Population dynamics and life cycle ofPisidium amnicum (Müller) (Bivalvia : Sphaeriidae) andValvata piscinalis (Müller) (Gastropoda : Prosobranchia)in the Saône river, a nine-year study

Abstract

The population dynamics and life cycle of two mollusc species Pisidium amnicum and Valvata piscinalis of the Saône river werestudied for nine years. The newborn of the former species appeared in April each year. The latter species produced two cohorts peryear until 2003. Spawning in spring led to the birth of the early cohort, which generally appeared in May, while spawning in thesummer led to the birth of the late cohort, which appeared from July to November. The increase of the mean annual temperature by1.9°C from 1998 to 2000 was followed by an increase in the density of mollusc populations, possibly triggering or amplifying interand intra specific competition for food. However, the 2003 heatwave was responsible for the sudden decline of the P. amnicumpopulation, then the more progressive decline of that of V. piscinalis after having strongly modified its demographic structure andits life cycle. In 2004 and 2005 V. piscinalis only produced one cohort per year. The observations carried out strongly suggest thatthe insufficiency of available resources was one of the possible causes of their failure to recover in 2004 and 2005. The increase ofglobal warming predicted by different models could result in the gradual disappearance of P. amnicum from the potamon and ofV. piscinalis from the freshwater systems most directly exposed to climatic warming, such as the Saône River.