Seasonal and depth variations in gametogenesis and spawning of Dreissena polymorpha and Dreissena bugensis in eastern Lake Erie

Abstract

Dreissena polymorpha and Dreissena bugensis are two exotic species of dreissenid mussels introduced into the Great Lakes within the past decade. Dreissena bugensis profunda is a morphologically distinct dreissenid taxon, representing either a third species or a phenotype of D. bugensis. In in situ mussels, D. polymorpha and D. bugensis from the wave zone spawned simultaneously, approximately 1 week before D. bugensis profunda from the epilimnion and hypolimnion.Dreissena bugensis profunda from the hypolimnion spawned at 9°C, which is below the previously accepted minimum dreissenid spawning temperature (12°C), and may explain why D. bugensis profunda is proliferating in the hypolimnion. The gametogenic index of all taxa was significantly correlated with temperature at all sites. The gametogenic index of D. bugensis profunda from the epilimnion and hypolimnion was significantly correlated with water protein and chlorophyll a content, respectively. In mussels transplanted to the epilimnion, D. bugensis and D. bugensis profunda spawned simultaneously between August 18 and 28, approximately 2 weeks before D. polymorpha. In mussels transplanted to the hypolimnion, D. bugensis and D. bugensis profunda spawned between July 27 and August 18 at a temperature of 9-10°C. Dreissena polymorpha neither spawned nor showed significant gametogenic development, which suggests that this taxon cannot successfully colonize hypolimnial waters.