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Materials and Methods <br />Experimental animals <br />Algae and rotifers <br />Monocultures of the freshwater algae Chlorella vulgaris (Carolina Biological Supply <br />Company, Burlington, North Carolina) and the rotifer Brachionus calyciflorus (Florida Aqua <br />Farms, Dade City, Florida) were maintained using prescribed methods (Hoff and Snell 1987). <br />Both organisms were maintained in a series of 20-L batch cultures with five target exposure <br />concentrations (0.0, 2.5, 5.0, 10.0, and 20.0 µg/L dissolved selenium). Each rotifer culture was <br />fed algae from the corresponding selenium exposure concentration (e.g., rotifers in the 20.0 µg/L <br />treatment were fed algae from the 20.0 µg/L treatment) two or three times daily. Abundance of <br />rotifers in batch cultures was quantified daily by subsampling. <br />' Razorback sucker <br />Razorback sucker larvae were obtained from the Grand Valley Propagation Facility (U.S. <br /> <br /> <br />Fish and Wildlife Service, Colorado River Fishery Project, Grand Junction, Colorado) and <br />transported to laboratory culture facilities at Colorado State University (Fort Collins, Colorado). <br />Culture-facility water temperature was 19°C. Razorback sucker larvae were reared in mass <br />cultures until approximately 75% of fish were observed feeding on live brine shrimp nauplii or <br />rotifers (10 days after hatching). Then, randomly-selected fish were transferred to exposure <br />beakers for acclimation to testing conditions. Larvae were approximately 12 days old (after <br />3 <br /> <br />