Zebra mussels are freshwater organisms, but do possess a limited ability to tolerate brackish conditions. North American populations will tolerate salinity levels up to 5 ppt for short periods of time, but they will not generally survive at constant salinity concentrations above this level. They have, for example, invaded the oligohaline portion of the Hudson River Estuary and are surviving in 2-5 ppt salinity (Walton 1996). Zebra mussels are able to tolerate salinities better when they have been acclimated to increasing levels over time (Wilcox and Dietz 1998).
In Europe, some subspecies of D. polymorpha appear to have higher tolerances, ranging up to salinities of 18.5 ppt, but D. polymorpha polymorpha (the subspecies which invaded North America) is found in European waters only in salinities <5 ppt (Karatayev et al. 1998). European populations of D. bugensis are also limited to waters with salinities <5 ppt (Karatayev et al. 1998).
Fong et al. (1995) have presented laboratory data suggesting that zebra mussels may be able to reproduce in brackish water. In the laboratory trials of Wright et al. (1996), tolerance to both higher temperatures and higher salinities was noted to increase with larval age in both zebra and quagga mussels.