Variations in the Reproductive Cycle of Dreissena Polymorpha in Europe, Russia, and North America

SYNOPSIS. The reproductive cycle of the zebra mussel {Dreissenapolymorpha) is highly variable throughout its range in Europe,Russia², and North America. The environmental factors influencingthis variation are poorly understood, but successful reproductionis occurring in areas where it was initially believed that adultzebra mussels could not survive (i.e., southern United States).The differences in mussel reproduction occurring from site-to-sitemake it difficult to predict timing of specific events, suchas the start of larval production, that are important in initiatingcontainment or control procedures. For example, the amount oftime required for a fertilized egg to develop into a juvenilemussel can be as short as 8 days, or as long as 240 days. Releaseof gametes by adults can be a highly synchronized event, focusedover a 1–2 week period, or it can be completely non-synchronized,occurring throughout the year. Zebra mussels in some localitiesstart spawning at water temperatures of 12–13°C, butdo not start until water temperatures reaches 22°C at othersites. While some of this variability in reproductive behaviorstems from mussel adaptation to local conditions, part is dueto difficulties in sampling these events. It is difficult todetermine reproductive success of a specific population becauseof the problems in separating locally produced larvae from larvaedrifting in from other areas. Further research is needed notonly on the relationship between reproduction and environmentat the community level, but also on the variability in responseof individual mussels.