Reproductive behavior of the freshwater mussel Dreissena polymorpha in Italy: a comparison between two populations

The gametogenic cycle of Dreissena polymorpha is described for two populations living in Lake Come and the Adda River in Northern Italy. Reproduction was monitored by histological examinations of gonads and by morphometric changes in oocyte nuclei from February 1998 to January 1999. Gonadal development commenced overwinter. Gamete deposition started in spring when the water temperature reached 13 degreesC. By May, the reproductive season had commenced but while in Lake Come it finished in August, in the Adda River depositions were observed until the end of October. The course of the Maturity Index and mainly the oocyte nuclei diameter analysis suggested the possibility of two main reproductive events in the lake zebra mussel population. In contrast, more continuous spawning was observed in the river mussels. In both sites, frequency distributions of nucleus size showed maturation of at least two gamete cohorts which supported successive depositions. The gonadal resting stage was longer in males than females. Oogenesis started soon after the end of the reproductive period while spermatogenesis commenced only 3 or 4 months later. This is the first study dealing with the Dreissena polymorpha reproductive cycle in the South Alps. Results are discussed and compared with regard to other populations living in the rest of Europe and North America.