Les mollusques de deux rivières franc-comtoises le Drugeon et la Clauge, comparaison entre les inventaires 1977-1978 et 2009-2010

The mollusc fauna of the Drugeon and the Clauge rivers, two small tributaries of the Doubs river, surveyed for the first time in 1977 and 1978, respectively, were re-sampled in 2009 and 2010 to asses long-term changes. Overall, the total species richness remained stable during the period (23 vs 22, respectively for the Clauge and 21 vs 19 species for the Drugeon). However, we observed a more important species turnover in the Clauge: four disappearances offset by three appearances including that of the invasive Potamopyrgus antipodarum and the lacustrine Pisidium hibernicum. Conversely the total number of individuals of the two rivers increased between surveys ranged from 5069 to 37241 (x 7.3) for the Drugeon and from 1375 to 15574 (x 11.3) for the Clauge. In this river Unio crassus, which is protected under French law, strongly regressed. The fall in discharge of the two rivers from 2003 due to local human activities but also partly to climatic warming is the most probable cause for the considerable increase in mollusc numbers observed in 2009-2010.