with empty shell material remaining. The aim of this study was to investigate whether empty and naturally degrading shells from M. margaritifera can cause false‐positive eDNA signals in water samples. Water samples were collected from outdoor stream channels (in Lemming, Denmark) with living freshwater pearl mussels or empty shell material (density ~10 individuals m−2) during a 3‐week experimental period. Living freshwater pearl mussels were collected from Hemgravs stream in Sweden and transported to Denmark according to permissions granted by the Swedish and Danish authorities. All water samples from stream channels containing empty shells were negative for eDNA indicating that eDNA traces in stream water are most likely to originate from living individuals located upstream of the sampling site. Water samples collected from stream channels containing living individuals of M. margaritifera were consistently positive for eDNA except for one sample (interpreted as a false negative). The study shows that positive eDNA signals for freshwater pearl mussels most likely reflect the presence of living individuals. Consequently, we suggest that eDNA should be used to locate remaining population fragments of M. margaritifera in deep and turbulent streams, providing a platform for faster and more efficient decision making when launching investigative and mitigation initiatives.
Highlights
Freshwater ecosystems are among the most impaired ecosystems on earth with species extinction rates exceeding those of most other types of ecosystem (Strayer & Dudgeon, 2010), and indicators of biodiversity are increasingly trending downward
Water samples collected from stream channels containing living individuals of M. margaritifera were consistently positive for Environmental DNA (eDNA) except for one sample
The freshwater pearl mussel is listed in Annex II and Annex V of the European Habitats Directive ensuring that European Union (EU) member states have legal obligations to protect and improve conditions for the remaining populations (Council of the European Communities, 1992)
Summary
Freshwater ecosystems are among the most impaired ecosystems on earth with species extinction rates exceeding those of most other types of ecosystem (Strayer & Dudgeon, 2010), and indicators of biodiversity (extinction rates, populations trends, etc.) are increasingly trending downward. The authors suggested that one plausible explanation could be DNA secretion from remaining shell material causing false-positive results Another similar example is the River Varde in Denmark, where a recent survey recorded eDNA in water samples from two sites with historical records of freshwater pearl mussel populations (Andersen & Wiberg-Larsen, 2017), but several conventional monitoring attempts (including diving) have not confirmed the presence of living individuals since 1999. These contradictory results between eDNA and conventional surveys in deep and turbid rivers can be: (i) a consequence of insufficient detection probabilities of conventional. We expected that the potential eDNA secretion from empty shell material would not produce detectable levels of eDNA in stream channel water, as the DNA present in shell material probably needs to be extracted by grinding followed by chemical extraction (Geist, Wunderlich & Kuehn, 2008)
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