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absence of a fright reaction and not quantify intensity of the <br />response. After conclusion of video interpretation, results were <br />tabulated for statistical analyses. <br />Scanning Electron Microscopy <br />Three fish from each exposure concentration were fixed and <br />preserved for SEM by immersion in 3o glutaraldehyde buffered to <br />pH 7.2 with sodium phosphate. Following preservation, <br />toxicant-exposed and control specimens from each experiment were <br />processed as a batch to eliminate bias from variable SEM <br />preparation techniques. Specimens were rinsed with phosphate <br />buffer and postfixed with 1% osmium tetroxide (Sigma Chemical <br />Company, St. Louis, Mo.}. Specimens were dehydrated in a graded <br />acetone series and critical-point dried in a Polaron apparatus <br />(Bio-Rad, Cambridge, Mo.). Individual specimens were mounted on <br />aluminum stubs with colloidal silver, sputter coated with gold <br />(20 nm thickness; Hummer VII sputtercoater, Anatech, Alexandria, <br />Va.), and examined using a Philips 505 scanning electron <br />microscope (Einhovn, Holland). For details of these procedures <br />see Lee (1993). <br />Specimens chosen for SEM confirmation were from controls and <br />the lowest toxicant concentrations that inhibited olfactory <br />ability in all 10 replicates, or if inhibition did not occur, <br />from the highest toxicant concentration. It was anticipated that <br />olfactory receptors from fish in toxicant-exposure concentrations <br />would show damaged olfactory receptors, whereas fish from <br />8 <br />