3.2 Focus on species
Our results are broadly congruent with the known distribution of the
French freshwater bivalve species (Prié, 2017). However, some species
were detected out of their known distribution range, noticeably
introduced and invasive species, but not only. Others were not detected
where they were expected, and valuable data was collected about poorly
known species.
3.2.1 Out-of-range species
Outstanding data has been collected with eDNA for conservation-concerned
species, such as the Thick-shelled River Mussel U. crassus(categorized EN by the IUCN and listed in the European habitat directive
annex II). The Thick-shelled River Mussel is widespread in the basins of
the Loire, Saône, Seine, and generally speaking in the north-eastern
half of France as shown in Figure 3a. Although its populations have
largely declined over the last century (Lopes-Lima, Kebapçı, & Van
Damme, 2014), there are many recent records of living populations
throughout its range. On the other hand, the species was not known south
of the Loire catchment area on the Atlantic coast (except for a few
specimens in museum collections from the Dordogne with doubtful
vouchers, unpublished data) and historical records downstream of the
Rhône catchment area have not been updated despite intensive research in
the Camargue wetland and Ardèche River. Data collected by eDNA analysis
confirmed the presence of the species throughout the north-eastern
quarter of its range and revealed its presence (i) upstream of the
Dordogne and Charente watersheds, south of the Loire drainage, where
field surveys confirmed its presence by the observation of a few live
individuals in 2018 and 2019, and (ii) in several sectors of the Rhône
River mainstream, particularly downstream, where it was not known from
recent data.
The Rhône River is one of the largest European rivers. The lower Rhône,
not surprisingly, remains a terra incognita for the malacologist.
It is a large, fast-flowing, deep, turbid, navigated river that is very
hard to sample. No scuba-diving nor extensive dredging campaign has been
performed to investigate freshwater bivalves species. The lower Rhône
flows in the Mediterranean area, and its valley constitutes a northward
extension of the northwestern Mediterranean subregion (ex. Darwall et
al., 2014). Its tributaries in the lower part are typical Mediterranean
streams on its right bank and rather alpine streams on its left bank.
Hence its tributaries, which are easier to sample, are very different
ecosystems from the Rhône itself. While the Rhône valley allows a
northward extension of the Mediterranean biome, the Rhône itself
provides a southward path for species of more northern affinities such
as the Thick-shelled River Mussel and the Swollen River Mussel U.
tumidus Philipsson, 1788. These new data could justify the
implementation of conservation measures (Natura 2000 network in
particular) on new stretches of the river.
Species of the genus Anodonta are sometimes difficult to identify
and are probably often misidentified in databases. Because these species
are widespread and common, validation processes of databases do not
focus on these potential misidentifications. In many cases, a barcoding
approach is needed to clarify the Anodonta populations’ taxonomic
status. As an example, the Italian Swan Mussel Anodonta
exulcerata Porro, 1838 has been recently re-described in Italy, based
on molecular data (Froufe et al., 2017). There is no reliable diagnostic
morphological character to identify it in the field (Riccardi et al.,
2019). It was considered endemic to Italy and Switzerland. In this
study, eDNA sequences of the species were detected in a few places in
the upper Loire River. The Italian Swan Mussel is a new species for
France, maybe introduced from Italy. It would for sure have been
overlooked without an eDNA highlighting.
The Chinese Pond Mussel Sinanodonta woodiana (Lea, 1834) is a
large species, introduced in France since 2007, which has rapidly spread
in the south of France and the Rhone basin (Adam, 2010). Data collected
by eDNA analysis confirmed the expansion of this easily identifiable
species in the field, and add a few localities that were not previously
known: downstream of the Tarn River, confirming the rapid colonization
of the Garonne basin, the Sèvre niortaise River, from where it has since
been observed with traditional methods, the Saône River, where it had
not been detected during the freshwater bivalves surveys carried out in
2016 by scuba-diving, and the Moselle River. On the other hand, eDNA
analyses show that it is probably still absent from the Meuse and
Dordogne watersheds, where the eDNA observation pressure is high, and
apparently from the Adour, Seine and Charente basins. For all these
basins, it would be urgent to implement protection measures to avoid the
colonization of the Chinese Pond Mussel. Indeed, it is now known that
this species represents a significant threat to native species
(Donrovich et al., 2017; Huber & Geist, 2019).
The Quagga Mussel Dreissena rostriformis bugensis (Andrusov,
1897) is a smaller species, similar in shell shape to the widespread
Zebra mussel Dreissena polymorpha (Pallas, 1771), and therefore
likely to remain unnoticed due to confusion or negligence. It has only
been known in France since 2010 when it was first detected in the Meuse
and Moselle basins (Bij de Vaate & Beisel, 2011). According to
previously available data shown in Figure 3b, it was thought to have
colonized only the Meuse and Moselle Rivers, with scattered data in the
Rhine and Rhône Rivers (Prié & Fruget, 2017). eDNA data shows that the
species has already spread considerably, being also introduced into the
Mediterranean coastal rivers (Lez River in Montpellier, probably via the
Bas-Rhône-Languedoc Canal), and into the Seine watershed (probably via
the Canal de Bourgogne), where it had never been observed with
traditional surveys. None of our samples from the Atlantic drainages
contained Quagga Mussel eDNA. These results highlights the value of eDNA
sampling for invasive species’ early detection and monitoring.
