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First published online October 18, 2006
Journal of Experimental Biology 209, 4254-4261 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02533

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Use of urea as a chemosensory cloaking molecule by a bony fish

John F. Barimo^* and Patrick J. Walsh

Division of Marine Biology and Fisheries, NIEHS Marine and Freshwater Biomedical Sciences Center, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149-1098, USA

View larger version (17K): [in a new window]   Fig. 1. Top view of experimental setup. Two behavioral arenas were cleared in the seagrass canopy where PVC stakes marked set distances from experimental shelter openings (25 and 50 cm). Two experimental shelters provided simultaneous delivery points for either an odorant or a negative (seawater) control in a modified Y-choice configuration with corresponding shelter assignments randomized. A submersible infrared camera was mounted on each shelter's rear end-cap and directed outward. Polyethylene tubing was connected to the odorant injection and water-sampling ports, with odorants and controls injected via peristaltic pumps to respective shelters. Baseline snapper activity was established prior to any odorant delivery. The order of odorants was randomized for each individual gray snapper to prevent treatment order effects, and the positive control was always delivered last. Overall tank flow was directed away from shelters with inflow pipes facing a reef-like structure, behind which was the tank drain pipe. Gray snappers generally hovered within the reef structure.

View larger version (7K): [in a new window]   Fig. 2. Behavioral responses to 25 µmol N l-1 ammonia-N (Amm), urea-N, a 1:1 ammonia-N/urea-N mixture (A/U Mix) and controls during 60 min observation periods. Clarified toadfish homogenate, a positive control, was delivered last. The behavioral index is the sum of snapper responses characterized by entry into the shelter, passing 0-25 cm from shelter opening, passing 25-50 cm from shelter, and hovering while visually scanning shelter interior, which were awarded 5, 2, 1 and 1 points, respectively. The baseline reflects activity prior to odorant delivery. Data were log(x+1) transformed for equal variance. The paired t-test was used to examine each treatment and its alternate seawater control, and significant differences are denoted by an asterisk. Differences among treatment group where determined with a repeated measures ANOVA (F=14.509, P<0.001, N=10) and significantly different groups are indicated by different letters.

View larger version (10K): [in a new window]   Fig. 3. (A) Number of approaches by snappers to <50 cm from the experimental shelter during the ammonia and urea treatments, separated into 10-min intervals. There were no significant differences among time intervals for either odorant with repeated measures ANOVA. (B) Ammonia concentration (mean ± s.e.m., N=10) from the calibration of the odorant delivery system. 33 mmol N l-1 NH4Cl was delivered for 30 min (1.67 ml min-1) followed by ambient seawater. The shelter's internal ammonia concentration (after mixing) was sampled via the access port.

View larger version (12K): [in a new window]   Fig. 4. (A) Response to a 2.5 µmol N l-1 mixture (1:1:1) of L-proline, L-alanine and L-glycine (Amino Acids; PAG); 2.5 µmol N L-1 PAG with 25 µmol N L-1 ammonia-N (Amm + Amino acids); 2.5 µmol N l-1 PAG with 25 µmol N l-1 ammonia-N + urea-N (1:1; A/U mix + Amino acids); and toadfish homogenate (positive control). Significant differences were found between treatment groups (F=7.393, P<0.001, N=8) as indicated by the different letters. (B) Behavioral assay to measure response to a 2.5 µmol N l-1 mixture (1:1:1) of PAG; 2.5 µmol N l-1 PAG with 25 µmol N l-1 urea-N; and seawater controls. Significant differences were found (F=9.293, P=0.005, N=6). Controls, the tabulation of the behavioral index, data transformations and subsequent statistics were identical to Fig. 2.