SynopsisFish groups were tested both in a circular and in a figure eight-shaped channel. In both cases fish showed a long lasting, constant direction swimming provided that illumination was maintained at a constant angle around the channel. In the circular channel, fish did not reverse direction, as would be expected, when light angle was shifted from one side to the other in the channel. However, direction reversals did occur when these illumination shifts were performed on the eight-shaped channel. We suggest that constant-oriented swimming reflects a sun-compass oriented behavior, but swimming at a constant angle in the circular channel produces an irreversible disarrangement of the inertial-orientation system, which does not occur in the eight-shaped channel due to the geometrical relationship between the light and the shape of the channel.

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