ESCAPE TRAJECTORIES IN ANGELFISH (PTEROPHYLLUM-EIMEKEI)

Although fish escape trajectories are linearly related to stimulus dir ection, they vary considerably after the initial turn away from the st imulus. Past studies of escape trajectories in fish and other animals have been analysed by employing linear plots of stimulus angle versus body turning angle. Here, we define escape trajectories as a circular variable, with 0-degrees as stimulus direction. Angelfish (Pterophyllu m eimekei) escape in non-random trajectories when the stimulus is pres ented laterally, within an angular zone of approximately 30-120-degree s (discriminating zone). The circular plot of escape trajectories show s a bimodal pattern that cannot be revealed by linear analysis. Angelf ish escape preferentially at 180-degrees and 130-degrees away from the stimulus, maximizing the distance covered from the stimulus and escap ing at the limit of their discriminating zone, respectively. Angelfish correct their responses when turning towards the stimulus, suggesting that escape trajectories are modulated by sensory feedback. Re-analys is of published work on other animals, by employing circular histogram s of escape trajectories, reveals multimodal patterns which are also n ot apparent from the linear plots. We suggest that the presence of mul tiple preferred trajectories may be adaptive in preventing predators f rom learning any single fixed pattern of response and compensating for it.