"It was nice to see them dead."

Biologists in Minnesota are at a loss to explain why zebra mussels (Dreissena polymorpha) are dying off in a portion of the Upper Mississippi River known as "Pool 8." As reported in this article from Wisconsin's LaCrosse Tribune, divers found piles of empty zebra mussel shells when doing a check of native mussel beds. There are now portions of the Upper Mississippi that have virtually no adults left, and populations have yet to rebound. While parasitic infection and low food supply are potential explanations, neither would be expected to cause such a significant and rapid population decrease.