A histological study of the gametogenic cycle of the freshwater mussel Dreissena polymorpha (Pallas, 1771) in Lough Derg, Ireland

The gametogenic cycle of three size classes of the freshwater mussel Dreissena polymorpha was investigated by histological examination during 2000 and 2001 at Lough Derg. Co. Limerick, Ireland. This study provided a full staging method for the study of the reproductive cycle of the zebra mussel. Gonads and gamete development were observed in mussels in all months and in all sizes of mussel from 6.0 mm up to 25.9 mm long. Gonads (non ripe) were only observed in a small percentage (3.4%) of individuals smaller than 6.0 mm and only in females. Thus, mussels become sexually mature and are able to reproduce from at least 6.0 mm long and during their first year, characterizing a fast-growing population. It also appeared that only one period of successful reproduction existed, as only during 3 months of the year was there synchronicity in the gonadal development of males and females. Variations in the sex ratio were observed with a shift from female dominance in September 2000 to equal proportions of male and female in October 2001. Gamete development in males appeared to be more dependent on external factors, such as water temperature, illustrating a potentially important difference in the reproductive physiology of male and female zebra mussel.