The Iowa Department of Natural Resources reports that zebra mussels have spread in the Iowa Great Lakes after the first discovery of the invasive pest in 2012 in East Okoboji Lake and Upper Gar Lake. The mussels, which can clog water intakes and smother native species, have spread to all the lakes in the chain but Big Spirit Lake, with homeowners reporting that the mussels have clustered on docks and hoists. The number of reports from East Okoboji Lake has increased significantly, the DNR reported. The D-shaped mussels are less than an inch long and have dark bands. They are filter feeders, which can help clear the water.