First record of the Asian clam Corbicula fluminea ( Müller , 1774 ) in the Guadiana River Basin ( southwestern Iberian Peninsula )

The paper presents the first record of Corbicula fluminea (Müller, 1774) in the Guadiana River Basin (Iberian Peninsula). This exotic bivalve was found in June 2006 on the Lacarón and Guadiana rivers with mean density is between 3.3 and 16.9 individuals per squared meter. Population structure and the dates of the first records suggest that the introductions may have occurred between 2003 and 2004. The potential impact of the invasion on native freshwater bivalves is discussed along with the need for monitoring and prevention of further introductions of non-indigenous bivalves in the Iberian Peninsula.

Recently two exotic species of freshwater bivalves, both from Southeast Asia, have been introduced into Iberian inland waters.The zebra mussel Dreissena polymorpha (Pallas, 1771) was first reported in Spain from the Ebro River basin, in the early 1980´s (Altaba, 1992).Corbicula fluminea (Müller, 1774) (Figure 1) was found for the first time in Iberian waters in 1980 in the Tagus River estuary (Mouthon, 1981).Later, the species was reported from the Douro River, Oporto, Portugal (Nagel 1989) as well as the Miño and the Douro rivers, Spain (Araujo et al. 1993).Both species are considered potential invasives because, they are characterized by early sexual maturity, high reproductive potential and a remarkable ability to adapt to the environments they colonize (Darrigran 1997).Such characteristics allow exotic bivalves to disperse quickly in great numbers (Darrigran and  At the Lacarón River, a quantitative survey of their density was carried out in two 100m x 100m areas.Random coordinates were chosen at which a sample of sediment (n= 10) from the river bottom was collected using an Ekman dredge that samples approximately 0.06 m 2 of sediment.The sediment was sieved by using a 1 mm 2 mesh size and the bivalves that were removed were counted and measured.Mean density (number of Individual's m -2 ) was estimated from the data.Shells were measured for anterior-posterior length using a digital caliper and length frequency distributions were constructed.The age of specimens was evaluated by using the age/length data from Cataldo and Boltovskoy (1998).
The Asian clam size frequency distributions at Lacarón site is shown (Figure 3).The population (n= 432) is composed of shells ranging from 12.53 to 25.72 mm in length with a modal size class of 18.1-19 mm.The approximate age distribution of each sample shows that the most of the individuals from the Lacarón sample (85.18%) are less than one year of age whereas the remainder (14.82%) is made up of one-year- No density or population structure data are available yet for the latter location, however it appears that, at these locations, native bivalves are very scarce.
The effects of exotic species on mollusks in the Iberian Peninsula freshwater ecosystems may be going unnoticed because of the lack of information on the distribution and abundance of the native fauna itself.However the decline of some bivalves as Margaritifera auricularia (Spengler, 1793) is partially attributed to the presence of zebra mussel, Dreissena polymorpha (Araujo and Ramos 2000).C. fluminea does not foul native bivalves in the same way that the zebra mussel (Ricciardi et al. 1998) do but it has potential to compete with adult and juvenile bivalves for space and food.

Figure 3 .
Figure 3. Asian clam size frequency distribution at Lacarón site .
length frequency for C. fluminea in the Guadiana River