The Ridgedbeak Peaclam Euglesa compressa (Prime, 1852) has been
formally discovered in France very recently (Mouthon & Forcellini,
2017), although the first data on its distribution date from 1989
(Mouthon & Taïr Abacci, 2012). Analysis of the eDNA has extended its
current range, with new data in the Meuse, upstream of the Seine, the
Saône, the Ardèche, and all along the lower Rhône River.
3.2.3 Rare and endangered species
Our large-scale sampling in France stressed out the worrying
conservation status of some species which are not currently considered
of conservation concern. The Depressed River Mussel P. complanatahistorically had a wide distribution in France, as evidenced by museum
collections (especially those of the National Museum of Natural History
in Paris, unpublished data), where three subspecies were recognized
(Bouchet, 2002). About a hundred records are available in France (see
Figure 3c). Out of these, 46 concern individuals observed living
recently. These data attest to the presence of the species in the
catchment areas of the Seine (about 15 stations), the Somme (one
station), the Meuse (two stations), the Moselle (one station) and the
Rhine (one station). There is no recent record of live specimens
elsewhere in France. eDNA data confirm the current presence of the
species in the Seine, the Meuse (9 stations out of the 52 sampled), the
Moselle (only one station studied), but also in the Saône (one station
out of the two studied), the lower Rhône (only one station out of the 65
sampled) and the Charente (only one station out of the 7 analyzed)
Rivers. Depressed River Mussel eDNA was not detected in any of the
localities sampled in potentially favorable sites (downstream ecosystems
on limestone substratum) in the Garonne basin. Similarly, although few
favorable stations were studied on the Loire drainage, none revealed the
species. The Depressed River Mussel seems to be extirpated from most of
its range in France. The only data on the lower Rhône is remarkable, as
no historical data are known for this sector.
The Witham Orb Mussel Sphaerium solidum is a typical downstream
species, living in large rivers, at depth, in well-oxygenated water,
preferably on sandy substrates. This kind of environment has been
extensively sampled in our study (estimated 250 sampling sites suitable
to the Witham Orb Mussel), but the Witham Orb Mussel has been detected
only once in our samples, close to one of the three places where it has
been last observed alive. Comprehensive data is available given its
easily identifiable thick shell, which can be preserved for years in the
river banks as shown in Figure 3d. But this data, mainly based on shell
observations, probably give an obsolete picture of the reality.
According to eDNA results, the Witham Orb Mussel appears to be
extirpated from the Aisne site by now (where sampling occurred about 10
years after its observation). Only a single population would remain, in
the lower Seine, which makes the Witham Orb Mussel the most threatened
freshwater bivalve species in France.
The Nut Orb Mussel S. rivicola is also a species characteristic
of downstream large rivers, probably less demanding in terms of the
substrate than the Witham Orb Mussel, preferring more muddy sediments.
It also seems a little less threatened, but its situation concerning
eDNA sampling throughout France is alarming. About a hundred records are
available, spread over most of the large French catchment areas.
Nevertheless, very few living populations are currently known. At least
one population seems to subsist downstream of the Loire, on a sector
that we did not sample in the framework of this work. eDNA was only
detected in a single site in the Rhône catchment (out of 65 sampled
sites) eDNA could be detected indicating that Nut Orb Mussels still
occurs there, but probably with very poor conservation status since it
is not known elsewhere in the basin. Living individuals have recently
been collected on the Meuse River, data confirmed by eDNA analysis.
Finally, there are several recent records of living individuals in the
Seine basin, but the eDNA sampling revealed the species only once in the
Seine basin, closed to a previously reported living population.
The False Orb Pea Mussel Euglesa pseudosphaerium Favre, 1927 is a
lowland rivers species that lives on stable swampy areas, in clear and
clean waters and is intolerant to pollution and water level fluctuations
(Welter-Schultes 2012). This kind of habitat has become very rare in
France, and so is the False Orb Pea Mussel. Welter-Schultes (2012) gives
it a wide distribution range, covering most of France, although it is
rare and localized everywhere. Mouthon and Kuiper (1987) recorded it
from only four sites in the Rhone Valley and report an additional
historical record near Toulouse (south-west France). Despite extensive
sampling, we didn’t detect its eDNA in the Rhone Valley. It has been
detected with eDNA analyses once in a small tributary of the Charente
River and once in Britany (north-west France). Although suitable habitat
for this species is under-sampled, eDNA data confirm the worrying
conservation status of the False Orb Pea Mussel.