Fig. 2. Diagram of the behavioral test apparatus. The transparent water container is placed in the center of a drum that rotates clockwise or counterclockwise. A black segment printed on white paper attached on the inside of the drum serves as a visual stimulus. When the black segment comes into view, the fish reacts with an `escape' response, i.e. by reversing its swimming direction. The column in the center of the container prevents the fish from swimming across the center. Above the apparatus is suspended a light-source, the intensity of which is controlled by neutral density filters. An infrared video camera was used to observe the behavior of the fish at low light levels.