Revision of the distribution of Corbicula fluminea ( Müller , 1744 ) in the Iberian Peninsula

This paper presents an overview of the chronology of Asian clam Corbicula fluminea invasions in the Iberian Peninsula and additionally presents the distribution of all relevant records published from 1981 (first record in the Peninsula) until 2008 in two geographic formats with references to invaded basins. In short, a total of six great basins and thirteen UTM 100x100 squares showed the presence of the species.

Freshwater ecosystems are the most severely impacted in the world and are experiencing a decline in biodiversity and modifications by human activity worldwide (Revenga and Kura 2003).In terms of biological perspective, these global changes include the homogenization of Earth's biota by means of invasive species (Ricciardi and Rasmussen 1998).In terms of both abundance and biomass, invasive bivalves of the genus Corbicula (Asian clam) are one of the most widespread species.The actual distribution of the Asian clam includes southeastern and eastern Asia, Australia and Africa (McMahon 1983).The species Corbicula fluminea (Müller, 1744) and Corbicula fluminalis (Müller, 1744) are among the most "efficient" freshwater invaders worldwide, appearing as exogenous species in watercourses of both North and South-America, Japan and Europe (Araujo et al. 1993).The IUCN Global Invasive Species Database considers the second species as synonym of the first, so hereafter both species will be considered as Corbicula fluminea.
At least eight alien freshwater molluscan species have been introduced into the Iberian Peninsula in the last century (the gastropods Melanoides tuberculata (Müller, 1774), Potamopyrgus antipodarum (J.E.Gray, 1843), Physella (Costatella) acuta (Draparnaud, 1805) and Gyraulus chinensis (Dunker, 1848), and the bivalves Dreissena polymorpha (Pallas, 1771), Mytilopsis leucophaeta (Conrad, 1831) and Corbicula fluminea (Müller, 1774)) and most are widespread in this area (García-Berthou et al. 2007).The first Iberian Peninsula record for the genus Corbicula was in the Tajo River basin at the beginning of the 1980´s (Mouthon 1981), since this time distribution data for the Asian clam have experienced an exponential increase until 2008 (Figure 1).Corbicula fluminea is present, at least, in more than six great Iberian basins (Figure 2 and Annex 1), and comprises 14.6% of the 100x100 km UTM (Universal Transverse Mercator coordinates) web of the Iberian Peninsula.This figure indicates that this species remains undetected or unreported only in the centre and south-eastern Iberian watercourses.
As a general rule introduced species can become more easily established in altered ecosystems (Clavero et al. 2004;Pérez-Quintero 2007).In highly variable Mediterranean-semiarid  freshwater habitats, like those of the Iberian Peninsula, small rivers with low flow rates are strongly influenced by two annual and predictable cycles: catastrophic winter floods and extended summer drought periods (Gasith and Resh 1999).For both reasons the actual distribution of the Asian clam in the Iberian Peninsula is related to human-impacted freshwater lentic sites like reservoirs and lotic habitats with permanent flow, like canalizations, irrigation tubes or docks (personal observations), but also to great rivers with permanent-deep sites.Both lentic and lotic sites provide the stable habitats in which the Asian clam can reduce the biotic stress related to the summer drought and so develop thriving populations.

Figure 1 .
Figure 1.Cumulative number of publications referring to the distribution of freshwater invasive molluscan species (filled circles) and to Corbicula species (empty circles) in the Iberian Peninsula, since the first species record in the twenty century.References for Corbicula publications appear in Annex 1, references to freshwater invasive molluscan species are from Table 1, Vidal-Abarca and Suárez 1985, García-Berthou et al. 2007 and author's database.

Figure 2 .
Figure 2. Distribution of Corbicula species across the Iberian Peninsula.Left: localities in which the species have been detected, with numbers inside brackets referring to river basins named inTable 1; right: UTM sites with presence of the Asian clam.
Annex 1. Chronology of the invasions of Corbicula fluminea in the Iberian Peninsula (w/d: without data).
* -first record in the basin