According to this press release at EurekAlert, new research indicates that calcium may be the key to why zebra mussels and quagga mussels (Dreissena polymorpha and Dreissena rostriformis bugensis) have not shown up some parts of the USA. Scientists looked at calcium concentrations from streams and rivers in the US to determine which regions had the greatest risk of mussel invasion. They found that low calcium levels meant an area was less likely to have either species. That's good news for New England, and bad news for those feeling optimistic about the Lake Mead invasion in the western USA.
The scientists surmise that this finding is due to the fact that the invasive mussels have a higher requirement for calcium than native mussel species. If you want to read more, the full article was published in the latest issue of Frontiers in Ecology and the Environment. The article abstract here. If, like me, you don't have full access to that journal, you can entertain yourself with the map that resulted from the analysis.