New records of Corbicula clams in French rivers

The aim of this paper is to present new data and review the current distribution of Corbicula spp. in France. Three morphotypes were recorded during the study “round form”, “light R form” and “saddle form”. During April 2009, Corbicula spp. were found in several watercourses in France, including three rivers (Canal de la Somme, Oise and Vilaine) where the taxa were not previously reported. In addition, this is the first report of form S in the River Seine and, the first record of the “light R form” in the River Gard. Molluscs of the genus Corbicula are considered to be a well established alien species in large rivers.


Introduction
Freshwater biodiversity is threatened by several mechanisms among which habitat loss and the introduction of non indigenous species are the two most important (Nepveu and Saint-Mexent 2002).Aquatic invasive species are of particular interest because of the major impact they have on both ecosystems and industrial installations.Currently, the invasion rate in French aquatic ecosystems, all functional groups taken into account, follows an exponential curve which indicates an acceleration of the phenomenon during the last decades (Devin et al. 2005).However, this acceleration could be an artefact, owing to the growing interest for the biology of invasions.In 2005, 43 freshwater non-indigenous species of macro-invertebrates were reported in France (Devin et al. 2005).Among them, molluscs of the genus Corbicula (Megerle Von Mühlfeld 1811) are one of the most successful groups and are of particular concern (Devin et al. 2005).Corbicula is a benthic filter-feeder and has multiple ecological impacts: e.g.competition with native bivalves or phytoplankton decline (Aldridge and Müller 2001;Hakenkamp et al. 2001;Schmidlin and Baur 2007).It is also considered as a biofouling organism (Swinnen et al. 1998).The aim of this paper is to give an overview of the current distribution of Corbicula in France, which includes new data.
The modern native range of the genus Corbicula includes South of Asia, the Middle East, Australia and Africa but fossils have been recorded in Europe, North America and Japan (reviewed in Araujo et al. 1993).
The first record of Corbicula outside its current original range was in 1924 in North America (British Columbia), it then rapidly spread across the country (McMahon 1982).The genus was first reported in South America around 1970 (Ituarte 1994).In Western Europe, Corbicula was then detected in 1980 in France, in the River Dordogne.That same year it was also observed in the Tagus estuary in Portugal (Mouthon 1981).Since then its European distribution has further extended (Hubenov 2001;Popa and Popa 2006;Morais et al. 2009).
Two main morphotypes are typically found in Europe.Here we refer to morphotypes R ("round form") and S ("saddle form") (Mouthon 2000;Pfenninger et al. 2002) that have usually been respectively identified as C. fluminea and C. fluminalis.However, this nomenclature does not satisfactorily reflect the taxonomic situation in the genus Corbicula in Western Europe (Renard et al. 2000;Pfenninger et al. 2002).The shell surface of forms R and S has well-marked concentric ridges that are more closely spaced and thinner in form S than in form R (Figure 1).Form R is round and broad, while form S is narrow.Generally, form R reaches larger sizes than form S (Swinnen et al. 1998).In form R the internal shell colour is white sometimes interspersed with purple marks while in form S it is deep purple (Figure 1).Additionally, a third, light-coloured form ("light R form", hereby named Rlc), was reported in the Haut-Rhône (Mouthon 2000).The Rlc form is morphometrically similar to form R, but the outer shell surface has a lighter colour.Moreover the inner shell surface is yellow-white with less purple marks (Figure 1) (Mouthon 2000).
The current distribution of taxa belonging to the genus Corbicula in French rivers is presented in Table 1.The present paper reports the recent discovery of Corbicula morphotypes R, Rlc and S in five rivers in France where they were never recorded previously (Table 1, Figure 2).

Results and discussion
On the 18 prospected rivers, one was not accessible (River Garonne) due to adverse hydrological conditions.Moreover no Corbicula individuals were found in the tidal River Léguer in Lannion (not shown on the map).Form R was found in all 16 remaining rivers (see Table 1 rivers with record date April or July 2009).Form S was found in the River Seine exclusively.In addition, we found at least two individuals of form Rlc in the River Gard where they occur in sympatry with form R. We highlight here the broad distribution of form R while forms S and Rlc seem restricted.However our survey underestimates the occurrence of form S, which was found for example in the rivers Moselle and Allier in previous studies (Bachmann et al. 1997;Vrignaud 2007).This is the first record of Corbicula (form R) in the Somme canal and therefore in the "Picardie" region.It is also the first record of Corbicula (form R) in the rivers Oise (region Seine-Normandie) and Vilaine (region Loire-Bretagne) and consequently the first observation of Corbicula clams in the Brittany region ("Bretagne") (Table 1, Figure 2).Corbicula form R was previously detected in the River Seine in 2000 (Mouthon 2000) and the presence of form S in this river has been suggested in a report by bij de Vaate et al. (2007).Here, we confirm the first official record of Corbicula form S in the River Seine in France.Moreover, we report the first record of form Rlc in the River Gard (Table 1, Figure 2).
These results confirm the widespread invasion of Corbicula into all main French basins, especially in large lowland rivers.This rapid dispersal of Corbicula in France is not surprising because the French basins are highly interconnected by a large number of canals which facilitate the progression of Corbicula spp.More specifically, the presence of the clams in the River Oise may be explained by the existence of the Aisne-Oise canal which allows the migration of these molluscs from the Aisne to the Oise.The presence of Corbicula clams in the River Aisne was first recorded in 2000 (Mouthon 2000).The Somme canal intersects with the River Aisne and therefore this connection probably allowed their passage from the Aisne to the Somme.The river Vilaine flows directly into the Atlantic Ocean, but is connected to the River Tarn by the Ille-et-Rance canal and by the River Rance, which were probably the dispersal routes of Corbicula to the River Vilaine.
April and July 2009, we conducted a sampling survey of Corbicula species in 18 rivers distributed in six French watersheds.Living specimens of Corbicula were collected by wading the bottom of the rivers with a handnet.We examined the morphology in order to classify each individual as form R, form Rlc or form S (Pfenninger et al. 2002).

Figure 1 .
Figure 1.External views of the shells of Corbicula form R (I), form Rlc (II) and form S (III).Photograph by Lise-Marie Pigneur.

Figure 2 .
Figure 2. Hydrological map of France showing previous and new records of Corbicula in French rivers.The new records are in red.Rivers sampled in this study are indicated in bold with the coloured circles referring to the presence of each form: form R in blue, form Rlc in red and form S in green.

Table 1 .
The invasion of Corbicula spp. in French large rivers and canals.The underlined locations present the new records of Corbicula in France.The morphotypes recorded in our samples are indicated.