Aquatic Invasions Records New records of Corbicula fluminea ( Müller , 1774 ) in Galicia ( Northwest of the Iberian Peninsula ) : Mero , Sil and Deva rivers

This paper presents new records of Asian clam Corbicula fluminea in the rivers Mero, Sil and Deva. Records from the Rivers Sil and Deva showed the increase of its spread in the Miño river basin. Findings in the Mero basin represented the first Iberian record of this species outside the Miño basin. Furthermore, it was the most north-western record in the Iberian Peninsula.


Introduction
The Asian clam Corbicula fluminea (Müller, 1774) was first reported in Iberian waters by Mouthon (1981).Since 1989 its presence has been known in Galicia, in the southern area of the Miño basin (Araujo et al. 1993;Antunes and Weber 1996;Sousa et al. 2007Sousa et al. , 2008a;;Ayres 2008).The Asian clam's distribution and density in the lower part of the Miño basin (i.e. the last 70 km comprising all of the international section (Spain-Portugal); Sousa et al. 2005Sousa et al. , 2007Sousa et al. , 2008aSousa et al. , 2008cSousa et al. , 2008d and 2008e and 2008e) is well studied.It is also known that there is both geographical expansion and population growth in this area but that no new records were described in Galicia apart from those in the southern part of the River Miño basin.
The Miño is the largest river in Galicia (approximately 300 km long) and the rivers Deva and Sil are tributaries of its basin located in the southwest of Galicia.The River Mero (northwest of Galicia) flows into the Atlantic Ocean next to the city of A Coruña; the Barcés is the Mero´s main tributary.Both rivers meet in the Cecebre reservoir.

Materials and methods
The first observations of C. fluminea were in June and August 2009 during sampling of freshwater bivalves in Galician rivers.Sampling, based on check sampling points was done to approximate the range extension of this alien species.The searching method utilized river checkpoints, wading and examination via a glass-bottomed bucket for initial observations.In the River Mero, maximum densities were also estimated using a quadrat (25×25 cm 2 ) in places with the highest abundance.All the individuals were counted and sieved using a mesh size of 2 mm.

Results and discussion
The River Deva Individuals were detected at the end of the River Deva next to the Frieira Dam in San Martiño de Valongo (Figure 2) where a low abundance of C. fluminea was found at one point.No dead shells or live individuals were found at the ten check points upstream (Figure 2).

The River Sil
In the River Sil a great number of dead shells were detected between the San Martiño dam (Petín) and Santiago dam (A Rúa) in a transect of 400 m (Figure 2).However, no live individuals were found.They were however located in deep places, in the reservoirs and in upstream areas.New sampling to detect live individuals should be implemented in this river area.

The River Mero basin
The C. fluminea (Figure 1   The Asian clam has traits that make it a successful invader: a short life span, high fecundity and high adaptability to colonize new environments (McMahon 2000).According to our data its spread across Galicia is increasing.
The new record in the River Mero was the first finding in Galicia outside of the River Miño basin and it should be noted that there are no natural connections between these two basins.The new records on the rivers Deva and Sil confirm the high spread of C. fluminea in the Miño basin and probably the fact that the River Miño estuary is a potential source for the introduction of C. fluminea into other aquatic ecosystems (Sousa et al. 2008a).An example of this is the high density and biomass of C. fluminea just 200 m below the Frieira (Sousa et al. 2008a).
The vector of introduction is unknown.On the one hand, it is known that in some shops in A Coruña C. fluminea is sold as an ornamental bivalve for aquariums (personal observation).On the other hand the lack of control for fishermen's gear, tackle and techniques may be the main cause of this spread.Furthermore, as local residents do not consider this species to be alien, they may spread it in an unconscious way.
Species belonging to the genus Corbicula have been described as having negative effects on ecosystems, especially to native unionid populations: bioturbation processes (Vaughn and Hakenkamp 2001) and filter feeding (Yeager et al. 1994) can disturb the habitat and affect the recruitment of juvenile native mussels.Moreover, both living and dead shells can give rise to biofouling problems (Pimentel et al. 2000) The M. margaritifera records in the River Mero are in accordance with the classical records of Macho (1878) but at present there is an extremely low abundance compared with other Galician rivers such as the Eo and Masma Rivers (Outeiro et al. 2008).In the River Mero basin, the Asian clam could be considered as an introduced opportunistic invader of habitats where freshwater pearl mussel populations have been reduced by anthropogenic effects, but its further spread represents a threat to other, less disturbed mussel habitats and to Galician freshwater ecosystems.
An effective control and management focused on environmental education is necessary in Galicia to prevent new invasions into other rivers, to avoid biofouling problems and to protect the native protected mussels, which are keystone species in stream ecosystems.
) population is located down-stream of the Cecebre dam from Apeadoiro Cambre-Sigras (point A) to Ponte da Agra (point B).From Ponte da Agra to the mouth of the river five more checks to test for the presence of C. fluminea were done.Two of them were carried out in tributaries (the Valiñas River and Rego da Gándara) with a negative result (Figure 1).Upstream of the Cecebre dam the presence of C. fluminea was not detected in the Rivers Mero and Barcés.The highest maximum density of C. fluminea was found in Apeadoiro Cambre-Sigras (estimated 2120 ind./m 2 ) in contrast to Ponte da Agra (estimated 1960 ind./m 2 ).The presence of the red swamp crayfish, Procambarus clarkii (Girard, 1852), was also found in point B and in the Cecebre reservoir too.Furthermore, upstream of the Cecebre dam, some individuals of the threatened bivalve Margaritifera margaritifera were found at one point in the river Mero and also at one point in the River Barcés (Figure 2).Margaritifera margaritifera is classified as Endangered (EN) on the IUCN Red List and is classified as Endagered in CATGEA.Furthermore it is also included in Habitat Directive (D 92/43/EEC) Annex II and mentioned in the Bern Convention Annex III.

Table 1 .
Records of Corbicula fluminea in the River Miño basin.

Table 2 .
Records of Corbicula fluminea and Margaritifera margaritifera in the River Mero basin.