4.1 Standardized eDNA sampling at a large scale
For the first time here, we have implemented eDNA surveys at a large
scale, with standardized sampling protocols and analyses. Our results
are therefore comparable, both in space and time: it is now possible to
reproduce our sampling in the future to monitor the evolution of bivalve
populations. This is particularly relevant for monitoring invasive
species: eDNA analyses are usually used to detect a given species’
occurrence. But it can also be used to argue for absence. Our
large-scale eDNA sampling revealed the occurrence of invasive species in
their previously known range and also in locations they were expected to
colonize considering the potential colonization routes (i. e. mainly
artificial channels connecting main watersheds, eg. Prié & Fruget,
2017). Contrarily, eDNA samples confirm the absence of the invasive
species where they were not (yet) expected. Re-sampling these places
regularly could allow documenting colonization. Our results also confirm
that some introduced species are not invasive. For example, the Long
Fingernail Clam Sphaerium transversum (Say, 1829) has been
recorded only twice in France with traditional methods, and eDNA only
added one isolated data.
On the other hand, large-scale eDNA sampling was valuable to detect rare
and endangered species. The False Orb Pea Mussel was detected in only
two sites in France, supporting its critical conservation status. Only
one new location could be added by our eDNA samples for the very
well-known Giant Freshwater Pearl Mussel (Prié et al., 2018).
Furthermore, some species such as the Depressed River Mussel, the Nut
Orb Mussel, and above all the Witham Orb Mussel were rarely detected
despite their supposed large distribution range in France, which flashes
a red light on their conservation status.