Reproductive biology of the Asiatic clams Corbicula fluminalis and Corbicula fluminea in the river Rhine

MORTON (1986) revised the genus Corbicula and distinguished two species, vi z., Corbicula fluminalis and Corbicula fluminea, each with a contrasting re productive strategy. In the river Rhine, where C. fluminalis and C. flumine a have coexisted in large numbers since their invasion in 1988, their repro ductive aspects were studied from April 1991 to January 1993. C. fluminalis is dioecious, with a small percentage (about 3%) of hermaphrodites, while C. fluminea is a simultaneous hermaphrodite, which incubates fertilised egg s within its inner demibranchs and releases pediveligers (200 mum shell len gth). C. fluminalis and C. fluminea exhibited different spawning periods: C . fluminea released its pediveliger larvae from May to September, when mean water temperature exceeded 15 degreesC, while C. fluminalis released its g ametes during October-December and March-April, when mean water temperature was between 6 degreesC and 15 degreesC. Both species showed two spawning p eaks per year, viz. October/November and March for C. fluminalis and May/Ju ne and September for C. fluminea. In both species, the second spawning peak was shorter than the first, and the percentage of spawning individuals was much lower during the second period. Spawning frequency of C. fluminea cor related positively with the chlorophyll-a content of the water. Body mass d evelopment also showed a close correspondence with spawning, with two peaks per annum. In contrast to C. fluminea, C. fluminalis body mass increased f rom December to March, when chlorophyll-a concentrations were very low, ind icating alternative food sources for this species other than algae (bacteri oplankton, detritus). C. fluminea, which shows brood care, allocated more e nergy resources (51% in May and 21% in September) to reproduction before sp awning than C. fluminalis (33% in October and 20% in March). The co-existen ce of C. fluminalis and C. fluminea in the Rhine branches is probably expla ined by their different reproductive strategies, reproductive periods and p ossibly different food preferences.