Over the summer of 2011, the water in Lake Michigan turned crystal clear. Shafts of angled light lit the lake bed, like searchlights from a U.F.O.; later, old sunken ships came into view from above. Pleasure was soon replaced by panic: lakes are not supposed to look like swimming pools. When biologists investigated, they found that the turbid swirls of plankton that typically grow in the lake by the million had nearly vanished—consumed gradually, they could only guess, by some ravenous organism.
The likely culprits were mollusks: the zebra mussel and its cousin the quagga mussel. The two species—Dreissena polymorpha and Dreissena bugensis—are thought to have originated in the estuarine basins of Ukraine, notably that of the Dnieper River. In the late nineteen-eighties, cargo ships, travelling from the Caspian Sea and the Black Sea, had dumped their ballast water into the Great Lakes, contaminating them with foreign organisms.