Depth effects on zebra mussel reproduction

Male and female Dreissena polymorpha from 2 m depth in Lake Iseo, northern Italy, spawned synchronously, and their gametogenic cycle followed the annual pattern previously observed in other Italian and European populations. Water temperature of 12degreesC and phytoplankton blooms triggered spawning, and seasonal variation in gametogenesis was related to photoperiod. Some mussels at 25 m depth always had active gonads, and reproduction continued all year, with no seasonal gametogenic phases. Hypolimnetic environmental conditions, such as slight variation in water temperature, darkness and low food availability, may cause this reproductive strategy. No evidence of hermaphroditism or modified sex ratios were noted at either depth. Gametogenesis and spawning ability of zebra mussels in the hypolimnion must be reconsidered.