Congener Dreissenid Mussels Differ in Their Survival and Behavioral Responses to Drought and Immersion in Sediments.

Quagga (QM) and zebra (ZM) mussels (Dreissena bugensis and Dreissena polymorpha) are invasive bivalves forming freshwater fouling communities of high economic and environmental importance in Europe and North America. They may experience immersion in soft sediments by sedimenting particles (mostly at deeper locations or areas with high hydrodynamic activity) or while attached to burrowing unionid mussels. On the other hand, mussels located near the water surface may encounter water level decreases and air exposure during droughts. We experimentally tested mussel survival and behavioral responses to immersion in sediments and water level reduction. Interspecific differences in these responses can help understand the ongoing replacement of ZM by QM in invaded communities and less common cases of their co-existence. ZM were more resistant to both stressors, surviving longer when immersed in sediments or exposed to air. Moreover, both species survived better in coarse versus fine sediments. QM re-emerged from sediments more efficiently than ZM. In turn, ZM relocated over longer downward distances facing water level reduction compared to the behavior of individuals tested at the constant water level. Thus, ZM seem better adapted to remain at their attachment sites, survive temporary adverse conditions, and cope with dangers typical for shallow nearshore areas. On the other hand, QM perform better on soft bottoms, abandoning their attachment sites to cope with immersion in sediments. These adaptations allow QM to dominate deep-water soft sediments, whereas ZM can find refuges in areas periodically exposed to air and on unionid mussels.