© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/10/306-0273 Internat. Rev. Hydrobiol. 96 2011 3 257–273 DOI: 10.1002/iroh.201111344 ERIKA BÓDIS* , 1, 2 , JÁNOS NOSEK 1 , NÁNDOR OERTEL 1 , BENCE TÓTH 1 and ZOLTÁN FEHÉR 3 1 Danube Research Institute of the Hungarian Academy of Sciences, Jávorka S. u. 14, H-2131, Göd, Hungary; e-mail: bodiserk@gmail.com 2 Szent István University, Faculty of Veterinary Science, Department of Ecology, Institute for Biology, POB 2, H-1400, Budapest, Hungary 3 Hungarian Natural History Museum, Baross u. 13, H-1088, Budapest, Hungary Research Paper A Comparative Study of Two Corbicula Morphs (Bivalvia, Corbiculidae) Inhabiting River Danube key words: bivalve, invasion corridor, Middle Danube, morphological and genetic variability Abstract In Europe, at least two Corbicula morphs (generally referred as C. fluminea and C. fluminalis) are known. However, their taxonomic and systematic position is controversial. In order to ascertain their taxonomy and systematics, we studied their characteristics in River Danube. Morphometric analysis confirmed the existence of two distinguishable morphs. Based on mtCOI sequence, morph-1 is identi- cal to C. fluminea, while morph-2 is an undefined species. According to the median joining analysis, morph-1 belongs to the C. fluminea clade, whereas morph-2 forms a distinct clade, which is known only from invasive populations. Differences were recorded in population structure and reproductive biology, too. It is probably correct to treat the two morphs as distinct taxa, but as long as the African and western Asian parts of the genus’ range are poorly studied, taxonomic position of morph-2 remains unclear. 1. Introduction In recent decades, the appearance and quick expansion of non-native species in aquatic systems was extremely accelerated. This acceleration can be explained by the increasing intensity of ship traffic due to the globalization of international trade, increasing recrea- tional activities, and the transformation of natural water systems, which is connected with the degradation of native biota and the vacancy of habitats (COHEN and CARLTON, 1998; EVERETT, 2000). Due to human activities and opportunistic life-strategy (good dispersion abilities, high ecological tolerance, high fecundity and growth rates, early maturity, and special repro- ductive biology), some bivalve species increase their distribution and become dominant elements in the benthic communities (SOUSA et al., 2008a, b, 2009). The dominant bivalve species deserve special attention since they may influence the nutrient cycles and water clarity with resulting important changes in the structure and function of the whole aquatic ecosystem (KARATAYEV et al., 2006). Corbicula fluminea is recognized as one of the most * Corresponding author