ESCAPE TRAJECTORIES IN ANGELFISH (PTEROPHYLLUM EIMEKEI)

Although fish escape trajectories are linearly related to stimulus direction, they vary considerably after the initial turn away from the stimulus. 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° as stimulus direction. Angelfish (Pterophyllum eimekei) escape in non-random trajectories when the stimulus is presented laterally, within an angular zone of approximately 30–120° (discriminating zone). The circular plot of escape trajectories shows a bimodal pattern that cannot be revealed by linear analysis. Angelfish escape preferentially at 180° and 130° away from the stimulus, maximizing the distance covered from the stimulus and escaping 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-analysis of published work on other animals, by employing circular histograms of escape trajectories, reveals multimodal patterns which are also not apparent from the linear plots. We suggest that the presence of multiple preferred trajectories may be adaptive in preventing predators from learning any single fixed pattern of response and compensating for it.